SABR tolerance after prostatectomy: pushing the boundaries of ultrahypofractionation

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

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.

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.

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