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Published online before print November 22, 2005, 10.1148/radiol.2381041832
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(Radiology 2005;238:184-191.)
© RSNA, 2005

Genitourinary Imaging

Prostate Cancer: Local Staging at 3-T Endorectal MR Imaging—Early Experience1

Jurgen J. Fütterer, MD, Stijn W. T. P. J. Heijmink, MD, Tom W. J. Scheenen, PhD, Gerrit J. Jager, MD, PhD, Christina A. Hulsbergen–Van de Kaa, MD, PhD, J. Alfred Witjes, MD, PhD and Jelle O. Barentsz, MD, PhD

1 From the Departments of Radiology (J.J.F., S.W.T.P.J.H., T.W.J.S., G.J.J., J.O.B.), Pathology (C.A.H.), and Urology (J.A.W.), University Medical Center Nijmegen, Geert Grooteplein zuid 10, NL 6500 HB, Nijmegen, the Netherlands. From the 2004 RSNA Annual Meeting. Received October 25, 2004; revision requested December 29; revision received January 3, 2005; accepted January 25; final version accepted February 28. Supported by a grant from the Dutch Cancer Society. Address correspondence to J.J.F. (e-mail: J.Futterer{at}rad.umcn.nl).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Purpose: To prospectively investigate the local staging accuracy of 3-T endorectal magnetic resonance (MR) imaging for prostate cancer by using whole-mount-section histopathologic analysis as the standard of reference.

Materials and Methods: This study was approved by the institutional review board, and informed consent was obtained from all patients. In 35 consecutive patients (median age, 62.3 years) with biopsy-proved prostate cancer, 3-T endorectal MR imaging was performed. High-spatial-resolution endorectal T2-weighted fast spin-echo images of the prostate were obtained in three planes. MR images were prospectively evaluated by two experienced radiologists and a third radiologist who was less experienced with regard to local disease extent by using five established extracapsular criteria. Whole-mount-section histopathologic analysis was the standard of reference. Evaluation was performed according to octant and patient. Sensitivity, specificity, positive and negative predictive values, overall accuracy, and interobserver agreement were calculated.

Results: Thirty-two patients who underwent radical prostatectomy were enrolled in this study. Accuracy, sensitivity, and specificity of local staging were 94% (30 of 32), 88% (seven of eight), and 96% (23 of 24), respectively, for both experienced radiologists, and these values were 81% (26 of 32), 50% (four of eight), and 92% (22 of 24), respectively, for the less experienced radiologist. There was substantial agreement between both experienced readers ({kappa} = 0.42–0.79) and moderate agreement between the less experienced reader and the experienced readers with respect to all extracapsular criteria. In regard to the three cases of minimal capsular invasion, two were detected by both experienced radiologists.

Conclusion: In this study, high accuracy for staging of prostate cancer at 3-T endorectal MR imaging, with moderate to substantial observer agreement, was demonstrated. In addition, minimal capsular invasion could be detected.

© RSNA, 2005


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Prostate carcinoma is the second most frequent cause of cancer-related death in men (1). The increase in the number of the aged, as well as the advent and the ever more frequent use of the prostate-specific antigen serum test for detection, has resulted in an increase in prostate cancer incidence (2,3). Determination of the tumor extension in prostate cancer is important not only to allow optimal choice between the various therapeutic options but also to influence prognosis and treatment (4–6).

Presently, the role of magnetic resonance (MR) imaging of the prostate for detection of extension beyond the capsule is being debated because of limited availability, high costs, and variability in results (7–9). A large heterogeneity in local staging performance exists (4,9–14). A frequently described limitation is the inability to demonstrate microscopic capsular penetration (7).

MR imaging at 1.5-T (the standard clinical field strength) with T2-weighted fast spin-echo sequences and combined endorectal and phased-array coils has enabled the acquisition of MR images of the prostate and its surrounding tissues with high spatial resolution. Although a relatively high spatial resolution can be achieved within a clinically acceptable examination time, reported accuracy values for staging vary from 54% to 88% (4,7–15). Jager et al (16) stated that staging with MR imaging in the preoperative work-up of prostate cancer is cost effective and should be performed with a high specificity (95%). In attaining this specificity, however, a low sensitivity (36%) has to be considered (17).

