By: Dan Sperling, MD

Multiparametric MRI (mpMRI) has demonstrated the ability to detect lesions that are suspicious of being aggressive cancer based on their size and tissue function parameters. There is a positive correlation between such lesions and a pathologic (biopsy-based lab analysis) Gleason score greater than 3+3.

However, with ongoing technologic advances and acquired reader experience, mpMRI often reveals small index lesions < 7 mm or even smaller. When pathology indicates that they are noncancerous, or of a Gleason grade 6 or lower, can they be safely monitored by imaging? If so, what is a reasonable interval between imaging?

This question was explored by a team comprised mostly of researchers at the National Cancer Institute/National Institutes of Health. They published the results of their study into this in the April, 2014 issue of the journal Diagnostic and Interventional Radiology.[i] The authors’ purpose was to determine the natural history (trajectory of growth) of small index lesions detected by mpMRI, to obtain lab pathology upon initial image detection, and to track the development of the lesions with a series of imaging over time. The “index lesion” was defined as the lesion(s) “with the highest cancer suspicious score…irrespective of size.” The authors used a scoring system similar to PI-RADS to evaluate the degree of suspicion. (To learn more about scoring for degree of suspicion, see https://sperlingprostatecenter.com/pi-rads-score/)

The team reviewed the records of 153 patients who had had at least two mpMRI sessions. They identified two study groups:

a)      Those who had no index lesion(s) <7 mm and

b)      Those with no index lesion(s) <5 mm.

In all cases, biopsies into the index lesions were guided by MRI-fusion guided biopsy; in other words, targeted biopsy based on MRI lesion identification. The growth rate of the lesions was calculated based on mpMRI follow-up.

Based on the biopsies, the data was as follows:

As demonstrated by imaging over a mean period of 2.31+1.56 years (for <7 mm)  and 2.40+1.77 years (for <5 mm), the lesions demonstrated “no significant change in size.” The article reports that these findings “held true on subset analyses of patients who had a minimum of 2-year interval follow-up with mpMRI.”

Citing the downward trend in prostate cancer aggression at diagnosis, based on screening and earlier detection, the authors point out the resulting overtreatment for lack of a better understanding of how low-risk, early stage tumors behave. Based on their results, the authors suggest that small index prostate lesions seen on imaging are either benign or are low-grade cancers that can be safely monitored. Thanks to the imaging, the authors provide quantitative measures of the low rate of growth of such lesions. Because they found no Gleason 7 cancer in lesions <7 mm, they propose a surveillance interval of at least two years without significant change.

Whether or not future treatment is needed, the idea of buying time to enjoy full quality of life will surely be appealing to many patients with small index lesions (as identified on mpMRI) that qualify for active surveillance for at least two years.