Transrectal ultrasound remains the most common modality used for prostate cancer imaging. However, transrectal ultrasound is often used as a way to identify the prostate and guide systematic biopsies rather than as an imaging modality for local staging [12
]. Arguably, this reflects the fact that transrectal ultrasound is usually performed by urologists rather than radiologists, and their experience and comfort level in evaluating images for extracapsular extension may be lower. Conversely, T2-weighted endorectal MR imaging is generally performed by a radiologist and has demonstrated repeated utility in tumor localization and prostate cancer staging [14
]. Concerns regarding cost and variability related to reader experience may account for the low utilization of endorectal MR imaging in patients with prostate cancer. Although costs remain similar to other pelvic MR imaging examinations, our results demonstrate similar accuracy between readers with varying experience levels with moderate interobserver agreement (κ
= 0.56). The relative similar accuracy may, at least partly, be explained by the progressive maturation in knowledge of radiologists when interpreting MR imaging studies of the prostate. Our results suggest that both T2-weighted endorectal MR imaging and B-mode transrectal ultrasound are equivalently accurate in the local staging of patients with prostate cancer, with the caveat that the urologist interpreting the ultrasound is well versed in prostate image interpretation. From a practical perspective, this suggests that urologists should gain more experience and place greater effort into staging prostate cancer during transrectal ultrasound-guided diagnostic biopsy or refer such patients for staging by endorectal MR imaging after a positive biopsy. The rationale to refer such patients to MR imaging is not only because it offers at least equivalent staging accuracy, but also because MR imaging provides a multiparametric evaluation with depiction of metabolic, vascular, and molecular changes in prostate cancer provided by spectroscopic, perfusion, and diffusion-weighted sequences, respectively. Endorectal MR imaging also provides an opportunity for assessment of locoregional adenopathy or bone metastases that cannot be assessed by transrectal ultrasound. It should be noted that such added benefits were not assessed as part of our study as many of these sequences were not obtained in the older scans. However, the anatomic T2-weighted endorectal scan parameters were similar for all patients.
The findings in our study are consistent with three relatively older reports demonstrating that T2-weighted endorectal MR imaging and transrectal ultrasound have similar accuracy in local staging of prostate cancer [5
]. However, our findings conflict with three other studies demonstrating higher accuracy for T2-weighted MR imaging [20
]. Given that ultrasound is highly operator dependent, this discrepancy may, at least partly, be explained by the extensive 26-year experience of our transrectal ultrasound reader who likely increased the overall transrectal ultrasound accuracy in our study. Additionally, demographic data were often available for the ultrasound reader, whereas MR imaging readers were blinded to such information. Hence, our results likely represent a “best-case” scenario for transrectal ultrasound. Conversely, the similar accuracy and moderate interobserver agreement seen for the three MR readers, despite variable experience levels, suggest that T2-weighted MR imaging may be a more robust and reproducible modality for local staging of prostate cancer. Timing of imaging may have also contributed to discrepancy between results. In our study, the ultrasound images were obtained prior to biopsy, whereas the majority of MR images were conducted postbiopsy; hence, postbiopsy hemorrhage and/or scarring may have conceivably further complicated MR image interpretation. There are other potential explanations for the differences in reported results, including variability in magnet strength, MR pulse sequences, types of MR coils used, quality of ultrasound equipment, and statistical techniques. With respect to the latter, it should be noted that many older studies failed to address the interdependence between prostate segments by using generalized estimating equations to account for the clustering effect of dividing the same prostate into right and left sides or into sextants. In a broader context, there are few recent data on the relative utility of endorectal MR imaging versus transrectal ultrasound for prostate cancer evaluation outside of staging. We are aware that serial MR imaging changes are correlated with disease progression in patients choosing active surveillance for management, but no such correlation can be shown for serial transrectal ultrasound [12
Our study has multiple limitations. It was a single-institution retrospective study, with the associated potential for bias. For example, referral of patients to MR imaging was not based on any fixed criteria, and it is conceivable that patients with grossly obvious locally advanced disease on transrectal ultrasound never underwent MR imaging or radical prostatectomy. However, even if our population was skewed by such selection factors to lower-risk disease, it would presumably have been equally confounding for both modalities since we only included patients who underwent both studies. Second, we had an insufficient number of cases of seminal vesicle invasion to conduct a separate analysis of endorectal MR imaging and transrectal ultrasound in predicting seminal vesicle invasion independent from extracapsular extension. A larger sample size would have yielded a more detailed analysis of extracapsular extension versus seminal vesicle invasion and may have shown a statistically significant difference between endorectal MR imaging and transrectal ultrasound. Third, only a single ultrasound reader was available for transrectal ultrasound interpretation; hence, it was not possible to analyze transrectal ultrasound interreader variability. Fourth, as this was a retrospective study, we were unable to investigate multiparametric endorectal MR imaging, a more comprehensive study that would also include gadolinium-enhanced dynamic sequences, diffusion-weighted images, and MR spectroscopic imaging. However, we contend that although multiparametric MR imaging may help increase detection of lesions, T2-weighted anatomic imaging is the most important parameter used for evaluating extracapsular extension secondary to the limited spatial resolution of the other parameters. Similarly, newer ultrasound techniques such as microbubble contrast agent imaging and elastography were not investigated with transrectal ultrasound; albeit, their usefulness in prostate cancer has not yet been convincingly demonstrated in the literature. Finally, this study was conducted in a single institution, a highly specialized tertiary care center, and the results may not be generalizable to community practices, especially those with less experienced ultrasound interpreters.
Our findings suggest that B-mode ultrasound demonstrates similar accuracy for depicting locally invasive disease as compared to T2-weighted MR imaging and should be performed more rigorously when evaluating for extracapsular extension. However, it must be reiterated that the findings suggest a best-case scenario for transrectal ultrasound as a result of a highly experienced sonographic interpreter. Additional MR parameters, access to demographic information, and evaluation of un-biopsied “clean” prostates may have potentially yielded results favoring MR imaging. Additionally, critical information about local lymph node and osseous metastases is not evaluated by transrectal ultrasound. However, we feel that the above MR imaging limitations are common and often representative of standard clinical practice because of the typical sequence in which transrectal ultrasound imaging and MR imaging are performed. More emphasis on the initially obtained transrectal ultrasound images for locoregional cancer spread may obviate the need for more expensive MR imaging at experienced centers.