For several years, whole-body MR imaging at high magnetic field strengths (>1.5 T) has been used for research purposes only. At present, however, high-field-strength MR imaging systems are becoming more widely available in routine clinical settings. Generally, use of a higher field strength increases the signal-to-noise ratio linear to the magnetic field strength (18), thereby affording the possibility of an increase in either the spatial or the temporal resolution of MR imaging. Other effects of clinical MR imaging at high field strengths are the increased susceptibility differences in tissues that cause magnetic field inhomogeneities, as well as possibly shorter T2 along with longer T1 relaxation times.

Preliminary results with endorectal MR imaging at 3 T in patients with prostate cancer contributed to increased spatial resolution of T2-weighted imaging with a voxel volume of 13 µm3 (19). To the authors' knowledge, the role of endorectal MR imaging at magnetic field strengths of 3 T in the evaluation of local staging in patients with prostate cancer prior to radical prostatectomy has not yet been reported. Thus, the purpose of our study was to prospectively investigate the local staging accuracy of 3-T endorectal MR imaging for prostate cancer by using whole-mount-section histopathologic analysis as the standard of reference.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Medrad, Pittsburgh, Pa, supplied the prototypes of balloon-mounted disposable endorectal surface coils and interface devices. The authors, however, had control of the data and the information submitted for publication.

Patient Characteristics
From July 2002 until July 2004, 35 consecutive patients with biopsy-proved prostate cancer underwent endorectal coil MR imaging examinations at 3 T. Patients who were scheduled for radical prostatectomy within 6 weeks (range, 2–42 days; median, 8 days) after MR imaging were included in the study. Exclusion criteria were previous hormonal therapy, lymph nodes positive for metastases at frozen section analysis during surgery, contraindications to MR imaging (eg, cardiac pacemakers, intracranial clips), and contraindications to endorectal coil insertion (eg, anorectal surgery, inflammatory bowel disease). The study was approved by the institutional review board, and informed consent was obtained from all patients.

Median patient age was 62.3 years, with a range of 51–72 years. Median prostate-specific antigen serum level was 8.9 ng/mL (range, 1–45 ng/mL), and median Gleason score was 6 (range, scores 4–7), respectively. MR imaging was performed at least 4 weeks after transrectal ultrasonographically guided sextant biopsy.

MR Imaging Acquisition Protocol
All MR images were obtained with a commercially available 3.0-T whole-body imager (Magnetom TRIO; Siemens Medical Solutions, Erlangen, Germany). A quadrature birdcage body coil was used for transmission, and prototypes of balloon-mounted disposable endorectal surface coils (Medrad) for 3-T MR imaging were used for receiving the MR signals. After digital rectal examination, the endorectal surface coil was inserted and inflated with demineralized water to a volume of approximately 60 cm3. Peristalsis was suppressed in all patients with an intramuscular injection of 1 mg of glucagon (Glucagen; Novo Nordisk, Bagsvaerd, Denmark) immediately before the start of the examination.

The protocol for acquisition consisted of a localizer and two fast turbo gradient spin-echo measurements for patient and coil positioning and high-spatial-resolution T2-weighted fast spin-echo imaging in three planes. The imaging parameters for the T2-weighted images were as follows: repetition time msec/(effective) echo time msec, 4000/109; flip angle, 180°; field of view, 280 mm; matrix, 512 x 256; number of sections, 15–18; section thickness, 4 mm; section gap, zero. The frequency direction was anteroposterior to decrease coil motion artifacts over the prostate. In addition, transverse high-spatial-resolution T2-weighted fast spin-echo images (matrix, 768–1024 x 512; field of view, 180 mm) were acquired with a voxel volume of 13 µm3, and this acquisition exploited the increased signal-to-noise ratio of the endorectal coil at 3 T. Possible biopsy-related hematomas were detected with images from a three-dimensional T1-weighted spoiled gradient-echo pulse sequence performed with the following parameters: 8.6/4; flip angle, 15°; number of sections, 32; section thickness, 1.5 mm; field of view, 130 mm; matrix, 128 x 256; and in-plane resolution, 0.51 x 0.51 mm. Total examination time for the aforementioned protocol and coil insertion was approximately 20–25 minutes.

