Small primary apocrine gland anal sac adenocarcinoma (AGASACA) tumors have a clinically relevant rate of locoregional lymph node metastasis.

Apocrine gland anal sac adenocarcinoma (AGASACA) is a highly relevant disease in dogs, with a high rate of lymph node (LN) metastasis during the course of disease. A recent study showed that risk for death and disease progression was significantly associated with primary tumor size less than 2 cm and 1.3 cm, respectively. The objective of this study was to report the proportion of dogs that have primary tumors less than 2cm in diameter, that are diagnosed with LN metastasis at presentation. This was a single site retrospective study of dogs that underwent treatment for AGASACA. Dogs were included if physical examination primary tumor measurements were available, abdominal staging was performed, and confirmation of abnormal lymph nodes by cytology or histology was done. Over a 5-year period, 116 dogs were included for review with 53 (46%) having metastatic LN at presentation. The metastatic rate for dogs with primary tumors < 2cm was 20% (9 of 46 dogs) compared to 63% (44 of 70 dogs) in dogs with primary tumors ≥ 2 cm. The association between tumor size group (< 2 cm vs. ≥ 2 cm) and the presence of metastasis at presentation was significant (P < 0.0001) with an OR of 7.0 (95% CI: 2.9 - 15.7). Primary tumor size was significantly associated with LN metastasis at presentation but the proportion of dogs that presented with LN metastasis in the < 2 cm group was relatively high. This data suggests that dogs with small tumors may still have aggressive tumor biology.