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(Radiology. 2000;215:27-40.)
© RSNA, 2000

How I Do It

Body CT and Oncologic Imaging1

Kenneth D. Hopper, MD, Kishor Singapuri, MD and Arkady Finkel, MD

1 From the Department of Radiology (K.D.H., K.S.) and College of Medicine (A.F.), Penn State University, PO Box 850, Hershey, PA 17033. Received September 17, 1998; revision requested November 2; final revision received June 8, 1999; accepted August 24. Address reprint requests to K.D.H. (e-mail: khopper@psu.edu).

Abstract

The most commonly used imaging modality in patients with cancer is computed tomography (CT). Whether to evaluate primary tumor or metastases to the neck, chest, abdomen, or pelvis, oncologic body CT has become invaluable to medical, gynecologic, and radiation oncologists. CT is the principal tool used to stage tumor, assess response, and guide radiation therapy. This review provides a discussion of how we optimize oncologic CT to best meet the needs of the patient with cancer.

Index terms: Biopsies, technology, **.12612 • Computed tomography (CT), **.1211 • Computed tomography (CT), helical, **.12115 • Neoplasms, CT, **.12113, **.12114, **.12115, **.12117 • Neoplasms, staging, **.1261 • Neoplasms, therapeutic radiology, **.1261 • Radiology and radiologists, How I Do It

Cancer imaging composes a substantial portion of most radiology practices. Conventional radiography, nuclear studies, and for neurologic and musculoskeletal tumors, magnetic resonance (MR) imaging are commonly ordered in patients with cancer. However, no single modality sees greater overall use in oncology than computed tomography (CT), especially when performed with intravenous contrast material. Whether to evaluate for metastasis to the lung from a sarcoma; to assess local invasion of a primary neck, chest, abdominal, or pelvic tumor; or to detect and/or assess treatment response in patients with metastatic disease, oncologic body CT has become invaluable to medical, gynecologic, and radiation oncologists.

Since its introduction, CT has played a substantial role in the treatment of patients with cancer. However, its use and capabilities have skyrocketed over the past decade because of three developments. First, increasingly powerful computers have allowed the use of a 512 x 512 matrix, complex and improved reconstruction algorithms, faster image reconstruction times, and workstations capable of doing direct dosimetry mapping for patients undergoing radiation therapy. These powerful CT workstations also allow virtual reality endoscopic examinations of the colon, airway, and paranasal sinuses to be performed.

Second is the development of spiral or helical CT. Not only are current scanning times only a fraction of what they were, but intravenous contrast material is optimized, and the overall image quality is substantially improved. In the patient with cancer, the entire chest, abdomen, or pelvis can be scanned during the intravenous administration of one bolus of contrast material in less than a minute with far better image quality and degree of contrast enhancement than is possible with conventional CT. Previously two, or even three, separately scheduled examination slots and doses of contrast material were required to complete this many examinations.

Third, high heat capacity x-ray tubes have been developed to allow spiral scans to be obtained at equivalent imaging parameters as those obtained with conventional CT. The advent of multisection scanners will further decrease acquisition times and increase linear and contrast resolution.

This review will focus on several aspects of oncologic CT. First, a variety of scanning, scheduling, and data reporting tips are provided. Second, the current literature of tumor measurement is reviewed. Third, the current use of CT in radiation therapy planning is discussed. Fourth, a guide to percutaneous tumor biopsy in which current CT scanner technology is used is provided.

THE ART OF CANCER IMAGING

Whether to examine a patient with cancerlike symptoms, to evaluate a primary tumor for local extension, to assess for metastasis, or to determine tumor response to treatment, CT plays a pivotal role in oncology. Many oncologists, however, do not do their best in providing a complete history. Detailed clinical information is important and substantially improves a radiologist's detection of diverse, subtle disease (1,2). In addition, many radiologists do not obtain appropriate histories; devote the attention needed to provide the best examination possible with available equipment; review old studies; simplify and annotate studies by providing prompt, easy-to-understand information to the oncologist; or provide the most accurate and complete tumor measurements possible. To do so improves the quality and accuracy of the information provided to the oncologist.

Many studies have been performed to demonstrate the optimal contrast material injection techniques with helical CT. The rapid scanning times of helical CT allow the torso to be effectively scanned during one examination at the peak of contrast enhancement. For this purpose, power injectors are mandatory. Spiral CT allows visualization of a greater contrast differential between normal tissue and tumor and, as a result, allows the depiction of more tumor foci (eg, in the liver). The greater the difference between tumor and normal tissue, the less the error of tumor measurement (3). However, spiral CT does not eliminate the problem of tumor foci with attenuation characteristics that are only minimally different from those of adjacent normal structures. A difference of at least 10–15 HU is needed for a lesion to be detectable (4–6).

There are three ways to improve the conspicuity of tumor foci. One is to center the display window between the attenuation of the lesion and that of the surrounding normal tissue (7). More practical, however, is the use of both nonenhanced and contrast material–enhanced CT (7). The use of nonenhanced CT is more useful with hypervascular or calcified lesions (5) but may not be cost-effective (8). This is especially true with the improved contrast resolution and lesion conspicuity of helical CT. Whereas we continue routinely to perform initial nonenhanced liver CT when scanning the abdomen, many users do not. Double- and even triple-phase contrast-enhanced spiral scans through the liver (Fig 1), kidney, and pancreas are very useful in finding the contrast material phase in which a lesion is most conspicuous (10–18). Our scanning protocols are summarized in Table 1, with certain items highlighted in the following paragraphs.