MR Image Evaluation
Prospectively, all MR images were independently read by three radiologists who were aware that patients had biopsy-proved prostate cancer and were scheduled for radical prostatectomy. They were unaware of the other clinical findings. The three radiologists had different levels of experience in interpretation of findings from prostate endorectal MR examinations. Radiologist A (J.O.B.) had 10 years of experience (total of approximately 700 studies), radiologist B (J.J.F.) had 3 years of experience (total of approximately 250 studies), and radiologist C (S.W.T.P.J.H.), who was considered less experienced than radiologists A and B, had 6 months of experience (total of approximately 30 studies). MR imaging studies were interpreted at a digital workstation (Impax; Agfa, Mortsel, Belgium). MR image evaluation was performed in three planes. All MR images were rated for overall quality as good, intermediate, or poor. Image quality was considered to be good if the images showed high anatomic detail and minimal artifacts and poor if poor anatomic detail or extensive artifacts disallowed the evaluation of the images. All other images were considered to be of intermediate quality.

The readers drew lesion and extraprostatic extension locations in standard schemes of the prostate for comparison with the whole-mount sections. The prostate capsule was divided into octants; that is, the prostate was split in half (from apex to base) and then further divided into four areas: right and left peripheral zone and right and left central gland. The most likely sites of capsular extension were identified in each octant and numbered on the drawings. T1-weighted images were used to rule out false-positive findings caused by postbiopsy hemorrhage; if a low-signal-intensity lesion on a T2-weighted MR image matched a high-signal-intensity lesion on the corresponding T1-weighted MR image, this area was considered to be a hematoma due to biopsy (20).

The presence of extracapsular extension was evaluated on the basis of five specific features described in the literature as highly indicative of extracapsular extension. These features were as follows: neurovascular bundle asymmetry, obliteration of the rectoprostatic angle, irregular bulging of the prostatic contour, tumor signal intensity within the periprostatic fat, and overt extracapsular tumor (8,21–23). The criterion used for determination of seminal vesicle invasion was abnormal asymmetric low signal intensity within the lumen on T2-weighted images (11,23).

The readers expressed the likelihood of each criterion—extracapsular extension and seminal vesicle invasion—with a five-point scale. A rating with a score of 1 indicated that extraprostatic disease was definitely not present; that with a score of 2, that disease was probably not present; that with a score of 3, that disease was possibly present; that with a score of 4, that disease was probably present; and that with a score of 5, that disease was definitely present. When a score of 4 or 5 was assigned, the criterion was considered to be present. When a score of 1–3 was assigned, the criterion was considered not to be present. Reading was performed at a high-specificity setting (24,25) for extraprostatic disease (ie, only if the reader was certain of stage T3 disease, the disease rated as this stage). This was done to prevent the withholding of potentially curative therapy because of the classification of false-positive stage T3 disease in a patient with actual stage T2 disease.

Histopathologic Analysis
Three urologists, including one author (J.A.W., with 17 years of experience) and two other urologists with 11 years and 4 years of experience, who had knowledge of the MR imaging results performed the prostatectomy procedures. The prostatectomy specimens were fixed overnight in 10% neutral buffered formaldehyde and were coated with India ink. Seminal vesicles were separated from the prostate and examined separately. Transverse whole-mount step sections were created at 4-mm intervals in a plane parallel to the transverse plane used to perform the T2-weighted sequence. All sections were routinely embedded in paraffin. Tissue sections of 5 µm were prepared and stained with hematoxylin-eosin. The presence and extent of cancer were outlined on the glass cover with the tissue section by an experienced genitourinary pathologist (C.A.H.) who had 12 years of experience and who was blinded to the imaging results. Staging of the prostatectomy specimens was performed according to the present TNM classification (26).

