Multiparametric MRI: The King of Prostate Cancer Imaging

The Radiological Society of North America (RSNA) was founded in 1915. It now has over 54,000 members. There are not only radiologists, but also other medical professionals and even medical physicists. At its annual gathering, its huge attendance makes it one of the largest medical conferences in the world. Thus, in the words of an old TV stockbroker ad, “When the RSNA talks, people listen.”

At the December, 2017 Annual Meeting, a team from the University of Chicago’s Department of Radiology presented an important message about multiparametric MRI (mpMRI) and its role after prostate cancer (PCa) treatment.  According to the paper by Patel, et al., “The use of multiparametric magnetic resonance (MR) imaging in prostate cancer therapy is increasing, as newer treatment methods and management approaches emerge.”[i]

Let’s think about that. The “old world” of PCa treatment was dominated by two whole gland (radical) therapies, surgery and radiation. The take-no-prisoners approach was based on the assumption that all PCa is multifocal; in other words, if you find a tiny spot of cancer in a biopsy needle, there must be many other microscopic PCa cell clusters scattered throughout the gland. If this were always the case, we wouldn’t need imaging to define tissue characteristics because no matter what, the whole gland would be removed or radiated. Sadly, such wholesale interventions often over-treated men with low risk PCa, leaving many with loss of bladder or bowel control, and the inability to achieve an erection.

The “new world” of prostate cancer treatment

The Patel presentation declares:

The mainstays of therapy—radiation and surgery—are being supplemented (and even replaced) by novel focal therapy methods. Laser and ultrasonographic ablation [HIFU], photodynamic therapy, electroporation, and cryoablation are the most common focal therapies, each with its own imaging findings.[ii]

Before I talk about imaging findings and why they are important, I find it exciting that the authors would talk about “replacing” radical treatments with focal therapies! It’s important to realize that there will continue to be a place for the traditional “mainstays” because focal therapy is not appropriate for every patient. What I believe the authors mean is that focal therapy replaces radical procedures for focal therapy candidates who were forced to choose between all or nothing when it came to PCa treatment. Less than two decades ago, a handful of pioneering urologists and radiologists who were offering men focal cryoablation were branded as heretics. Thankfully, focal therapy is now recognized as a legitimate replacement for whole-gland treatments…for the right patients. This is indeed a “brave new world.”

Now, onto the role of mpMRI findings in the confirmation and follow-up of focal treatment effects.

The importance of imaging after treatment

We know that mpMRI has tremendous value in six areas:

  1. Detecting prostate cancer
  2. Real time MRI-guided biopsy that far surpasses TRUS biopsy in terms of accuracy and risk reduction
  3. Image-based monitoring (together with biomarkers) for patients on Active Surveillance
  4. Thermometric tracking for safety and effectiveness during focal ablation
  5. Confirming post-ablation treatment effects
  6. Following patients over time after focal ablation

With regard to numbers 5 and 6, the Patel presentation underscores the importance of mpMRI after treatment. They stipulate that dynamic contrast enhanced (DCE) MRI is the imaging sequence of choice to evaluate immediate treatment success by confirming lack of tumor blood flow in the zone of ablation. Just as important, using both DCE MRI and diffusion-weighted imaging for longer term follow up is the best way to achieve early detection of any possible recurrence. A key element is also knowing patterns of recurrence: after radiation, it is most likely at the prior tumor site, but after prostatectomy it’s more probable where the urethra is rejoined with the bladder after the gland is removed.

Early detection of recurrence and salvage therapy

Unlike prostatectomy, which removes the prostate from the body, both radiation and focal therapy treat the gland while it is still in place. If there is localized recurrence after treatment – that is, the cancer has not yet begun to spread beyond the prostate gland – it is possible to administer what is called a “salvage” therapy with intent to cure. In the case of post-radiation recurrence, no more radiation can safely be given, and removing the radiated gland (salvage prostatectomy) is complex and comes with a higher risk profile. Therefore, image-guided ablation is the salvage therapy of choice. Multiparametric MRI makes it possible to detect suspected recurrence as early as possible. In some cases, a focal ablation may still be feasible, especially if the post-focal ablation recurrence is a small lesion in a new area of the gland.

Patel et al. performed a welcome service in presenting the value of mpMRI after treatment of prostate cancer. They made it clear that when it comes to the reigning image modality for prostate cancer, mpMRI is heir to the throne. Long live the king!

[i] Patel P, Mathew MS, Trilisky I, Oto A. Multiparametric MR Imaging of the Prostate after Treatment of Prostate Cancer.  2017 Radiographics. 2018 Jan 26:170147.

[ii] Ibid.