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  		Figure 1. Transverse spiral CT images obtained in a 46-year-old woman with a history of osteosarcoma with a hepatic lesion found on a previous screening CT scan. Arterial (top images) and portal venous (bottom images) phase scans (20- and 65-second scan delay, respectively) demonstrate this lesion (arrow) to be a hemangioma. Because the arterial phase study (5-mm section thickness) began at the inferior hepatic edge, the displayed arterial images have some portal venous enhancement. The diffuse intense arterial phase enhancement with quick return to isointensity is a less common appearance of a cavernous hemangioma and occurs in approximately 8% of such lesions (9). The left two images were photographed with soft-tissue (width, 374 HU; level, 92 HU) and the right two with higher contrast liver (width, 200 HU; level, 125 HU) settings. P = posterior, R = right.

 

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  		TABLE 1. Oncologic Body CT Protocols
 
Several investigators (19–22) have used automated computer techniques to reduce the volume of contrast material required for hepatic CT and to optimize opacification. With one technique, scanning is initiated only when an operator-selected threshold of hepatic enhancement is reached (eg, 50 HU). However, this technique requires 8–10 seconds to actually initiate scanning after the threshold has been reached.

Another method of optimizing the use of contrast material is the use of saline solution loaded behind the contrast material (Fig 2). This technique, called the "saline push" (23) (Hopper KD, Oh YZ, Mosher TJ, Kasales CJ, TenHave TR, Weaver JS, unpublished data, 1998), clears residual contrast material from the syringe, the tubing, and the patient's arm by forcing all contrast material into the central circulation. The saline push technique can be used to reduce the amount of contrast material required by up to 50 mL or can be used to provide increased opacification.



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  		Figure 2a. Photographs of a power injector show contrast material followed by saline push. (a) After the contrast material is drawn into the injector syringe, the technologist carefully loads 50 mL of injectable saline solution, colored blue for demonstration. All air is then evacuated from the syringe, and the tubing is attached and filled. With a continuous, steady, and slow motion, the technologist then inverts the injector such that its nozzle is pointed directly at the floor. (b) During this inversion, the saline solution flows over the top of the contrast material, and their positions in the syringe are reversed. During the injection, the saline solution clears the syringe, tubing, and patient's arm of contrast material.

 


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  		Figure 2b. Photographs of a power injector show contrast material followed by saline push. (a) After the contrast material is drawn into the injector syringe, the technologist carefully loads 50 mL of injectable saline solution, colored blue for demonstration. All air is then evacuated from the syringe, and the tubing is attached and filled. With a continuous, steady, and slow motion, the technologist then inverts the injector such that its nozzle is pointed directly at the floor. (b) During this inversion, the saline solution flows over the top of the contrast material, and their positions in the syringe are reversed. During the injection, the saline solution clears the syringe, tubing, and patient's arm of contrast material.

 
Current spiral scanners with a 180° linear interpolator show little degradation when going from a pitch of 1.0 to 1.5. Using a pitch of 1.5 allows one to use a routine section thickness of 8 mm in the body while scanning from the thoracic apex to the adrenal glands in one breath hold or from the diaphragm to the base of the pelvis in one scan. The routine use of a thinner section decreases measurement variability (3,24) and allows the detection of smaller pulmonary nodules and hepatic lesions than those visible with a 10-mm section thickness. A combined thoracic-abdominal-pelvic CT scan can be obtained by using an 8-mm section thickness, a pitch of 1.5, two 24–28-second breath holds, a 5-second intervolume breathing delay, and a single bolus of contrast material. For patients unable to hold their breath this long, a pitch of 2.0 can be used to decrease the scanning time, or the patient can be allowed to breathe quietly during image acquisition. This latter approach works well in young children and patients who are very ill, unconscious, or uncooperative.

An increased pitch allows the use of thinner sections, which provides more views through the area of interest and improved image quality. In the lung, the use of an 8-mm instead of a 10-mm section thickness decreases the size of a pulmonary nodule that can be missed to approximately 1.5 mm in diameter versus approximately 3.0 mm (25,26). For the neck, pancreas, adrenal glands, and kidneys, a 4- or 5-mm section thickness can be used. Exact section thicknesses possible with spiral CT will vary with the equipment used. When examining the chest or combined chest, abdomen, and pelvis, we use a scanning delay of 40 seconds. This allows an even distribution of contrast material throughout the mediastinal vessels and heart and allows the liver to be scanned in its portal venous phase. When scanning the abdomen and pelvis together, we will not allow patients to void from when they begin drinking oral contrast material until the end of the CT examination. This causes the bladder to be distended with urine, which allows the abdomen and pelvis to be scanned in a single contrast-enhanced spiral acquisition (65-second scanning delay, 8-mm section thickness, pitch of 1.5). This not only decreases the time necessary for these examinations but also allows the pelvis to be scanned with the bladder distended during the vascular phase of contrast enhancement (Fig 3). If bladder distention is not important, the patient can be allowed to void just before CT scanning, and the abdomen and pelvis can then be scanned together with the bladder collapsed.



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  		Figure 3. Transverse contrast-enhanced abdominal CT scan of the pelvis in a 43-year-old woman with International Federation of Gynecology and Obstetrics, or FIGO, stage IIB cervical cancer. She was not allowed to void prior to CT, which caused the bladder (B) to be distended with urine. The practice of not allowing patients to void prior to abdominal or pelvic CT not only decreases the time necessary for these examinations but also allows the pelvis to be scanned with the bladder distended during the vascular phase of contrast enhancement. The arrows indicate opacified vessels.

