(Radiology. 2001;219:203-206.)
© RSNA, 2001
Breast Electron Boost Planning: Comparison of CT and US1
Melanie C. Smitt, MD,
Robyn L. Birdwell, MD and
Don R. Goffinet, MD
1 From the Departments of Radiation Oncology (M.C.S., D.R.G.) and Diagnostic Radiology (R.L.B.), Stanford Hospital, Stanford University Medical Center, 300 Pasteur Dr, Stanford, CA 94305. Received September 20, 1999; revision requested October 20; final revision received August 1, 2000; accepted September 6. Address correspondence to M.C.S. (e-mail: melanies@reyes.stanford.edu).
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ABSTRACT
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PURPOSE: To compare computed tomography (CT) with ultrasonography (US) for depiction of the biopsy cavity.
MATERIALS AND METHODS: Thirty-two consecutive patients who underwent radiation therapy following lumpectomy with a planned electron boost were examined. At the time of simulation for whole-breast radiation therapy, all patients underwent planning CT (CT 1) at 3-mm section intervals. At the time of electron boost simulation, US was performed to define the biopsy cavity. In 17 cases, a second CT examination (CT 2) was performed at the time of electron boost simulation. CT and US studies were reviewed jointly and assigned a cavity visualization score (CVS) of 1 (cavity not visualized) to 5 (all cavity margins clearly defined).
RESULTS: The median CVS at CT 1 was 5; at CT 2, 4; and at US, 4. For patients who underwent all three studies, the median CVS at CT 1 was 5; at CT 2, 4; and at US, 4. Factors related to CVS at CT 1 were homogeneous versus heterogeneous appearance (score, 5 vs 4), surgery-to-CT interval (
30 days, 5; 31–60 days, 4; >60 days, 4), and cavity size (>15 cm3, 5; <15 cm3, 4). In all cases, cavity volume decreased somewhat during the CT 1–to–CT 2 interval.
CONCLUSION: CT performed at the time of whole-breast simulation can be used to plan electron boost fields, with cavity visualization similar to that at US.
Index terms: Breast, CT, 00.1211 • Breast, US, 00.12986, 00.12989 • Breast neoplasms, 00.32 • Breast neoplasms, therapeutic radiology
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INTRODUCTION
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The results of several randomized studies (1) and large institutional experiences (2,3) have established the effectiveness of breast conservation therapy for early-stage breast cancer. Optimization of local tumor control is important in reducing the need for salvage mastectomy and potentially decreasing the prevalence of distant metastasis. Large studies now support the role of boost doses of radiation in reducing the risk of local recurrence in patients with negative inked margins of resection (4,5). However, controversy regarding the cost-effectiveness of routine electron boost exists (6), and some long-term cosmetic decrement is associated with the higher radiation dose.
Clinical methods of delineating the boost volume for either electron fields or interstitial implants have been shown to be inaccurate (7–9). Ultrasonography (US) has been used successfully to identify the biopsy cavity, particularly when radiation has not been substantially delayed after surgery (10–13). Clips placed at the time of lumpectomy also can aid conventional or computed tomography(CT)–based planning (14–16), but the use of surgical clips is not uniform and does not enable one to completely define the cavity edges in three dimensions. Results of the 1994 Patterns of Care Facility Survey in Radiation Oncology (17) demonstrated an increasing use of dedicated CT scanners or CT simulators at radiation therapy facilities. CT-based delineation of the biopsy cavity would offer several advantages over US, including convenience of location within the radiation therapy department and ease of data transfer to treatment planning systems for dose optimization with conventional (18) or intensity-modulated treatment (19).
Preliminary experience suggests that CT can be used to define the tumor bed for the electron boost (20) and to preplan interstitial brachytherapy (16,21). However, the image quality and use of CT without surgical clips, as well as the optimal timing for this purpose, have not been well described relative to US. This study was performed to compare the depiction of the biopsy cavity with CT and US in a series of patients who underwent breast conservation therapy.
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MATERIALS AND METHODS
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Thirty-two consecutive patients who were undergoing breast conservation therapy with a planned electron boost following lumpectomy for stages Tis to T2 breast cancer were examined during the initial time following our institution’s acquisition of a commercial CT simulator. The pertinent clinical characteristics of the patients are listed in Table 1. At the time of initial radiation therapy simulation (with tangential fields), the patients were positioned in an Alpha cradle (Smithers Medical Products, Akron, Ohio), and a radiopaque marker was placed over the lumpectomy scar. The patients then underwent CT scanning at 3-mm intervals with an AcQSim PQ5000 CT simulator (Marconi Medical Systems, Cleveland, Ohio) (CT 1). On the day of simulation of the electron boost field, the patients, in the treatment position previously described, underwent US and marking of the biopsy cavity (11). US was performed by using either a Sonoline unit (Siemens, Issaquah, Wash) with a 7.5-MHz transducer or an Ultramark 9 HDI unit (Advanced Technology Laboratories, Bothell, Wash) with a 5–10-MHz bandwidth transducer.
