Top Line: What acute and late toxicity can be expected after SBRT to the prostate fossa?
The Study: Post-prostatectomy radiation remains the stalwart of conventional fractionation for prostate cancer. Initial results of GU003 showed that moderately hypofractionated prostate fossa radiation (62.5 Gy in 25 fractions) had non-inferior patient-reported toxicity at 24 months compared to conventional fractionation. Ultra-hypofractionation isn’t as far up the clinical trial ladder, but there have been several early phase trials evaluating this approach. A common theme has been favorable acute toxicity but, in some cases, concerning late toxicity. SCIMITAR was a multicenter, phase 2 trial of adjuvant or salvage prostate fossa SBRT. Patients had adverse features (positive margin, pT3/T4, Gleason 8+, tertiary Gleason 5), a rising PSA > 0.03, or a Decipher score >0.45. MRI and bone scan were required and PET scan was strongly encouraged. The standard RTOG CTV was used. The fossa PTV was prescribed 30-34 Gy in 5 fractions. Of 100 patients, 69% were treated on a standard CT-based linac and 31% were treated on an MR-linac. A SIB boost to the prostate bed (median 40 Gy) was used in 27%, and 27% also received elective nodal radiation (median 25 Gy in 5 fractions). ADT was given in 41%. Acute and late toxicity was generally mild. The rates of acute grade 2 GU and GI toxicity were 9% and 5% while grade 3 GU and GI toxicity were 1%. Late grade 2 toxicity was <10% and late grade 3 toxicity was 1%. While it appeared that toxicity was less after MR-guided treatment, only 9.7% of those patients received elective nodal treatment compared to 30.4% of CT-guided patients. On multivariable analysis, elective nodal treatment was associated with increased toxicity.
TBL: The doses used in this phase 2 trial of post-prostatectomy SBRT resulted in favorable acute and short-term late toxicity. | Ma, Int J Radiat Oncol Biol Phys 2022