 
The speed of reconstructions of current spiral scanners allows the acquired volumes to be reconstructed more than once. We have found it an especially useful technique to reconstruct the volume a second time with edge-enhancing interpolators and algorithms when evaluating the lung (27) or bone structures (Figs 4, 5). As lung and bone images can be filmed automatically, the double reconstruction of spiral volumes to enhance detail in bone or pulmonary parenchyma requires only the 5–8 seconds needed for the technologist to type in the commands. In the thorax, we use this opportunity to change the reconstruction interval from 8 to 7 mm (8-mm section thickness), which allows the routine display of 1.0-mm pulmonary nodules (25,26). Some centers routinely use bone windows in all patients with cancer. We and others (28) have not found this time- and cost-effective.



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  		Figure 4. Transverse follow-up CT scans show a new spiculated mass (arrow) in the left lung in a 77-year-old man with a previous right pneumonectomy for non-small cell carcinoma. The left two images demonstrate this metastatic tumor with use of the routine standard interpolator and algorithm for evaluation of the mediastinum. The spiral volume was reconstructed a second time with edge-enhancing interpolators and algorithms (right two images) and yielded a sharper display of the pulmonary parenchyma. P = posterior, R = right.

 


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  		Figure 5. Transverse follow-up diagnostic abdominopelvic spiral CT scans show a spinal lesion (arrows) in a 23-year-old man with a history of a previously resected pelvic Ewing sarcoma. The left two images show magnified views of this spinal-left transverse process lesion on the standard interpolator and algorithm images. The right two images, however, show the volume reconstructed a second time with edge-enhancing interpolators and algorithms. P = posterior, R = right.

 
A common error in filming oncologic CT images is photographing them needlessly small. All current scanners allow needless and redundant text on each image to be eliminated, which causes a net increase in image size (Fig 6). In addition, when technologists size a thoracic, abdominal, or pelvic CT scan for reconstruction, they can exclude the lateral subcutaneous fat and tissues, which further increases image size (Fig 6). We have incorporated these two changes into our practice by increasing the number of frames per sheet of film from 12 to 16 or 20. This not only yields the benefit of substantial film savings, but it also, we believe, makes these complex tumor cases easier and faster to review. However, the use of smaller fields of view, or larger images, causes some of the subcutaneous tissues and any associated subcutaneous disease to be excluded from the image (Fig 6, H).



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  		Figure 6. A, B, Anteroposterior CT scout images show a new right hilar metastatic lymph node (arrow in A) in a 65-year-old woman with known ovarian cancer. Transverse spiral CT images are displayed in C-E, which show rectangular matrices with all text and the gray scale shown on every image, and F-H, which show square matrices with the gray scale and most of the text removed. Three different fields of view (B, arrows) were chosen for reconstruction: 1, (C, F) maximum skin-to-skin; 2, (D, G) minimum skin-to-skin; and 3, (E, H) outside rib to outside rib at the lowest level of pulmonary parenchyma. Each image was photographed onto film with the same frame size of four on one. The film images were then photographed for the illustration with exactly the same magnification for each. H, The use of the minimum field of view necessary (outside rib to outside rib) and a square matrix results in the largest image per frame.

 
Oncologic CT scans are frequently complex. Radiologists should mark these studies by highlighting hard-to-see disease and annotating measured tumor sites. We have found red grease pencil and circles are best for this purpose, as red is the easiest color to see on a view box, and our clinical colleagues can easily find red by using room light. Circles around specific findings or foci are better than lines because one's eye is immediately drawn to the center of a circle. Specific measured tumor sites should be numbered on the film, and calipers (electronic on a workstation) (Fig 7) should be used for all measurements (29). Regardless of the experience of the radiologist, measurement by means of gestalt is substantially wrong 28% of the time (24). Annotations in this manner simplify the oncologists' job of reviewing the study on what may be only one or two view boxes in a well-lit office or examining room. Paging through the multiple sheets of film, they can find quickly the particular annotated images they want to see.



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  		Figure 7. Transverse spiral CT scans show metastatic lung cancer, with measured tumor sites marked, in a 62-year-old woman. The careful numbering (1-6) of measured tumor sites on the film simplifies the oncologist's job in reviewing these studies and improves the accuracy and speed of the radiologist in remeasuring the same sites at subsequent examinations. P = posterior, R = right.

 
These particular annotations also save the interpreting radiologist time in two ways. The first is that on follow-up studies, it is easy for the radiologist to visualize where the tumor sites were previously measured; and, for complex masses, there is instant display of the exact level at which the site was measured. The second way is through decreased consultations with the oncologist. When oncologists, through their own review of their fully annotated films, can answer all or most of their questions about a patient's study, the patient's need for further consultation with a radiologist is decreased. Patients benefit through more accurate measurements on follow-up studies and through the increased understanding their oncologists have of their complex CT examination results.

Film annotation, however, is not sufficient to transmit exact measurements to the oncologist. We have found that a simple table rubber stamped onto the CT scan jacket (Fig 8) solves this problem well. Its three columns listing the site measured plus old and new measurements allow the oncologist, once the table is completed, to quickly evaluate the patient's overall tumor burden and response to treatment. This table also greatly simplifies the job of the radiologist on follow-up studies, for which the new measurements from the previous CT study are copied in order as the old measurement on the current one. We have also adopted the practice of actually dictating this table into the typed report (Fig 9). This simplifies the dictating process and presents the data in a concise manner. Once sites are identified and numbered, it is important to maintain the numbering system and order, as frequently the measurements are copied onto regional or national oncologic forms for entry into a multicenter treatment database. For this reason, too, any changes in previous measurements must be carefully described in the new report.



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  		Figure 8. Photograph shows a simple table stamped onto the patient's CT scan jacket, which allows meticulous delineation of each tumor measurement.

