Retrospective Study Evaluating the Outcome and Efficacy of Stereotactic Body Radiation Therapy for the Treatment of Metastatic Abdominal Lymph Nodes in Dogs With Apocrine Gland Anal Sac Adenocarcinoma.

Local treatment for dogs with regional lymph node metastasis secondary to apocrine gland anal sac adenocarcinoma (AGASACA) includes nodal extirpation or radiotherapy. Stereotactic body radiation therapy (SBRT) may provide a definitive intent treatment option for macroscopic nodal disease when surgery is declined or the disease is deemed inoperable. Twenty-five dogs receiving SBRT to the metastatic sacroiliac lymph nodes were retrospectively evaluated. Dogs were staged according to the previously published TNM staging system with 3 stage IIIa, 14 stage 3b, and 8 stage IV. The overall median survival time (MST) was 451 days and the stage did not significantly impact survival (p = 0.31). The overall median event-free survival time was 246 days. Significant positive prognostic factors included male sex, higher dose per fraction, and higher total dose (p = 0.034, 0.0035, 0.0047). Dogs receiving 6-7.5 Gy per fraction with a total dose of 30-37.5 Gy outperformed dogs receiving other protocols. Twelve dogs experienced gait changes in the hind limbs during the late radiation effects period. Resolution of hypercalcemia in 5 dogs was inconsistent and transient. The best response was complete in 21%, partial in 58%, and stable disease in 17% at a median of 100 days. Three dogs (12%) developed progression of treated lymph nodes at 157, 498, and 644 day. Eight dogs (32%) had recurrence of their primary (untreated by radiation) anal sac masses. SBRT was determined to be an effective alternative to surgical excision; however, more investigation is needed to determine the cause of gait changes in the late toxicity period.