In the first 17 patients, repeat CT scanning (CT 2) was performed on the day of US, again with positioning in the Alpha cradle. Thirty-two CT 1, 17 CT 2, and 30 US images were available for analysis; 15 patients underwent all three studies. All imaging studies were performed because of accepted clinical indications.
Transverse CT and US images were evaluated jointly by one radiation oncologist (M.C.S.) and one diagnostic radiologist with expertise in breast imaging (R.L.B.). The biopsy cavity depicted on the CT images was contoured section by section on the CT simulator workstation. All available CT and US images were assigned one of the following cavity visualization scores (CVSs): 1, cavity not visualized; 2, cavity visualized but margins indistinct; 3, cavity visualized with some distinct margins; 4 cavity visualized with all but one margin distinct; and 5, all cavity margins clearly defined. The cavity volumes on the contoured CT images were computed on the workstation. Although US measurements were obtained, these represented maximum dimensions. Because the cavities could assume varying or irregular shapes and automated volume calculation was not available for the US images, corresponding cavity volumes for the US studies could not be easily calculated.
The image characteristics of the cavities were assessed. For CT studies, the cavities were classified as homogeneous or heterogeneous. For the US studies, the cavities were labeled cystic, complex primarily cystic, complex, or ill defined/edematous. The images obtained in a patient with a CVS of 5 at both CT and US are shown in Figure 1. CT images with scores of 4 and 3 and US images with scores of 4 and 2 are shown in Figures 2 and 3.
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Figure 1a. Transverse (a) CT 1 and (b) US images of the breast cavity in a patient with a CVS of 5 at both studies. The cavity (arrows) has a homogeneous pattern at CT and a cystic pattern at US.
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Figure 1b. Transverse (a) CT 1 and (b) US images of the breast cavity in a patient with a CVS of 5 at both studies. The cavity (arrows) has a homogeneous pattern at CT and a cystic pattern at US.
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Figure 2a. Transverse (a) CT 1 and (b) US images of the breast cavity in a patient with a CVS of 4 at both studies. The cavity (arrows) has a mildly heterogeneous appearance at CT and a complex primarily cystic appearance at US. Crosses denote the locations of the anterior and posterior depth measurements.
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Figure 2b. Transverse (a) CT 1 and (b) US images of the breast cavity in a patient with a CVS of 4 at both studies. The cavity (arrows) has a mildly heterogeneous appearance at CT and a complex primarily cystic appearance at US. Crosses denote the locations of the anterior and posterior depth measurements.
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Figure 3a. Transverse (a) CT 1 and (b) US images of the breast cavity in a patient with a CVS of 3 at CT and of 2 at US. The cavity (arrows) has a heterogeneous appearance at CT and an ill-defined appearance at US.
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Figure 3b. Transverse (a) CT 1 and (b) US images of the breast cavity in a patient with a CVS of 3 at CT and of 2 at US. The cavity (arrows) has a heterogeneous appearance at CT and an ill-defined appearance at US.
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In this patient series, radiation therapy electron fields were based on US delineation of the cavity. The extent of the cavity was marked on the patient’s skin at the time of US to determine the field borders; a record of the depth from the skin surface to the anterior and posterior cavity walls in several areas was obtained to clinically determine the energy to be used. Formal treatment planning, with isodose generation, was not routinely used. Therefore, it was not possible to directly compare treatment plans for the electron fields determined with US guidance with those that might have been determined with CT guidance.
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RESULTS
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For all patients, the median CVS (range, 1–5) was 5 at CT 1, 4 at CT 2, and 4 at US. Thirty patients underwent both CT 1 and US; for these patients, the median CVS was 5 at CT 1 and 4 at US. For the 17 patients who underwent both CT 1 and CT 2, the median CVS was 5 at CT 1 and 4 at CT 2. For the 15 patients in whom all three studies were performed and available for direct review, the median CVS was 5 at CT 1, 4 at CT 2, and 4 at US. This information is summarized in Table 2. In one patient, in whom the interval from surgery to CT 1 was more than 4 months, no cavity was visualized at either study. CT 2 was not performed after review of the initial 17 cases.
Several features appeared to correlate with CVS at CT. For a cavity volume greater than 15 cm3 at CT 1, the median CVS was 5 compared with a median CVS of 4 for a volume smaller than 15 cm3. Homogeneous appearance was associated with a higher CVS—5—compared with a CVS of 4 for heterogeneous appearance. The combination of smaller cavity volumes and a higher percentage of heterogeneous appearances contributed to the lower scores at CT performed at the time of electron boost (CT 2) compared with those at CT performed at the time of initial simulation (CT 1). The median cavity volume at CT 1 was 22 cm3 34 days after surgery; the median cavity volume at CT 2 was 10 cm3 at 78 days. Fifty-nine percent (19 of 32) of the CT 1 scans versus 36% (six of 17) of the CT 2 scans had a homogeneous appearance.