 


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  		Figure 9. Photograph shows a radiology report. Tumor measurements are easily dictated into the radiology report and actually simplify the dictation process. The organized display of new versus old measurements allows the oncologist to quickly evaluate the patient's overall tumor burden and response to treatment. This table also simplifies the job of the radiologist in evaluating follow-up studies.

 
The incorporation of filmless radiologic and oncologic imaging will require creative solutions to provide similar information to the oncologists and to ourselves when comparing or measuring follow-up CT studies. Picture archiving and communication systems, or PACS, must allow the creation of key image files, simple annotation functions, and the dynamic exportability of both the primary study, the annotated key images, and the tumor measurements to clinics and oncologists' offices. One novel approach is that of the visual report in which key images are incorporated into the written report itself (30,31).

Patients with cancer, especially those with metastatic disease, frequently require numerous diagnostic tests, in-hospital treatments, and innumerable clinic appointments. For working patients, this has a serious effect on their schedules. For the patient too ill to work, frequently another family member must assist in getting them to and from these many visits. For these patients and for those who must travel a distance, we have adopted with great success the policy of performing all radiologic examinations the same day as the doctor visit or treatment. The patient is scheduled by the nurses in the oncology department and arrives fasting in the early morning at the laboratory for any required blood tests. These are performed, and the results are reported to the oncologist over the next few hours. If a bone scan is required, the next stop is to have the radiopharmaceutical injected. While the patient is waiting for nuclear imaging, any additional radiography (eg, chest radiography, skeletal survey) and, if needed, electrocardiography, is performed. After completion of the bone scan, the patient reports for CT. At the completion of CT scanning, the patient is sent to lunch while the films are printed and read. On his or her way to the clinic, the patient will pick up the now interpreted radiographs, including the CT scan with carefully annotated measurements. The patient reports to the clinic with all of the radiographs. The treating oncologist has all of the laboratory and radiographic test results available at the time of the patient's visit. The patient and family are inconvenienced for only one long morning rather than having to make repeated visits to the hospital. Working in a referral center, we find this has also helped to eliminate the difficulty of patients undergoing radiographic or CT studies elsewhere and arriving to see one of our oncologists either without the study, or, more commonly, its interpretation.

THE ACCURACY OF TUMOR MEASUREMENT

The size of tumor foci as evident on diagnostic imaging studies is a commonly used criterion in assessing the response of patients to therapy (32–34). According to the World Health Organization criteria (29,33), two dimensions of each lesion are obtained—the largest diameter plus its greatest perpendicular— as these diameters provide the best overall representation of the amount of tumor present (29,34). For CT, both of these measurements are obtained on the transverse section that demonstrates the largest tumor diameter. These dimensions are then multiplied together, yielding a two-dimensional area of each tumor focus. The area measurements of all tumor foci are then added together to yield a number representative of the patient's overall tumor bulk (35). With treatment, serial imaging studies are obtained and foci are measured to assess changes in the amount of tumor present. In general, tumor growth of 25% or more or new site of tumor portends a poor prognosis, which is indicative of treatment failure. Tumor regression of 50% or more is categorized as a partial response, whereas complete disappearance is considered a complete response or remission (Table 2).


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  		TABLE 2. Response Categories for Patients with Cancer
 
The history behind the development of these measurement and response criteria centers on a desire by oncologists to standardize and assess various treatment regimens for a given tumor. This has focused attention on the methodology and accuracy of oncologic measurement. The greatest variability in assessing tumor size, as might be expected, is by gross palpation. In a study of 26 oncologists, Lavin and Flowerdew (36) found such variability in tumor assessment by palpation alone that they recommended that a 100% increase in size be used to indicate tumor progression. Moertel and Hanley (37) found that the error possible with palpation increased with larger masses and that measurement error between investigators was far greater than sequential measurements by the same investigator.

When radiographic studies can be used to assess tumor response, more reliable and reproducible measurements are obtained. In a 1965–1966 study, Gurland and Johnson (38,39) evaluated chest radiographs of 40 lesions, mostly pulmonary nodules, assessed by two radiologists and eight clinicians. In this study, they found extensive measurement variation between readers and between some multiple measurements by the same reader. They also found that there was a greater percentage of measurement variability with smaller rather than larger lesions and that the addition of the greatest perpendicular diameter to the greatest diameter only increased measurement error. On the basis of their study results, they recommended that only the maximum diameter be used to assess tumor size and that at least a 50% decrease in measurable tumor be used to indicate a treatment response.

Other authors (40,41) have supported the use of only the maximum diameter in assessing tumor response to treatment. These studies were, as that of Gurland and Johnson (38,39), largely based on the measurement of pulmonary nodules on chest radiographs. However, only approximately half of primary pulmonary tumors are measurable on a chest radiograph (42). The difficulty in delineating tumor margins, differentiating bulky mediastinal disease from normal structures, and excluding the effects of atelectasis and fibrosis after radiation therapy seriously hinders the use of the chest radiograph alone as a means of assessing tumor.

Most investigators (24,35,37,43) during this period found that the best method of determining tumor size radiographically was by using the measurement of the tumor's maximum diameter plus its perpendicular. To account for the extensive interobserver variability (20% or more) of radiographic measurement, they advocated the response criteria currently in use today (Table 2). Hayward et al (35) also recommended that up to eight lesions be measured. For each tumor site, the two measurements are multiplied together, which yields an area. This makes the assumption that all tumor foci are rectangular. A patient's tumor volume, or bulk, is the sum of these areas.

