3362 - Treatment Choices and Toxicity Outcomes in Patients with a High Risk PROSTOX Score

12:45pm - 02:00pm PT

Hall F

Screen: 27

POSTER

Presenter(s)

Joanne Weidhaas, MD, PhD - UCLA Radiation Oncology, Los Angeles, CA

A. U. Kishan¹, K. McGreevy², D. Telesca², and J. B. Weidhaas¹; ¹Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, ²Department of Biostatistics, Fielding School of Public Health, University of California, Los Angeles, Los Angeles, CA

Purpose/Objective(s): PROSTOX is a genetic test that predicts for the risk of moderate (grade >2) late genitourinary (GU) toxicity following stereotactic body radiotherapy (SBRT) for prostate cancer. Importantly, genetic predictors of late GU toxicity for patients treated with SBRT versus those treated with conventionally fractionated radiotherapy (CFRT) have been shown to be unique, affording radiation treatment alternatives for patients with a “high-risk” PROSTOX test. In this prospective registry study, for the first cohort of patients who received high-risk PROSTOX results, we evaluated patient treatment choices as well as toxicity rates. We hypothesized that a majority of PROSTOX high-risk patients would not choose to receive SBRT, and that toxicity rates for other forms of radiation would be lower for these patients.

Materials/Methods: Prostate cancer patients being offered SBRT who underwent PROSTOX testing and received high-risk results were included in this study (n=98). Each patients’ definitive treatment choice was recorded, and proportions choosing each treatment were calculated. Late GU toxicity rates were calculated in patients with a minimum of 12 months of follow up who received definitive radiotherapy, whether this was SBRT, CFRT, moderately hypofractionated radiotherapy (MHFRT), or high dose rate (HDR) brachytherapy (n=68). We calculated the proportion of late grade >2 GU toxicity events for each regimen and derived exact binomial 95% confidence intervals. We then compared toxicity rates between patients receiving SBRT and MHFRT or CFRT using Fisher’s exact test.

Results: Of the 98 individuals included, 26 (26.5%) proceeded with SBRT, 46 (46.9%) received either MHFRT or CFRT, 18 (18.4%) underwent a radical prostatectomy (RP), and the remaining 8% received HDR brachytherapy or other treatments (focal ultrasound ablation or cryotherapy). With a minimum follow up of 12 months, the overall late grade >2 GU toxicity rate in patients receiving radiotherapy was 36.8%. However, substantial differences existed across treatment groups. Only 7.1% of patients receiving MHFRT or CFRT experienced late grade >2 GU toxicity (95% CI =1.5-19.5%), whereas for patients who continued with SBRT the rate of late grade >2 GU toxicity was 83.3% (95% CI=62.6-95.3%). These differences were statistically different between MHFRT/CFRT and SBRT (Fisher’s exact p=3.6 x 10^-10).

Conclusion: These results from prospectively tested patients support prior retrospective data suggesting that patients identified as genetically at high-risk for toxicity after SBRT do not appear to be at higher risk of toxicity after MHFRT or CFRT. Analyses of additional high-risk patients and further studies of genetic biomarkers for toxicity to alternative fractionation regimens are ongoing.