CVSs and cavity volumes as related to the surgery-to-CT 1 interval are listed in Table 3. The highest visualization scores generally were obtained for the studies performed during a surgery-to-CT 1 interval of 30 days or less. The CT 1 CVS scores were as good as or better than those for US performed at all time intervals. However, boost volumes based on findings of CT performed within 30 days after surgery might be extremely large. In the current series, the median cavity volume for these studies was approximately 39.0 cm3 compared with 15.4 cm3 for CT 1 performed 31–60 days after lumpectomy.
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DISCUSSION
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The results of numerous randomized studies have shown that breast conservation therapy with radiation results in disease-free and overall survival rates that are equivalent to those for modified radical mastectomy. In several breast conservation series, long-term local recurrence rates of 2%–7% have been reported when the microscopic surgical margin status of the lumpectomy specimen was assessed and a negative final margin status was observed before radiation therapy (2,3). Although boost doses of radiation to the tumor bed following whole-breast treatment were not required in the National Surgical Adjuvant Breast Project B-06 study (1), they have been shown to produce statistically significant improvements of 1.0%–2.5% in local control (4,5).
To our knowledge, localization of the tumor bed based on clinical criteria, without imaging information, has resulted in a geographic miss of the tumor bed in a substantial percentage of cases in all published comparative series. The use of surgical clips at the time of excision and/or of imaging with US or CT at the time of radiation therapy appears to improve localization of the biopsy cavity. The use of surgical clips, however, is not a standard surgical practice in many communities and has not been shown to improve local recurrence rates (22). For US visualization, the patient is usually required to undergo imaging in a diagnostic facility, with the radiation oncologist present to check field placement; local radiologic expertise in cavity delineation may vary. It has been suggested that US visualization may be difficult or operator dependent (21,23). In one study, US, compared with surgical clips, underestimated the size of the tumor bed (24).
The increasing use of multisection CT for three-dimensional treatment planning has prompted the acquisition of dedicated CT scanners or CT simulators in radiation therapy departments. In addition, although breast conservation treatment has been quite successful with conventional planning, recent study results suggest that there is some benefit in using three-dimensional planning tools in terms of normal tissue sparing and targeting the breast and regional nodes (18). The use of CT to target the tumor bed would be relatively attractive in terms of the ability to acquire all anatomic information for planning within the radiation therapy department, the ability to transfer anatomic information directly to the treatment planning computer for isodose planning, and the acquisition of data for optimization of whole-breast fields simultaneously. However, to our knowledge, there is relatively little published information on the use of CT, in the absence of surgical clips, to visualize the biopsy cavity for radiation therapy planning, especially as compared with US. This information may be useful also in ongoing studies to examine the potential of interstitial brachytherapy as the only treatment modality in breast conservation.
In the present study, the CT images acquired at the time of initial simulation (CT 1) had CVSs as good as or better than those of US images obtained at the time of boost planning. The CT images obtained at the time of boost planning (CT 2) were less useful, presumably owing to changes in cavity characteristics and size that are associated with an increased interval from surgery and to effects of radiation on the soft tissues. However, the CT 1 images obtained within 30 days after surgery were associated with large cavity volumes. Because these cavity volumes decreased substantially within another month, we believe that boost fields based on very large cavities should be avoided. Either the initiation of therapy could be delayed, or, if a delay is not feasible, a second CT or US scan could be obtained. It is not clear whether cavity visualization with US or CT will be as successful when regimens that require delays of several months until radiation therapy are routinely used. In these circumstances, use of surgical clips or CT simulation early in the course of therapy may be needed for adequate localization.
In the present study, qualitatively, certain situations were associated with greater difficulty in delineating biopsy cavities at CT. A subareolar location sometimes caused difficulty in distinguishing retroareolar tissue from the cavity. Dense breasts also caused difficulties, particularly when an external marker was not used to identify the biopsy incision. Occasionally, large axillary seromas were contiguous with the biopsy cavity at CT 1. In general, these masses had resolved or were no longer contiguous at CT 2 and/or US.
In summary, CT performed at the time of whole-breast simulation can be used to plan electron boost fields and achieve cavity visualization similar to that achieved with US. The optimal time for planning CT appeared to be 31–60 days following surgery. At that point, cavity visualization was quite good, and the cavity volume through treatment was relatively stable. Cavity volumes reliably decreased somewhat during whole-breast radiation, and, thus, depiction of large cavity margins at CT 1 may not be necessary for electron boost planning. US also is useful; however, the choice of imaging modality can be determined according to individual practice considerations, including convenience, local expertise, and cost.
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FOOTNOTES
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Abbreviation: CVS = cavity visualization score
Author contributions: Guarantors of integrity of entire study, M.C.S., R.L.B.; study concepts and design, M.C.S., R.L.B., D.R.G.; literature research, M.C.S.; clinical studies, M.C.S., R.L.B.; data acquisition, M.C.S., R.L.B.; data analysis/interpretation, M.C.S., R.L.B.; manuscript preparation, M.C.S., R.L.B.; manuscript definition of intellectual content, M.C.S., R.L.B.; manuscript editing, M.C.S., R.L.B., D.R.G.; manuscript revision/review, M.C.S., R.L.B., D.R.G.; manuscript final version approval, M.C.S., R.L.B., D.R.G.
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ABSTRACT
INTRODUCTION
