By: Dan Sperling, MD

Multiparametric MRI (mpMRI) of the prostate has increasing use as a method of monitoring prostate cancer patients on Active Surveillance (AS). As the term implies, AS is a strategy for deferring prostate cancer treatment by keeping track of possible tumor progression to more aggressive disease, or possible tumor growth in volume. Prior to the availability of 3T mpMRI, patients on AS were recommended to have an annual TRUS biopsy in addition to PSA blood tests at intervals of 6-12 months. However, it is greatly advantageous to have a 3T mpMRI as a way to determine whether a repeat biopsy is necessary, because this type of imaging can register both progression and growth of prostate cancer.

Two studies underscore the utility of imaging to track AS success in order to avoid repeat biopsies. The first is reported by a team from Johns Hopkins Medical Institutions. [i] The study group was comprised of 96 men on AS, each of whom had a baseline mpMRI within 1 year of beginning AS. The image results were analyzed to identify those whose tumor was “invisible” on three MRI functional parameters, including “no signal abnormality on T2-weighted images, no focal restricted diffusion, and no perfusion abnormality on dynamic contrast-enhanced images.”

The team found that 84 patients had an MR-visible tumor, but 12 did not (MR-invisible). The two groups were followed according to the conventional protocol of repeat biopsies, and compared according to the risk of biopsy-proven adverse pathology (increase in risk factors leading to possible disease reclassification/upgrading of prostate cancer). The median follow-up was 23 months. The table below shows the data:

In other words, those whose tumor was invisible on the baseline mpMRI had a lower risk of developing adverse pathology, a condition that could determine when AS was no longer safe and therefore lead to a treatment decision. The authors concluded, “The MR-invisibility of tumor on MP MRI could be of prognostic significance in monitoring men in AS with potential benefit of tailoring the frequency of surveillance biopsies and reducing the number of unnecessary biopsies.”

The second study was published by a Korean research team.[ii] They retrospectively analyzed 464 prostate cancer patients who were initially diagnosed with clinically localized prostate cancer, Gleason grade < 6 and PSA < 10 ng/mL. All patients eventually had radical prostatectomy, but were scanned with mpMRI prior to surgery. In other words, the post-surgery gland specimens were available to compare with the pre-surgery MRI results.

The researchers divided the cases into three groups:

• Group A (238 men who were eligible for AS at the time of diagnosis)
• Group B (59 men who were not eligible for AS and had MR-invisible tumor
• Group C (167 men who were not eligible for AS and had MR-visible tumor

They then examined the post-surgical pathology findings (based on gland specimens) of all patients to determine whether the disease outcomes were favorable (low-to-moderate risk, organ-confined tumor) or unfavorable (higher risk disease or capsular penetration). Finally, they then correlated the outcomes for the patients who were not eligible for AS (Groups B and C) with their tumor visibility or invisibility on mpMRI. The results are summarized in this table:

They found that there were no statistically significant differences between Groups A and B, whereas Groups A and C were significantly different. Thus, tumor invisibility on mpMRI was a predictor of organ-confined disease (favorable disease) for patients who did not meet AS qualifications. The authors concluded, “Multiparametric magnetic resonance imaging could help to determine treatment modality for the low-risk prostate cancer patients who consider active surveillance even if they did not meet active surveillance criteria.”

The two studies line up in agreement that tumor invisibility on mpMRI indicates a lower risk for unfavorable disease at the time of treatment. The second study is especially optimistic for patients who have organ-confined disease but do not meet strict requirements for AS. Together, these studies reinforce the advantages of undergoing mpMRI before making a treatment decision, including AS.

[i] Magnetic resonance-invisible versus magnetic resonance-visible prostate cancer in active surveillance: a preliminary report on disease outcomes. Dianat SS, Carter HB, Peinta KJ et al. Urology. 2015 Jan;85(1):147-53.

[ii] Low-risk prostate cancer patients without visible tumor (T1c) on multiparametric MRI could qualify for active surveillance candidate even if they did not meet inclusion criteria of active surveillance protocol. Lee DH, Koo KC, Lee SH et al. Jpn J Clin Oncol. 2013 May;43(5):553-8.