Malignant anal sac melanoma in dogs: eleven cases (2000 to 2015)

To report the signalment, clinical presentation, treatments pursued and outcomes of dogs with malignant anal sac melanoma.

Medical records from five institutions from January 2000 through December 2015 were reviewed and dogs with cytologically- or histologically-confirmed malignant anal sac melanoma were identified. Signalment, clinical signs, staging, cytology, histopathologic analysis, surgical and non-surgical treatments were extracted from the medical records. The referring veterinarians and owners were contacted for follow-up data.

Eleven dogs were included and survival data was available for all. The most common clinical signs were bloody anal sac discharge and perianal licking. Initial treatments pursued included surgery (n=8), chemotherapy (n=1), and palliative treatment with pain medications and stool softeners (n=2). In an adjuvant setting, melanoma vaccine was pursued following surgery in three dogs and chemotherapy in one dog. Regardless of treatment, progression-free survival (mean 92 · 5 days) and overall survival times (median 107 days) were short.

Dogs in this case series had a guarded to poor prognosis regardless of treatment. Ten of 11 dogs were euthanased due to local or distant disease progression. Only 1 of 11 dogs was alive one year after diagnosis. An understanding of tumour behaviour in this location could lead to improved survival times with earlier diagnosis and treatment.