Data Analysis
The MR imaging–predicted extraprostatic extension was compared with the findings at histopathologic analysis by one radiologist (J.J.F., with 3 years of experience) after assignment of scores and evaluation of the data were performed. The T2-weighted fast spin-echo MR images were aligned with the whole-mount sections. The morphologic characteristics of the central gland and peripheral zone—apex and base of the prostate, cysts, calcifications, and urethra—were used as landmarks. Aligning of MR images and whole-mount sections is considered difficult (10). Although no literature is available on this subject, to our knowledge, we were confident that our alignment was within 8-mm accuracy (eg, two sections). If the detected extraprostatic extension in the whole-mount section was within 4 mm from the aligned MR image–detected location and on the correct side, this was considered a match.

Statistical Analysis
A finding was considered true-positive in a case in which the imaging results were correlated with the histopathologic findings, as mentioned previously. The sensitivity, specificity, positive and negative predictive values, and overall accuracy for the prediction of extraprostatic criteria, tumor stage, extracapsular extension, and seminal vesicle invasion were calculated by dichotomizing the readings. When a score of 4 or 5 was assigned, these features were considered to be present. When a score of 1–3 was assigned, the features were considered not to be present. This analysis was performed according to patient and according to octant.

The statistical analysis included the evaluation of the interobserver agreement by using nonweighted {kappa} statistics. The following qualitative terms were used to describe the strength of the various values of {kappa}: 0–0.20, poor agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, substantial agreement; and 0.81–1.00, near-perfect agreement (27). Two-tailed tests were used to calculate all P values; a P value of .05 or less was considered to represent a statistically significant difference. All statistical analyses were performed with software (SPSS, version 9.0; SPSS, Chicago, Ill).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Surgical Specimens
In three patients, performance of the standard of reference was not available because the urologist did not remove the prostate as a result of lymph node metastases, and on the basis of this finding, the urologist decided not to resect the prostate. These patients subsequently underwent a combination of radiation and/or hormonal therapy. One of these three patients had clear invasion of the seminal vesicles at MR imaging, and this finding was reported to the urologist. The urologist then performed a biopsy of the seminal vesicle, and results were positive for cancer. These three patients were excluded from further analysis; thus, 32 patients were included in our study. Eight of 32 patients had extracapsular extension; in three of these patients, seminal vesicle invasion was observed. In the remaining patients, disease was confined to the prostate (stage T2a disease in nine patients and stage T2b disease in 15 patients). The image quality was good in 29 patients and intermediate in three patients. In six patients, postbiopsy artifacts were visible on T1-weighted images; however, the artifacts were not in the area of capsular extension or seminal vesicle invasion.

Staging
The staging results, with scores assigned according to octant, are presented in Table 1. Two hundred fifty-six capsular sites were evaluated for extracapsular extension on the MR images. Of these 256 sites, 13 had capsular extension at histopathologic evaluation. Of these 13 sites, one was not identified as a site of possible capsular extension on MR images by all three readers. The penetration depth of this extension was only 0.5 mm. In three sites, the penetration depth of the capsular extension was less than 2 mm (Fig 1). Two of these sites were identified by the two experienced readers (readers A and B).


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  		Table 1. Staging and Statistical Analysis Results for Three Readers according to Octant

 


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  		Figure 1a: Images in 62-year-old patient with prostate-specific antigen serum level of 8.2 ng/mL and Gleason score of 6 show histologically confirmed stage T3a prostate carcinoma. (a) Transverse T2-weighted fast spin-echo MR image (4000/109) shows suspicious area of low signal intensity (open arrows) in the right peripheral zone. Experienced radiologists identified this area as irregular border (solid arrows) of the prostate capsule and classified the disease as stage T3a. (b) Sagittal T2-weighted fast spin-echo MR image (4124/109) demonstrates disrupted prostate capsule in the same patient (arrow). (c) Histopathologic section reveals stage T3a disease with minimal capsule penetration (arrow) in the right peripheral zone.

