Introduction

Consensus is not established regarding optimal therapy for canine apocrine gland adenocarcinoma of the anal sac (AGAAS). The purpose of this study was to better define local treatment options by comparing outcome in dogs with AGAAS treated with differing radiotherapy (RT) protocols.

Methods

Dogs with AGAAS undergoing radiotherapy at a PetCure hospital between 2015 and 2020 were studied retrospectively. Clinical data were abstracted from the medical records; adjuvant therapies were noted. Outcome was assessed using medical records and electronic follow up. Treatment related toxicities were classified according to VRTOG criteria. NCSS v. 12.0.4 was used for statistical analyses.

Results

Fifty-seven dogs were included. There were 4 RT treatment groups: hypofractionated (HF), stereotactic (SRT), conventional with surgical resection (CFRT), and fiducial guided SRT with surgical resection (SRT/Sx). The median RT-related survival time (start of RT to rescue, death or last contact) was longest in dogs that had surgery followed by CFRT or SRT (p=0.017). Statistically equivalent numbers of dogs in all groups received various chemotherapy drugs, but systemic therapy did not result in improved outcome. The most common side effects were VRTOG grades 1 and 2 acute radiodermatitis and proctitis. Six dogs had severe late term RT side effects including rectal stricture, resulting in euthanasia in 3 cases.

Conclusion

This study suggests that dogs with AGAAS that undergo surgery and CFRT or SRT have superior outcomes. Prospective studies are needed to develop techniques to minimize late term RT side effects in the rectum. Chemotherapy did not provide a definitive benefit in this study.