Multiparametric MRI as a Predictor of PSA Response in Patients Undergoing Stereotactic Body Radiation (SBRT) Therapy for Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> In order to maximize the therapeutic ratio, it is important to identify adverse prognostic features in men with prostate cancer, especially amongst those with intermediate risk disease, which represents a heterogeneous group. These men may benefit from treatment intensification. Prior studies have shown pretreatment mpMRI may predict biochemical failure in patients with intermediate and/or high-risk prostate cancer undergoing conventionally fractionated external beam radiotherapy and/or brachytherapy. This is the first study to evaluate pretreatment mpMRI findings as a marker for outcome in patients undergoing SBRT. <h3>Materials/Methods</h3> We identified all patients treated at our institution with linear accelerator-based SBRT to 3625 cGy in 5 fractions, with or without androgen deprivation therapy (ADT) from November 2015 to March 2021. All patients underwent SpaceOAR placement followed by pre-treatment MRI. Post-treatment PSA measurements were obtained 4 months after SBRT, followed by every 3-6 months thereafter. A two-sample t-test was used to compare preoperative mpMRI features with clinical outcomes. <h3>Results</h3> 123 men were included in the study. The table below summarizes the pretreatment patient characteristics. Patients with higher pretreatment diameter of the largest intraprostatic lesion, number of prostate lesions, and maximal PI-RADS score were associated with higher PSA nadir and longer time to PSA nadir (p<0.0001). When separated by ADT treatment, this association remained for patients who were not treated with ADT (p<0.001). In patients who received ADT, the pretreatment MRI variables were each significantly associated with time to PSA nadir (p<0.01) but not with PSA nadir (p>0.30). <h3>Conclusion</h3> Our experience shows a significant association between mpMRI features and PSA outcome in patients treated with SBRT with or without ADT. Since PSA nadir has been shown to correlate with biochemical failure, this information may help radiation oncologists better counsel their patients regarding outcome after SBRT and can help inform future studies regarding who may benefit from treatment intensification with, for example, ADT and / or boost to dominant intraprostatic lesion.