Future role of ultrasound and MR imaging in prostate cancer

In response to the article of Szopiński et al.⁽¹⁾ we want to compliment the authors for a very thorough and sound review article. The authors listed the most significant developments and trends in prostate ultrasound, and mentioned the increasing role of MRI in prostate cancer diagnosis and staging.

In our opinion, despite the impressive recent developments of Ultrasound we see MRI already presently and even more in the future as the primary imaging modality for detection, staging, active surveillance, and local recurrence of prostate cancer⁽²⁾.

Yet, Ultrasound will maintain its established role in real time image guidance during transrectal (TRUS) prostate biopsies. Otherwise, Ultrasound cannot match the image quality and accuracy of MRI. Besides the superior soft tissue contrast and high resolution of MR images, it offers a comprehensive tissue assessment: anatomic/topographical with T2-W images for example, pathophysiologic (tissue/tumor, perfusion/diffusion) with dynamic contrast enhanced MR and diffusion weighted imaging (DWI), and metabolic/molecular with Spectroscopy. Besides being relatively operator independent and reproducible, MR imaging has been proven to have superior sensitivity, specificity, and accuracy in detection and staging of prostate cancer^{(3, 4)}.

Especially the improvement of diffusion weighted imaging (DWI and DTI), and Spectroscopy make MRI the most powerful and comprehensive imaging modality for prostate cancer. This is reflected in the increasing number of referrals by urologists and other clinicians over the last 10 years, with a +7% increase in prostate MRI exams per year in the US over the last 4 years (making prostate MR the fastest growing MR exam). In the age of widely accessible internet and public medical knowledge about proper management of disease, well informed patients more and more themselves request prostate MRI, especially before repeat biopsies. The increasing demand of prostate MR results in decrease of costs and will consequently further increase the numbers of referrals to MR.

Although the costs for MR will further decline, MR will never be as inexpensive as ultrasound. Therefore, we see the role of ultrasound in TRUS guided biopsies, with MRI/ US image fusion, as the future standard of care. MR-guided biopsies will be reserved for patients with repeat negative biopsies, since those are compared to TRUS biopsies, costly and time consuming.

Fig. 1.

62-year-old patient, PSA = 9; bilateral several positive biopsy cores; Gleason score = 7; with clinically und sonographically suspected extraglandular extension (ECE) of a large bilateral prostate cancer. MRI was ordered by the urologist to confirm ECE. A. 3 Tesla MRI axial T2-W image and B. corresponding CAD color coded axial dynamic contrast enhanced – DCE image through the mid third of the prostate. Diffuse T2-W signal changes suggest diffuse cancerous changes bilateral. Yet, there is no suspicious signal seen beyond the intact, regular and smooth prostate capsule (yellow arrow). The neurovascular bundles (red arrows) are unremarkable. The DCE images shows a large bilateral tumor involving most of the left and right peripheral zones, with preserved prostate capsule and without evidence of ECE. The mriT2c stage was confirmed by pathology after successful prostatectomy (pT2c)

Finally, promoted by the latest controversial discussions about PSA and prostatectomies, more and more patients with small and low Gleason score cancers choose active surveillance rather than aggressive treatment. Currently, there are about 30% of the patients (in our institution and US national average) on active surveillance. In this particular patient cohort, MRI is the only clinical tool to meaningfully observe the cancer over time: MRI can assess for small yet clinically significant interval changes of the cancer over time, which would necessitate intervention (e.g. surgery or radiation therapy), can guide a targeted repeat biopsy, and consequently can facilitate appropriate treatment decision.

In the coming years, prostate MRI will be state-of-the-art and the routine clinical care for prostate cancer pre- and post-treatment assessment and management, while ultrasound will be essential for MR-US fusion biopsies.