Body CT for tumor measurement began at the time the current tumor assessment criteria were adopted and gained immediate widespread popularity (44,45). CT is more reproducible and able to delineate two dimensions of most tumor foci. It is frequently easier to differentiate tumor from surrounding normal tissue and from adjacent noncancerous disease, especially with the use of intravenous and oral contrast material. With the depiction of actual tumor shapes, oncologists can view and measure a patient's actual tumor bulk. With conventional radiographic measurements, all tumors are measured in two dimensions, and these measurements are multiplied together, with the assumption that each focus is then rectangular. When both dimensions cannot be measured on conventional radiographs, correlation of a large mass on conventional radiographs is related to a normal structure, such as the ratio of bulky mediastinal disease to thoracic width in the assessment of Hodgkin disease (46,47).

Although two-dimensional measurements provide a method of quantifying the total amount of tumor volume on imaging studies, they are somewhat subjective and can be complicated by superimposed human error ranging from 5% to 10% (37). The use of two-dimensional measurements can also grossly misrepresent the actual size of tumor foci, which are often irregular or infiltrative in configuration (48). Tumor can encase normal structures, which, when included within measured tumor bulk, will lead to overestimation of tumor size or underestimation of tumor response as the surrounding tumor shrinks. In addition, tumors in which the longest dimension is in the cephalocaudal direction will be grossly underestimated with the use of measurements from only the transverse plane.

To solve these problems, many investigators (49–53) have recommended the use of three dimensions—maximum diameter, its perpendicular, and length—to assess tumor size. They have done this because of the assumption that an average tumor lesion is not rectangular but rather ellipsoid. Using both formulas for an ellipsoid ({pi}/6 x L x W x H, 1/2 x L x W x H), Tomayko and Reynolds (49) found both were superior for estimating tumor mass (r = 0.93). The use of maximum diameter (r = 0.66) and area (small masses, r = 0.89; large masses, r = 0.41) was inferior.

Many other investigators (44,54–65), however, have used a sum of areas approach to calculate tumor volume. Described by Breiman et al (54), the actual volume calculation is performed by mapping out the desired area on every CT section by means of a cursor and is multiplied by the section thickness to yield a section volume. The individual section volumes are then added together to obtain an overall site volume. This technique, although time-consuming, has proved to be accurate to within plus or minus 3%–8%.

Several authors (63–65) have compared the sum of areas method of tumor volume assessment with the ellipsoid method, and they have found the sum of areas method to be statistically better. Using several volume formulas (sphere, ellipsoid, rectangular), Albright and Fram (65) found the sum of areas, or planimetry, to be superior (P < .001).

The last method of tumor volume determination has been the use of volumes as calculated by means of a computer workstation (66–68). Although the tumor site is defined as in the sum of areas approach, the computer interpolates between individual sections to create the actual tumor shape (Fig 10). This method is the most accurate in determining a patient's total tumor volume. However, like the sum of areas method, the use of workstation-interpolated actual volumes still requires some operator time in defining tumor edges. Several investigators (59,69–72) are working on methods to fully automate this process. CT workstations that are easier to use and the optimization of scanning protocols and display techniques will be necessary before this approach gains widespread acceptability and reproducibility (73). A simple alternative is to provide coronal or sagittal reconstructions.



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  		Figure 10. Computer-interpolated volumes, as shown here in a 25-year-old woman with primary hepatocellular carcinoma, provide the most accurate method of measuring tumor size. Orange indicates normal liver, red indicates tumor, and white indicates inferior vena cava and portal vein. The top left image is an anteroposterior view, the top right image is a view from the top of the liver, and the bottom two images are views from the bottom of the liver. INF = inferior, SUP = superior.

 
The authors of a few studies have used three-dimensional volume measurements (sum of areas, interpreted volumes) to assess tumor response to therapy. In 19 patients with hepatic tumors, Friedman et al (55) found that in only 41% of the 28 follow-up examinations did the three-dimensional tumor volume measurement agree to plus or minus 20% with the original radiologic interpretation of tumor progression, stability, or improvement. Quivey et al (44) performed manual three-dimensional volume analysis on follow-up CT scans in 20 patients with pancreatic cancer and found it accurate (±5%) and reproducible. They also noted that the ability to outline the contours of the tumors accurately was highly dependent on the CT scan quality and the presence of intravenous contrast material to help distinguish the tumor mass from adjacent normal soft tissue. Hopper et al (68) found the use of actual tumor volumes would have changed the response categories (Table 2) in 52 of 294 follow-up CT scans in patients with metastases.

Interpolated volumes clearly can provide a more accurate representation of the actual amount of tumor present and thus a more accurate assessment of the response of patients to therapeutic regimens. Reported data (68) reveal a significant difference in how patient response would be classified depending on the method of measurement used if current response categories were held constant. Three-dimensional tumor volume estimates appear they would give the most accurate representation of overall tumor burden, as they do not incorporate normal structures in the calculation of volume and do not presume tumor bulk to be symmetric or cylindric. However, the sophisticated computer systems required to perform such calculations may not be available at smaller institutions.

It may seem inappropriate to use traditional categories for classifying patient response when actually measuring tumor bulk with the use of CT volume measurements. The widespread use of a tumor's maximum diameter and its perpendicular was adopted, and response categories based on the variability of measurements on conventional radiographs were created (Table 2). Clearly, further evaluation of the utility of three-dimensional tumor volume measurements in the assessment of patient response is warranted. Since the addition of such measurements to preexisting oncologic protocols would drastically alter outcomes and prove disruptive to clinical practice in which conventional categorizations are used, a prospective evaluation through the serial assessment of newly diagnosed disease would be required. New response categories would have to be developed if CT volume measurements were to be adopted universally.

