PD22-10: High volume biopsy core involvement predicts for elevated PSA nadir following definitive SBRT in intermediate risk prostate cancer

Jonathan Lischalk*, Astrid Sanchez, Christopher Mendez, Todd Carpenter, Moses Tam, Anthony Corcoran, Matthew Witten, Seth Blacksburg, Aaron Katz, Jonathan Haas, New York, NY

Introduction: Prostate SBRT has become one of the fastest growing radiation modalities used to treat localized prostate cancer in the United States. Randomized data including the HYPO-RT-PC, RTOG 0938, and PACE B trials have established its efficacy and safety. Given the larger radiobiological dose delivered using SBRT relative to conventional dose escalated regimens, the ablative capability of such SBRT is thought to be superior. As such, PSA nadirs following SBRT have been shown to be lower than those observed with conventional fractionation schedules. Here we explore the relationship between burden of disease based on pretreatment prostate biopsy core involvement and PSA nadir following definitive SBRT.

Methods: A large single institutional database of 5,412 patients was interrogated to identify all those who had detailed prostate biopsy information available. Patients were included who underwent definitive five-fraction SBRT for localized prostate cancer sans nodal irradiation prior 2019. Patients who received androgen deprivation therapy were excluded from analysis. “High-volume” core involvement was defined as = 50% of biopsy cores involved with adenocarcinoma, as defined by the NCCN. All patients were treated using a robotic radiosurgical platform with fiducial tracking.

Results: A total of 2,034 patients were identified with a median age of 67 years. The risk group breakdown was as follows: low (n = 610, 30%), intermediate (n = 1,353, 67%), and high (n = 71, 3%). The most common (n = 1,842, 91%) prostate SBRT dose utilized was 3,500 cGy in 5 fractions. High volume core involvement stratified by risk group was as follows: low (n = 82, 13.4%), intermediate (n = 214, 15.8%), and high (n = 6, 8.5%). With a median follow up of 16 months, there was a statistically significant increase in PSA nadir within the intermediate risk cohort found to have high-volume versus low-volume core involvement (0.9 vs. 0.8, p = 0.001).  No significant difference in PSA nadir between high- and low-volume disease was observed in the low risk cohort.

Conclusions: To our knowledge, this is the first large scale analysis of prostate biopsy core involvement as it relates to PSA outcomes following curative SBRT. High volume core involvement is associated with higher PSA nadir following curative prostate SBRT for intermediate risk disease.

Source of Funding: None