SABR tolerance after prostatectomy: pushing the boundaries of ultrahypofractionation

Objective: To evaluate the feasibility and tolerance of ultra-hypofractionated SABR (stereotactic ablative radiation therapy) protocol following radical prostatectomy.

Patients and methods: We included patients undergoing adjuvant or salvage SABR between April 2019 and April 2023 targeting the surgical bed and pelvic lymph nodes up to a total dose of 36.25 Gy (7.25 Gy/fraction) and 26 Gy (5.2 Gy/fraction), respectively, in 5 fractions on alternate days with an urethra sparing protocol. Acute and late adverse effects were assessed using the CTCAE v5.0. Pearson's chi-square test for categorical variables was used to compare characteristics and possible associations among different subgroups.

Results: Adjuvant radiation therapy (ART) was administered to 40 high-risk patients (detectable post-surgery PSA, Grade Group 4/5, nodal involvement, R1/R2 resection margin), while salvage radiotherapy (SRT) was delivered to 60 patients with rising PSA levels post-undetectable values. Elective nodal irradiation was performed in 57 patients, with 11 additional patients receiving a simultaneous integrated boost (total dose: 40 Gy in 5 fractions) for macroscopic nodal disease. Twenty-four high-risk patients underwent 24-months androgen deprivation therapy (ADT). Treatment was well-tolerated with minimal toxicity. The maximum grade of SABR-related toxicity observed was grade 3. Acute gastrointestinal (GI) toxicity included seven cases of grade 2 and one of grade 3, while acute genitourinary (GU) events were limited to grade 2 in eight patients. Early-late toxicity included two cases of grade 3 and seven of grade 2 for GI, and 11 cases of grade 2 for GU. No toxicity above grade 3 was reported. With a median follow-up of 24 months (6-60 months), 14 patients experienced disease recurrence.

Conclusions: Ultra-hypofractionated adjuvant/salvage SABR appears feasible and safe. Longer follow-up is needed to validate observed outcomes.