CT-BASED RADIATION THERAPY PLANNING

The clinical practice of radiation therapy requires that a precise dose of ionizing radiation be delivered to carefully designated volumes of tissue. In the curative setting, the treatment volume may include tumor, surgical bed, and areas at risk for recurrence. In treating such tissue volumes, treatment planning is performed to ensure that critical structures such as the spinal cord, kidneys, lungs, optic nerves, and brainstem are not treated beyond functional tolerance. The aim is to accomplish a durable cure while preserving normal organ function. In the palliative radiation setting, therapy is used to control pain, maintain luminal patency, preserve skeletal integrity, and restore organ function. For both curative and palliative radiation therapy, maintaining a high quality of life is of paramount importance (74).

Radiation treatments are often limited by the tolerance of surrounding normal tissue. The prescribed dose for a given tumor must be based on the relative probability of tumor control versus normal tissue complications, and a balance between the two gives a measure of a treatment's therapeutic ratio. The therapeutic ratio is defined as the percentage of tumor cures obtained at a given level to normal tissue complications (75). The radiation oncologist, therefore, needs to have precise knowledge about a patient's anatomy so that tumor volumes and their relationships to normal tissue are clearly delineated.

The photon beams produced by current linear accelerators have substantial penetrating ability. The radiation must penetrate healthy tissue before and after passing through tumor. As a result, intervening and surrounding normal tissue are inevitably irradiated. Although the use of multiple fields has helped to lessen morbidity associated with treatment, the availability of CT has (a) increased the ability to depict anatomic structures for better delineation of the tumor volume and healthy critical organs, (b) allowed planning in a three-dimensional setting, (c) permitted accurate placement of beams and field-shaping devices, and (d) allowed more accurate prediction of dose at various sites (76).

Prior to the advent of CT-based treatment planning and fluoroscopic simulation, localization meant definition of the target area by reference to surface markers by using both clinical and radiologic information. Simulation was performed to check treatment feasibility and to confirm the accuracy of a proposed field size and beam direction. This required placing the patient in the treatment position and selecting beam entry portals that exactly mimicked the proposed treatment on the actual machine. Treatment was simulated by both light and x-ray beams. Radiographic techniques with wire markers and dyes (eg, nephrograms to outline kidneys) were often used to facilitate identification of structures, as conventional radiographs provided only bony detail. Computation of dose distribution was performed manually by using isodose charts or by using computer guidance. These were often presented to the radiation oncologist on a life-size cross-sectional contour of the patient.

When diagnostic CT scans became increasingly available, they were incorporated into the treatment planning process as an additional procedure after conventional radiographic simulation. This included the placement of radiopaque tubing or wire on the patient's skin where the ports were marked at simulation. Oral and intravenous contrast material further helped delineate internal structures with a higher degree of accuracy. The required changes in the fields were made, if necessary. Portals were verified and modified, and treatment was performed (77).

In the past decade, computerized treatment planning has undergone a revolution. The method of simulation and treatment planning described earlier has undergone major changes. The widespread use of CT, MR imaging, fusion software, new hardware and software packages, and three-dimensional displays and graphics all have combined to bring three-dimensional treatment planning to new heights in terms of planning patients' radiation treatments (78).

Three-dimensional treatment planning to deliver fractionated curative radiation therapy is becoming a standard procedure in many radiation therapy facilities. After review of the patient's data and a decision to treat is made, the optimal treatment position is determined. Immobilization devices to assist in maintaining position through a course of fractionated radiation therapy are used as necessary. Orthogonal radiographic simulation films may be obtained prior to CT scanning. The treatment position is recreated in the CT scanner with the help of a laser isocenter projection system as described earlier for the simulator (Fig 11). Flat couches, like the ones used in a linear accelerator, are used during imaging. Cross-sectional images with a section thickness of 4 mm and table increments of 3 mm, although this may vary, are obtained through the volume of interest. Normal anatomic structures and tumor are outlined on each section by the radiation oncologist. The images are loaded into a planning computer. After contouring of a discrete target such as a prostate or pancreatic tumor, computer selection of an isocenter (a point in the target where the beam centers, called "central axes") is performed. The isocenter anterior and lateral entry points are marked on the patient's skin or immobilization device, and the patient leaves the CT scanner. With the help of the treatment planning computer, a three-dimensional reconstruction is obtained, and digitally reconstructed radiographs are generated. On a digitally reconstructed radiograph, the tumor volume and other normal structures that were outlined are superimposed on the film (79).



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  		Figure 11. Photograph shows the orthogonal localization in three planes allowed by current CT-based radiation therapy systems. (Photograph used with permission.)

 
The next step in the treatment planning process is virtual simulation. Beams can be established from any angle or direction. The most important display is known as the beam's eye view. This shows the anatomy of the patient, as initially outlined section by section, from the point of view of the source of radiation in the treatment unit. This takes into account field divergence, collimator rotation, gantry rotation, and treatment couch angulation. By using the beam's eye view, optimal beam directions can be used to minimize morbidity associated with treatment. In addition, field-shaping devices can be designed to conform the field to the target of interest and to better avoid critical tissues such as the spinal cord. Furthermore, dose-volume histograms display dose received to percentage volume of tissue treated. This includes both target tissues and normal tissues that are contoured in. This also allows comparison of different field arrangements so that the treating physician can choose the optimal treatment plan.

Once the treatment plan is agreed on, it is implemented. The patient is positioned and aligned with the help of laser lights in the treatment room. One to four fields typically are treated, and each field is treated separately. Once a field is set, the gantry angle, field size, collimator rotation, and field-shaping and other beam-modifying devices are verified before the treatment is administered. Although the treating technologist leaves the room during treatment, both voice and visual contact are maintained.