 


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  		Figure 1b: Images in 62-year-old patient with prostate-specific antigen serum level of 8.2 ng/mL and Gleason score of 6 show histologically confirmed stage T3a prostate carcinoma. (a) Transverse T2-weighted fast spin-echo MR image (4000/109) shows suspicious area of low signal intensity (open arrows) in the right peripheral zone. Experienced radiologists identified this area as irregular border (solid arrows) of the prostate capsule and classified the disease as stage T3a. (b) Sagittal T2-weighted fast spin-echo MR image (4124/109) demonstrates disrupted prostate capsule in the same patient (arrow). (c) Histopathologic section reveals stage T3a disease with minimal capsule penetration (arrow) in the right peripheral zone.

 


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  		Figure 1c: Images in 62-year-old patient with prostate-specific antigen serum level of 8.2 ng/mL and Gleason score of 6 show histologically confirmed stage T3a prostate carcinoma. (a) Transverse T2-weighted fast spin-echo MR image (4000/109) shows suspicious area of low signal intensity (open arrows) in the right peripheral zone. Experienced radiologists identified this area as irregular border (solid arrows) of the prostate capsule and classified the disease as stage T3a. (b) Sagittal T2-weighted fast spin-echo MR image (4124/109) demonstrates disrupted prostate capsule in the same patient (arrow). (c) Histopathologic section reveals stage T3a disease with minimal capsule penetration (arrow) in the right peripheral zone.

 
The staging results, with scores assigned according to patient, are shown in Table 2. The overall accuracy in prostate cancer staging (T3 disease vs T2 disease) was 94% (30 of 32) according to patient for the experienced readers A and B and 81% (26 of 32) for the less experienced reader C. No statistically significant difference was present among readers A, B, and C.


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  		Table 2. Staging and Statistical Analysis Results for Three Readers according to Patient

 
The results of the readers' observation of diagnostic features of extracapsular extension are given in Table 3. The mean sensitivity values for the individual criteria of extracapsular extension ranged from 44% to 88%. All readers obtained a high sensitivity for irregular bulging (Fig 1) of the prostatic contour. High specificity was established for all criteria (Table 3).


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  		Table 3. Results of Readers’ Observation of Diagnostic Features for Extracapsular Extension

 
In the three patients in whom seminal vesicle invasion was observed, results were confirmed with findings at analysis of the specimen removed at prostatectomy. All readers identified the invasion of seminal vesicles, and this result yielded an accuracy of 100% (Fig 2 ).



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  		Figure 2a: Images in 66-year-old patient with prostate-specific antigen serum level of 4.5 ng/mL and Gleason score of 6 show seminal vesicle invasion. (a) Coronal T2-weighted fast spin-echo MR image (4130/109) obtained through the prostate and seminal vesicles. Area of low signal intensity (arrow) is present in base of prostate and extends into seminal vesicle (arrows in b). This finding was confirmed with whole-mount-section histopathologic analysis. (b) Sagittal view in same patient.

 


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  		Figure 2b: Images in 66-year-old patient with prostate-specific antigen serum level of 4.5 ng/mL and Gleason score of 6 show seminal vesicle invasion. (a) Coronal T2-weighted fast spin-echo MR image (4130/109) obtained through the prostate and seminal vesicles. Area of low signal intensity (arrow) is present in base of prostate and extends into seminal vesicle (arrows in b). This finding was confirmed with whole-mount-section histopathologic analysis. (b) Sagittal view in same patient.

