Frequency of Metastasis in Dogs With Anal Sac Adenocarcinoma

Ravinder S. Dhaliwal, DVM, MS, DACVIM (Oncology), DABVP (Canine and Feline), MedVet Silicon Valley, San Jose, California

ArticleLast Updated August 20233 min read
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In the Literature 

Jones AE, Wustefeld-Janssens BG. A relatively high proportion of dogs with small apocrine gland anal sac adenocarcinoma (AGASACA) primary tumours present with locoregional lymph node metastasis. Vet Comp Oncol. 2023;21(2):327-331. doi:10.1111/vco.12890

The Research …

Apocrine gland anal sac adenocarcinoma (AGASACA) represents 17% of perianal tumors in dogs.1 These tumors are locally aggressive and highly metastatic to regional lymph nodes (LNs). Treatment can be challenging due to local invasiveness, rapid metastasis to regional LNs, and association with paraneoplastic hypercalcemia. Reported metastatic rates in dogs with AGASACA are variable, with 36% to 90% of affected dogs having measurable metastatic disease at the time of diagnosis.2,3

In this study, records of dogs with a histologic diagnosis of AGASACA with documented primary tumor size and intra-abdominal staging were retrospectively reviewed. Of 116 dogs, 46 had tumors <2 cm and 70 had tumors ≥2 cm. Crossbreed dogs (37%) were most common; other represented breeds included Labrador retrievers, German shepherd dogs, golden retrievers, and Siberian huskies. Abdominal ultrasonography (85%) was the most common imaging method used for staging, followed by CT (11%) and CT enhanced by positron emission tomography (4%).

Overall rate of nodal metastasis was 46%. Of dogs with primary tumors <2 cm, 20% had regional nodal metastasis compared with 63% of dogs with primary tumors ≥2 cm; tumor size was significantly associated with LN metastasis. If the tumor size cutoff was changed to <1.3 cm, LN metastasis was noted in 21% of dogs compared with 58% of dogs with tumors ≥1.3 cm. Of dogs with tumors <2 cm and no nodal metastasis at initial evaluation, 19% developed LN metastasis at a median of 179 days. Of dogs with tumors ≥2 cm, 15% developed LN metastasis at a median of 205 days.  

… The Takeaways

Key pearls to put into practice:

  • Dogs with AGASACA may have a small primary tumor (≤1 cm) at presentation; however, staging may reveal a large volume of metastatic nodal burden. The regional nodal bed should be evaluated in dogs presented with small-sized AGASACA. In the current study, the fraction of dogs that developed LN metastasis with tumors <2 cm was relatively high. Number of LNs affected is a prognostic indicator4; LNs should be measured at the greater long-axis diameter.

  • Abdominal imaging is essential in clinical staging of dogs with AGASACA, regardless of tumor size. Abdominal ultrasonography is an acceptable modality, but CT is preferred to evaluate the regional nodal bed and distant metastasis when available.

  • The literature is in discordance on whether hypercalcemia is prognostic in dogs with AGASACA.2,3,5 In this study, presence of hypercalcemia was not prognostic. Of interest, 69% of dogs in this cohort were neutered males, and 31% were spayed females. Although this demographic is similar to another study,6 previous studies have reported a female predisposition or equal sex distribution.2,3,5,7-9

  • This study did not report whether adjuvant chemotherapy was administered, which may have delayed development of nodal metastasis in dogs with no evidence of metastasis at initial presentation.