Figure 12 shows the example of a 75-year-old man with prostatic adenocarcinoma and a transplanted kidney. At CT-based simulation, the transplanted kidney, prostate, seminal vesicles, bladder, and rectum were contoured (Fig 12a). Digitally reconstructed radiographs were obtained (Fig 12b). A virtual simulation was performed on the treatment planning computer. With the help of beam's eye view, a four-field technique with anterior, posterior, and right and left lateral beams was fashioned. Areas to be blocked in each of the fields were marked. This is illustrated in the design of the anterior field (Fig 12c). A transverse CT image through the middle of the prostate shows the orientation of all four radiation therapy fields (Fig 12d). On the basis of the calculations, whereas the prostate received 70 Gy, the kidney dose was 0 Gy. The patient, therefore, underwent a course of curative radiation therapy.



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  		Figure 12a. Images obtained in a 75-year-old man with prostatic adenocarcinoma and a transplanted kidney. Purple indicates bladder, yellow indicates seminal vesicles and prostate, blue indicates transplanted kidney, and green indicates rectum. (a) Transverse CT scan shows the contoured region of the prostate and seminal vesicles (yellow), bladder (purple), and rectum (green). (b) An anteroposterior digitally reconstructed radiograph was obtained. The blue overlying the yellow prostate is a combination of prostate and rectum (green). (c) A coronal beam's eye view image was then created, areas to block were marked, and the design of the anterior field was completed. (d) Transverse CT image through the middle of the prostate shows the orientation of all four radiation therapy fields to be used in this patient. The use of these treatment fields resulted in a 70-Gy dose to the prostate and a 0-Gy dose to the transplanted kidney.

 


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  		Figure 12b. Images obtained in a 75-year-old man with prostatic adenocarcinoma and a transplanted kidney. Purple indicates bladder, yellow indicates seminal vesicles and prostate, blue indicates transplanted kidney, and green indicates rectum. (a) Transverse CT scan shows the contoured region of the prostate and seminal vesicles (yellow), bladder (purple), and rectum (green). (b) An anteroposterior digitally reconstructed radiograph was obtained. The blue overlying the yellow prostate is a combination of prostate and rectum (green). (c) A coronal beam's eye view image was then created, areas to block were marked, and the design of the anterior field was completed. (d) Transverse CT image through the middle of the prostate shows the orientation of all four radiation therapy fields to be used in this patient. The use of these treatment fields resulted in a 70-Gy dose to the prostate and a 0-Gy dose to the transplanted kidney.

 


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  		Figure 12c. Images obtained in a 75-year-old man with prostatic adenocarcinoma and a transplanted kidney. Purple indicates bladder, yellow indicates seminal vesicles and prostate, blue indicates transplanted kidney, and green indicates rectum. (a) Transverse CT scan shows the contoured region of the prostate and seminal vesicles (yellow), bladder (purple), and rectum (green). (b) An anteroposterior digitally reconstructed radiograph was obtained. The blue overlying the yellow prostate is a combination of prostate and rectum (green). (c) A coronal beam's eye view image was then created, areas to block were marked, and the design of the anterior field was completed. (d) Transverse CT image through the middle of the prostate shows the orientation of all four radiation therapy fields to be used in this patient. The use of these treatment fields resulted in a 70-Gy dose to the prostate and a 0-Gy dose to the transplanted kidney.

 


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  		Figure 12d. Images obtained in a 75-year-old man with prostatic adenocarcinoma and a transplanted kidney. Purple indicates bladder, yellow indicates seminal vesicles and prostate, blue indicates transplanted kidney, and green indicates rectum. (a) Transverse CT scan shows the contoured region of the prostate and seminal vesicles (yellow), bladder (purple), and rectum (green). (b) An anteroposterior digitally reconstructed radiograph was obtained. The blue overlying the yellow prostate is a combination of prostate and rectum (green). (c) A coronal beam's eye view image was then created, areas to block were marked, and the design of the anterior field was completed. (d) Transverse CT image through the middle of the prostate shows the orientation of all four radiation therapy fields to be used in this patient. The use of these treatment fields resulted in a 70-Gy dose to the prostate and a 0-Gy dose to the transplanted kidney.

 
Although this method of treatment planning and delivery is becoming fairly standard, rotational techniques or the use of larger numbers of fields to deliver conformal therapy are coming into vogue. This requires complex field shapes and modulation of beam intensity. With increasing complexity of treatment, implementation will require computer control of the linear accelerator and necessitate enhancement of computer management skills by both treating physicians and radiation therapy technologists (80). Whereas the treatment planning methodologies at the current time show considerable sophistication compared with those of the past, much needs to be done to minimize setup errors. Contouring target organs that are static on CT scans appears to be accurate, but one must bear in mind the variation of organ position and shape as a result of breathing, heartbeat, swallowing, peristaltic motion, variable filling of bladder or gastrointestinal tract, and weight gain or loss. Therefore, ongoing research is needed to continue refinement of localization and take these factors into consideration (81).

CT-GUIDED PERCUTANEOUS BIOPSY

The complete oncology-radiology service must offer accurate, prompt percutaneous biopsy procedures. Adequate diagnosis usually requires histopathologic confirmation for the oncologist to decide the optimal treatment approach. The combination of spiral CT with new innovations in percutaneous biopsy techniques has substantially increased their diagnostic yield and decreased procedural time. For most biopsies of the brain, neck, chest, spine, retroperitoneum, and pelvis, spiral CT is the guiding modality of choice. The speed and multisection capabilities of spiral CT make it at least equivalent to ultrasonography (US) in hepatic biopsy. What follows is not a complete review of CT-guided percutaneous biopsy; rather it is intended to highlight the unique advantages of spiral CT and to offer some specific guidelines based on the authors' experience.