 
Interobserver Agreement
The results of the interobserver agreement analysis are displayed in Table 3. Assessment of interobserver variability showed moderate to substantial agreement ({kappa} = 0.42–0.79) for ratings in regard to extracapsular extension for all three readers. Agreement was best for ratings in regard to tumor signal intensity within the periprostatic fat and irregular bulging of the prostatic contour, and agreement was the worst for ratings in regard to obliteration of the rectoprostatic angle.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
The most important findings of our study were the high sensitivity of 88% in the staging of prostate cancer, the retention of a specificity of 96%, and the detection of minimal capsular extension through the prostate capsule by experienced readers. Two of three cases of minimal capsular penetration were recognized by the experienced radiologists (readers A and B). This finding suggests a role for prostate MR imaging at 3 T. Although we did not perform a comparative study between 1.5-T MR imaging and 3-T MR imaging, the results of this study indicated that with the 3-T technique the accuracy in staging was increased to a level that was higher than that reported for 1.5-T imaging, with a range in accuracy of 54%–88% (8–11,15).

So far, only four studies about in vivo prostate imaging at high field strengths have been published, as far as we know. Kim et al (28) used a relatively large external transceive coil for prostate imaging without exploring staging and localization performance with MR imaging. In our study, we used an endorectal receiver coil for imaging of the prostate, and use of this coil resulted in images with a high in-plane resolution and a voxel volume as small as 0.13 mm3. This resolution and voxel volume would be very difficult to achieve within a reasonable time without the use of a local coil. Sosna et al (29) reported results of volume assessment of the prostate at 3-T MR imaging with an external phased-array coil. They concluded that in vivo volume determinations were very close to ex vivo imaging volume determinations. They, however, did not investigate performance of MR imaging for staging. Bloch et al (30) showed a proof of principle of MR imaging of the prostate at 3 T with an endorectal coil in a limited study with six volunteers.

In a concurrent study by Fütterer et al (19), patients with prostate cancer were examined by using an endorectal coil at 3-T MR imaging. It was shown that the increased signal-to-noise ratio with an endorectal coil at 3 T has great potential to improve either the spatial or temporal resolution of T2-weighted (dynamic) contrast material–enhanced and spectroscopic MR imaging of the prostate. In the current study, we applied this signal-to-noise ratio to obtain images with high in-plane resolution for clear delineation of the prostate capsule. Because of this increased resolution, we were able to display minimal capsular extension in two of three cases. Findings in reports from the early 1990s suggested that minimal capsular penetration of less than 1 mm does not adversely affect the surgical cure rate (31,32). No recent data in the literature, however, have confirmed these results.

With the demonstrated ability to detect minimal capsular penetration, the question arises about whether the progression from stage T2 disease to stage T3 disease should be used as the basis on which clinicians should choose between surgical resection of the prostate or a combination of radiation therapy and hormonal therapy for treatment. If minimal capsular penetration is detected, curative surgery will be withheld in a patient in whom the treatment for the disease is potentially curative. Perhaps the reported early detection of stage T3 disease, when it is treated as stage T2 disease, does not affect patient outcome. The effect of the detection of minimal capsular penetration, as well as possible implications in treatment, has to be evaluated in future studies.

In our study, we found a mean sensitivity of 62% (range, 44%–88%) and a high specificity for the criteria of extracapsular extension with moderate to substantial interobserver agreement. Outwater et al (23) found only poor accuracy for the extracapsular criteria at 1.5-T MR imaging. This difference in accuracy could be caused by the increased in-plane resolution in our study (voxel volume of 0.61 µm3) versus that of 0.18 µm3 in the study of Outwater et al. All the radiologists read the images with the focus on high specificity (24,25). To prevent overstaging, only if the reader was certain of stage T3 disease was the disease rated as such.

The results of the current study showed that there was a difference in accuracy according to experience of the reader; these results also showed a difference particularly in sensitivity (88% vs 50%). This difference was also found in other studies performed at 1.5-T MR imaging (14,33). The sensitivity and specificity at 1.5-T MR imaging found by Yu et al (33) were 17% and 94% and 54% and 95% for the less experienced and experienced readers, respectively. This difference in sensitivity values was much smaller in the present study, where sensitivity and specificity were 50% and 92% and 88% and 96% for the less experienced and experienced readers, respectively.

