After treatment of the first 3 patients on DL1 was observed to be well tolerated, treatment on DL2 was initiated. Both patients treated at DL2 (42.5 Gy in 5 fractions) experienced dose limiting toxicity (grade 3 urinary incontinence for 7.5 week duration and hematuria for a 4 day duration). This prompted de-escalation and expansion on dose level 1 as it was considered the MTD. GI toxicity was minimal and the most severe toxicity was grade 2 proctitis lasting 2.5 weeks. Biochemical failure was observed in one patient at 33 months post treatment.
In conclusion, the maximum tolerated dose (MTD) of focal, salvage SBRT for in-gland recurrence after definitive, fractionated, external beam radiotherapy (EBRT) is 40Gy in 5 fractions. The most concerning toxicities observed were GU, and GI toxicities were minimal. One biochemical failure was observed after the end of the 2-year study period, comparing favorably to prior published results.
A detailed manuscript reporting the results of NCT03253744 is published in Practical Radiation Oncology (Patel et al., PRO 2023; PMID: 37442430), and a 4-minute video summary of the trial design and principal findings can be found at the following link.
Written by: Krishnan R. Patel, MD, Assistant Research Physician, Radiation Oncology Branch, Center for Cancer Research, National Cancer Institute, Bethesda, MD
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