Automated biopsy devices (biopsy guns) have dramatically improved diagnostic yield and simplified image-guided percutaneous biopsy. Multiple, high-quality tissue cores with little shear or fragmentation can be obtained quickly, even by a novice. Diagnostic rates of 90%-95% easily can be achieved to include adequate lymphoma classification, all with less trauma (fewer punctures) than that associated with fine-needle biopsy (82–90).

With automated biopsy devices, inexpensive ($6–$10 each), specifically sized coaxial needles have become widely available (91–93). There are several advantages of using coaxial needles. First, after placing a coaxial needle, multiple biopsy specimens are obtained quickly. Second, the solid steel trocar of these diamond- or round-tipped needles is easily visible at US and CT. Third, because only one coaxial needle need be placed rather than multiple punctures made with a biopsy needle to obtain several cores of tissue, it is less traumatic (Fig 13). Fourth, because it is a larger needle, a coaxial needle can be moved, or torqued, to fan cores throughout a greater proportion of the tumor.



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  		Figure 13. Photograph shows a coaxial needle form fitted for a current automated biopsy device.

 
Spiral CT offers unique advantages for percutaneous biopsy. Needle localization is possible with a single three- to four-section spiral scan during one breath hold (Fig 14). Current spiral image processing times of 3–4 seconds per section mean that localization is close to real time. Fluoroscopy is now being offered within the gantry (94) or as an added option. One novel development is the incorporation of a spiral volume with an articulated arm that interacts with the scanned volume much like the use of a US transducer. As the arm is moved or rotated over the previously scanned portion, the needle guide is oriented for proper needle placement. There are also certain to be further developments in spiral CT and biopsy techniques that will continue to improve and simplify the percutaneous biopsy procedure.



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  		Figure 14. Transverse spiral CT scans show a large abdominal mass (*) in a 57-year-old woman who was subsequently shown, with results from this percutaneous biopsy, to have B-cell lymphoma. During needle localization, several transverse spiral CT sections were obtained consecutively through the needle and demonstrate its tip (arrow). P = posterior, R = right.

 
However, despite the technologic advances of spiral CT and biopsy devices, patient positioning is critical to simplify and maximize these procedures (95,96). We have found the anterior oblique decubitus position with the biopsy side down to be ideal for many thoracic and upper abdominal biopsies. When in this position, the dependent lung partially collapses and largely ceases to move with respiration (Figs 15, 16). Adrenal and distal pancreatic biopsy can be performed easily, and biopsy of deep pulmonary nodules and mediastinal masses can be performed quickly with less trauma to the patient. Occasionally, water windows are created to avoid vital structures (Fig 17). Various positions and the injection of saline solution can also be used to move masses or tumors into a more satisfactory, less traumatic position for percutaneous biopsy.



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  		Figure 15. Transverse spiral CT scans obtained in a 55-year-old man with non-small cell primary lung carcinoma and two failed attempts at transbronchial biopsy. With the patient in a right lateral decubitus position that limited motion of the right lung, a single 19-gauge coaxial needle was placed into the mass (*) by using spiral CT guidance. Two 22-gauge aspirates and two 20-gauge cores were obtained subsequently and yielded the final diagnosis. A = anterior.

 


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  		Figure 16a. (a) Transverse CT scan shows non-small cell lung carcinoma in a 57-year-old woman. Arrows indicate a mass. (b) Transverse CT scan shows biopsy was much easier to perform with CT when the patient was rolled toward her left. Biopsy was then accomplished with no complications.

 


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  		Figure 16b. (a) Transverse CT scan shows non-small cell lung carcinoma in a 57-year-old woman. Arrows indicate a mass. (b) Transverse CT scan shows biopsy was much easier to perform with CT when the patient was rolled toward her left. Biopsy was then accomplished with no complications.

 


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  		Figure 17. Transverse CT scan shows CT-guided adrenal biopsy in a 63-year-old man with primary lung cancer. The kidney was adjacent to the vertebral body, but by injecting 20 mL of saline solution a safe path was created and the needle passed into the left adrenal gland (arrows). L = left.

 
SUMMARY

This review has addressed spiral body CT in oncologic imaging. Cancer and cancer screening, without doubt, make up the greatest proportion of most CT practices. It is important we all understand the needs of not only the oncologists but also the patients. These patients require a complete, meticulous approach to imaging techniques, and tumor measurement is essential. Gestalt and generalizations must give way to a detailed exact description of a patient's tumor and measurement of tumor volume.

The advent of spiral CT has had a dramatic effect on oncologic imaging. The future holds tremendous promise of continuing this trend of rapid development. Fluoroscopic CT and CT biopsy systems are now or soon will be available from most vendors. The years 1999 and 2000 will witness the rapid introduction of multisection CT with its promise of dramatically improved contrast resolution. The speed of such computer systems will also allow nearly instantaneous reconstruction times and the versatility of obtaining volume measurement reconstructions easily and quickly. CT virtual reality is showing great promise in the airway and colon and may become valuable clinical adjuncts.

In conclusion, oncologic body CT has advanced tremendously since its initial introduction in the 1970s. Whether to evaluate primary tumor or metastases to the neck, chest, abdomen, or pelvis, oncologic body CT has become invaluable to medical, gynecologic, and radiation oncologists. Radiologists should work to maximize the quality of their oncologic practice and to incorporate new CT capabilities as they quickly become available.

Footnotes

**. Multiple body systems Back

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