Evaluating The Tolerability Of A Simultaneous Focal Boost To The Gross Tumor In Prostate Sabr: A Toxicity And Quality-Of-Life Comparison Of Two Prospective Trials

319 Background: Dose-escalated SABR to the whole prostate may be associated with better outcomes, but at a risk of increased toxicity. An alternative approach is to focally boost the dominant intraprostatic lesion (DIL) seen on MRI. We report the toxicity and quality of life (QOL) outcomes of two phase II trials of prostate and pelvic SABR, with or without a simultaneous DIL boost. Methods: The first trial treated patients with high-risk prostate cancer (PCa) to a dose of 40 Gy to the prostate and 25 Gy to the pelvis in 5 fractions. The second trial treated patients with intermediate-risk and high-risk PCa to a dose of 35 Gy to the prostate, 25 Gy to the pelvis, and a DIL boost up to 50 Gy in 5 fractions. Acute toxicities, late toxicities and QOL were assessed. Results: 30 patients were enrolled in each trial. In the focal boost cohort, the median DIL D90% was 48.3 Gy. There was no significant difference in acute grade ≥2 GI or GU toxicity between the two trials, or cumulative worst late GI or GU toxicity up to 24 months. There was no significant difference in QOL domain scores or minimally clinical important change between the two trials. Conclusions: Prostate and pelvic SABR with a simultaneous DIL boost was feasible and did not lead to a significant change in toxicity or QOL when compared to a cohort that did not receive a focal boost. Further follow-up will be required to assess long-term outcomes. Clinical trial information: NCT02911636.