I-N33

Signalment (JPC# 4037525-00): Dog

HISTORY: Dermal mass over right temporalis

HISTOPATHOLOGIC DESCRIPTION: Haired skin: Expanding the dermis, elevating the overlying epidermis, and compressing adjacent adnexa is an unencapsulated, well demarcated, moderately cellular, multilobulated neoplasm composed of polygonal cells arranged in islands, trabeculae, and cords on a moderate fibrovascular stroma. Neoplastic cells have distinct cell borders, a moderate to abundant amount of pale, eosinophilic, granular to lacy cytoplasm often with variably sized clear vacuole(s), a round to oval to pleomorphic nucleus with coarse chromatin, 1-4 distinct nucleoli and occasional intranuclear cytoplasmic invaginations. Anisocytosis and anisokaryosis are marked, there are 4-6 mitotic figures per 2.37mm2 and many multinucleated cells. Admixed with neoplastic lobules are large foci of lytic necrosis composed of abundant necrotic debris, viable and necrotic neutrophils, fibrin, hemorrhage, and edema (cystic degeneration). Fibrous connective tissue separating neoplastic lobules contain pale, smudgy sclerotic collagen, eosinophilic fluid (edema), fibrin, hemorrhage, and variable numbers of lymphocytes, plasma cells, macrophages often with intracytoplasmic brown globules (hemosiderin), accumulations of canary-yellow pigment (hematoidin), and rare foci of mineralization. The superficial dermis contains low numbers of often perivascular and periadnexal lymphocytes, plasma cells, neutrophils, and macrophages, hemorrhage, and dilated lymphatics. There are few small intracorneal pustules composed of abundant aggregates of neutrophils and cellular debris as well as focal ulceration of the epidermis with subjacent small caliber vessels and reactive fibroblasts (granulation tissue) as well as hemorrhage, fibrin, edema, neutrophils and histiocytes.

MORPHOLOGIC DIAGNOSIS: Haired skin: Clear cell adnexal carcinoma, breed unspecified, canine.

SYNONYMS: Follicular stem cell carcinoma; clear cell hidradenocarcinoma; clear cell epitrichial carcinoma; clear cell ductal carcinoma

GENERAL DISCUSSION:

Epitrichial (apocrine) glands give rise to many benign and malignant tumors
1. Adenomas include cystadenoma (I-N06); simple or complex (mixed) secretory adenoma; and canine, feline, or solid-cystic ductular adenoma (I-N07)
2. Carcinomas include cystadenocarcinoma; simple or complex (mixed) adenocarcinoma (I-N06); ductal adenocarcinoma; solid-cystic adenocarcinoma; clear cell adnexal carcinoma (I-N33); and apocrine gland anal sac adenocarcinoma (I-N08)

PATHOGENESIS:

Unknown; rare undifferentiated adnexal neoplasm

TYPICAL GROSS FINDINGS:

Slow growing dermal to subcutaneous mass
Metastasis to regional lymph nodes and lungs possible

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Large polygonal cells arranged in lobules and nests with areas of cystic degeneration
Moderate to abundant finely vacuolated cytoplasm
Subtle tubules lined by 1-3 layers of cells may be present
Sparse to moderate stroma with chondroid metaplasia (rarely reported) and occasional nests of small mesenchymal cells (“follicular papillary bodies”)
Dermal papilla-like structures (proposed as due to an “inductive effect”)
Lymphatic invasion possible

ADDITIONAL DIAGNOSTIC TESTS:

Immunohistochemistry: CAM5.2, CK7/8, CK13 positive
Cytology:
- High cellularity with loosely arranged individualized neoplastic cells with moderate to marked pleomorphism, multinucleation, and criteria of malignancy
- Cells are oval to polygonal to spindle shaped with variably abundant blue-gray cytoplasm with wispy cytoplasmic borders
- Nuclei are round to ovoid with coarse chromatin and prominent, eccentric nucleoli

DIFFERENTIAL DIAGNOSIS:

For histologic findings:

Balloon cell melanoma: very similar cytologically and histologically; melanin pigment, large clear polygonal cells and spindle shaped cells may be present in both; balloon cell melanomas are not positive for CK and often have junctional activity
Clear cell basal cell carcinoma: rare variant with large clear to finely granular polygonal cells, lack of pleomorphism, and frequent necrosis

COMPARATIVE PATHOLOGY:

Not reported in other species

References:

Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Saunders Elsevier; 2016: 714, 719, 721.
Raskin RE, Conrado FO. Integumentary system. In: Raskin RE, Meyer DJ, Boes KM, eds. Canine and Feline Cytopathology A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Saunders Elsevier; 2023: 82.