Role of prostate magnetic resonance imaging in active surveillance
Active surveillance (AS) has emerged as a beneficial strategy for management of low risk prostate cancer (PCa) and prevention of overtreatment of indolent disease. However, selection of patients for AS using traditional 12-core transrectal prostate biopsy is prone to sampling error and presents a challenge for accurate risk stratification. In fact, around a third of men are upgraded on repeat biopsy which disqualifies them as appropriate AS candidates. This uncertainty affects adoption of AS among patients and physicians, leading to current AS protocols involving repetitive prostate biopsies and unclear triggers for progression to definitive treatment. Prostate magnetic resonance imaging (MRI) has the potential to overcome some of these limitations through localization of significant tumors in the prostate. In conjunction with MRI-targeted prostate biopsy, improved sampling and detection of clinically significant PCa can help streamline the process of selecting suitable men for AS and early exclusion of men who require definitive treatment. MRI can also help minimize the invasive nature of monitoring for disease progression while on AS. Men with stable MRI findings have high negative predictive value for Gleason upgrade on subsequently biopsy, suggesting that men may potentially be monitored by serial MRI examinations with biopsy reserved for significant changes on imaging. Targeted biopsy on AS also allows for specific sampling of concerning lesions, although further data is necessary to evaluate the relative contribution of systematic and targeted biopsy in detecting the 25–30% of men who progress on AS. Further research is also warranted to better understand the nature of clinically significant cancers that are missed on MRI and why certain men have progression of disease that is not visible on prostate MRI. Consensus is also needed over what constitutes progression on MRI, when prostate biopsy can be safely avoided, and how to best utilize this additional information in current AS protocols. Despite these challenges, prostate MRI, either alone or in conjunction with MRI-targeted prostate biopsy, has the potential to significantly improve our current AS paradigm and rates of AS adoption among patients moving forward.