Anal sac adenocarcinoma
These tumors are highly metastatic and locally invasive and should be approached cautiously.
Affected dogs usually present with perianal pruritus, anal licking, tenesmus, constipation, scooting, dysuria, hindlimb weakness or acute paralysis (associated with vertebral metastases).
Many clinical symptoms are referable not only to the primary perianal mass but also the cystic metastatic sublumbar lymph nodes which press on vital organs. Secondary anal sacculitis and perineal dermatitis may also be present due to self-trauma.
Manual palpation of the anus in affected dogs may reveal a large mass (sometimes up to 100cm), but some are quite small and cannot be palpated. Regional lymphadenopathy in metastatic cases occurs in many cases, and pulmonary metastasis in about 10% of cases.
Diagnosis is based on presenting clinical signs, careful digital examination of the anus, rectum and anal glands, and histological analysis of biopsied material. Fine-needle aspirates of the anal gland are well tolerated in most dogs.
A thorough radiographic study should be performed to exclude metastases in visceral and thoracic organs.
Hematological analysis may reveal hypercalcemia of malignancy (normal range 1.1-1.3 mmol/L). A calcium:phosphate ratio higher than 60 suggests soft-tissue mineralization and impending renal failure, and these cases should be treated aggressively. Disrupted calcium homeostasis is a potential complication following the treatment of long-standing cases. Secondary hypertrophic osteopathy has also been reported.
At the time of diagnosis, approximately 50% of dogs have hypercalcemia and 80% have metastases within regional lymph nodes.
Treatment usually involves a combination of therapies including surgery, radiation therapy and chemotherapy.
Surgical resection is the mainstay of treatment, including cytoreductive surgery on regional lymph nodes. In dogs with visible metastases within regional lymph nodes, omentalization of regional sublumbar lymph node is recommended for long-term palliation of tenesmus and dysuria. Post-operative infections and fecal incontinence are common complications.
Chemotherapy drugs include melphalan (7 mg/m2 x 5 days every 3 weeks orally indefinitely), actinomycin-D (0.7 - 0.9 mg/m2 every 3 weeks intravenously), doxorubicin (30 mg/m2 every 2 - 3 weeks intravenously), mitoxantrone (5 mg/m2 every 3 weeks for five treatments) or nitrosylcobalamin.
Since approximately 20% of anal sac adenocarcinomas express Kit and platelet-derived growth factor receptors, toceranib has also been trialed with favourable responses when given at 2.8 mg/kg daily.
Corticosteroids such as prednisolone are less effective.
Fecal softeners such as lactulose will help to reduce fecal straining.
- Veterinary Practice News
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