Minimal late radiation toxicity and transient early toxicity following postoperative definitive intent conformal radiation therapy (20 × 2.5 Gy) for canine apocrine gland anal sac adenocarcinoma

Postoperative radiation therapy (RT) may be beneficial for dogs with anal sac apocrine gland adenocarcinoma (ASAC). Clinically significant late toxicities have been reported in up to 65% of dogs with perianal tumors following non-conformal definitive RT, particularly when fractions of 3 Gy or higher are prescribed. The primary objective of this prospective, descriptive study was to evaluate tolerability of a novel 3D conformal RT (3DCRT) protocol in a group of dogs. Dogs with ASAC were prospectively enrolled if clients elected RT following surgery. The planning target volume was prescribed 50 Gy in 2.5 Gy fractions using 6 MV photons and administered over 26 days. Early and late radiation toxicities were graded according to standardized criteria. Thirteen dogs were initially enrolled but 1 was excluded due to a high risk of anesthesia-related mortality. Seven dogs presented with early stage disease. Median follow up time was 771 days (91-2223). Transient grade 3 dermatitis and anusitis developed in all dogs, with resolution within 4 weeks. Two dogs developed transient grade 2 late colitis. Locoregional failure in the irradiated field was documented in one dog at 738 days. All-cause median survival time was 771 days (95% confidence interval: 510 → 2223 days). Findings indicated that this fractionation may be safely administered to the canine anus and pelvic canal using 3DCRT, although acute toxicity should be anticipated. Further prospective studies are needed in order to confirm long-term tolerability and efficacy.