Despite the promising results of our study, some limitations need to be considered. The correlation between findings at MR imaging and the findings at corresponding whole-mount-section histopathologic analysis was difficult to determine in most patients. This difficulty is a frequently encountered problem. The angle at which the whole-mount sections were cut in the prostate specimens was never exactly the same compared with the angulation of the MR images obtained in vivo. After the prostate specimens are fixed in formaldehyde, the specimen shrinks and deforms. We tried to overcome these problems by using all whole-mount sections in the assessment of the corresponding level.

Because prostate-specific antigen serum levels and Gleason scores were low, only four patients with seminal vesicle invasion were included in this study. In one patient with seminal vesicle invasion who was excluded from this study because no results of analysis with the standard of reference were obtained, the urologist changed the treatment on the basis of MR imaging findings, and such a change suggests that the technique used in this study may influence a decision about treatment and patient outcome. Such a suggestion, however, requires confirmation in a future study.

The number of patients included in this study may be considered rather low. In their meta-analyses, Engelbrecht et al (12) and Sonnad et al (17) showed that studies with 50 patients or fewer had more favorable results than did studies with a higher number of patients. In an article titled "Clinical Efficacy of MR Needs Rigorous Study" (Diagn Imaging 1990;12:69,71,161), Kent, however, estimated that 30–70 patients would be required for a comparison of the diagnostic accuracy of MR imaging with that of a reference standard. In a future study, a multicenter approach has to be used to confirm our promising preliminary results.

The role of endorectal MR imaging in local staging of cancer of the prostate is still controversial. A number of articles indicated the poor capability of MR imaging in the staging of cancer of the prostate (7–9). May et al (9) even suggested that treatment decisions should not be altered as a result of endorectal MR imaging findings. In contrast, Jager et al (16) developed an analytic model for decision making that supported the opinion that MR imaging for staging in the preoperative work-up of prostate cancer is cost effective and should be performed with a high specificity. Langlotz et al (24) and Langlotz (25) emphasized this need for high specificity to ensure that as few patients as possible will be unnecessarily turned down for potentially curative therapy on the basis of false-positive MR imaging results. D'Amico et al (34) suggested using endorectal MR imaging in patients with intermediate risk only; in this group, the probability of extraprostatic disease is high enough to warrant the use of MR imaging. If the prevalence of extracapsular disease is, for example, 30% and MR imaging is performed with a specificity of 97% and a sensitivity of 33%, then only in one of 10 patients will MR imaging results affect the treatment. This could be a reason why urologists do not use MR imaging as a modality for staging. If the results of our study are reproducible, this number could be extrapolated to one of three or four.

With consideration of the thus far published data, results of this study indicate that 3-T endorectal MR imaging of the prostate in staging is of additional value in patients with prostate cancer. MR imaging of the prostate at 3 T should be performed only in patients with intermediate risk of having extraprostatic disease (34), and it should be performed with high-specificity readings by experienced genitourinary radiologists (14). Nonetheless, it remains difficult for less experienced readers to interpret MR images of the prostate; however, at 3-T MR imaging, it may be anticipated that interpretation will improve.

In conclusion, in this study, high accuracy for staging with moderate to substantial observer agreement at 3-T MR imaging was demonstrated. In addition, minimal capsular invasion could be detected. These outcomes suggest a future role for high-field-strength MR imaging in the staging of prostate cancer.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge Vincent Cuijpers, MSc (Department of Pathology), and Yvonne Hoogeveen, PhD (Department of Radiology), for their contributions.


   FOOTNOTES
 
See Materials and Methods for pertinent disclosures.

Author contributions: Guarantors of integrity of entire study, all authors; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, J.J.F., G.J.J.; clinical studies, all authors; statistical analysis, J.J.F., G.J.J.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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