In dogs, perianal glands (ie, circumanal or hepatoid glands; hepatoid refers to the histologic/cytologic resemblance of cells to hepatocytes) are modified sebaceous glands located circumferentially around the anus; on the skin of the prepuce, perineum, tail, and pelvic limbs; and along the dorsal or ventral midline as far cranial as the neck.2-4 These glands (and their associated tumors) occur only in dogs and should not be confused with anal glands/sacs present in multiple species.
The 3 types of perianal gland tumors are perianal gland adenomas, epitheliomas, and perianal gland adenocarcinomas.2 Of these, perianal gland adenomas are most common, comprising >80% of all perianal gland tumors and ranking third among the most frequently diagnosed tumors in male dogs.5 Predisposed breeds include cocker spaniels, poodles, schnauzers, beagles, English bulldogs, Siberian huskies, Samoyeds, and crossbreed dogs.4,6,7
Perianal gland adenomas are most commonly found on the skin (typically circumferentially around the anus but can be in any location where there are perianal glands) and are benign, typically slow-growing tumors that can be single or multiple, nodular or multinodular, ulcerative, and/or infected.4,6,8
On cytology, perianal gland adenomas have a predominant population of large, cuboidal- to polygonal-shaped epithelial cells found in cohesive cell clusters with granular, pink–blue cytoplasm. Nuclei are round, usually single, with distinct nucleoli. Cellular pleomorphism is minimal.3 Varying numbers of reserve (basaloid) cells are also typically present, but the proportion of reserve cells is not an indicator of biological behavior.9 Tumors predominantly composed of reserve cells (>90%) are classified as epitheliomas.8
Pathogenesis and occurrence of perianal gland adenomas are not completely understood and appear multifactorial. Development and progression are dependent on sex hormones.6,7 Intact male dogs ≥8 years of age are at increased risk.4,6,7 Spayed dogs are also at significantly greater risk compared with intact female dogs.10 Androgen and estrogen may be both deleterious and protective, respectively, in etiology.10 Perianal gland adenomas often fully or partially regress (≤95% of cases) following neuter, indicating the influence of testosterone on these tumors.4,11,12
A presumptive diagnosis of perianal gland tumor can often be made based on signalment, patient history, and physical examination findings, especially when the tumor is located in the hairless perianal skin, a common location for this tumor type.6 Fine-needle aspiration of the mass and subsequent cytologic examination can support diagnosis and help rule out other tumors (eg, anal sac adenocarcinoma, inflammatory lesions).6
Cytology typically does not help differentiate between benign and malignant perianal gland tumors (ie, perianal gland adenoma vs perianal gland adenocarcinoma) because perianal adenocarcinomas can be either well or poorly differentiated.2 Incisional or excisional biopsy with histopathology is typically recommended to determine the biological behavior of the tumor.2,6
Neutering and surgical resection are the recommended treatments for perianal gland adenomas.13 Staged surgery may be preferable, and neutering is recommended to remove the androgenic stimulation in patients with large tumors. Tumors shrink and facilitate surgical excision as testosterone concentrations decline.6,13 Neutering does not lead to resolution or regression of perianal gland adenocarcinoma in intact male dogs.2
Alternate treatments (eg, cryosurgery, carbon dioxide laser ablation, electrochemotherapy, estrogen therapy, cyclosporine) for perianal gland adenoma management can have varying results.6,14,15