JPC SYSTEMIC PATHOLOGY
ENDOCRINE SYSTEM
January 2022
R-N14

Signalment (JPC# 2830560):  A 12-year-old spayed female greyhound

HISTORY:  This dog had a mass in the mammary gland area.

HISTOPATHOLOGIC DESCRIPTION:  Haired skin, mammary gland:  Expanding and effacing the mammary gland, incorporating and compressing adjacent ducts, and elevating the overlying epidermis is a 1 x 2 cm, unencapsulated, infiltrative, multilobulated, moderately cellular neoplasm composed of polygonal to fusiform cells arranged in islands and solidly cellular areas supported by fine fibrovascular stroma, while lobules are separated by variably thick bands of collagen. Neoplastic cells have variably distinct cell borders and abundant, eosinophilic, often highly vacuolated cytoplasm.  Larger cytoplasmic vacuoles (up to 50 um in diameter) peripheralize the nucleus.  Nuclei are vesiculate with one prominent magenta nucleolus.  Mitoses average 5 per 2.37mm² (10 HPF).  There is moderate anisokaryosis and anisocytosis and scattered single cell death.  Focally, cyst-like spaces are lined by attenuated to cuboidal neoplastic cells and contain homogenous eosinophilic material and cellular debris. Within the neoplasm are multifocal areas of necrosis, hemorrhage, mineralization, and acicular cholesterol clefts, and there are small foci of cartilaginous metaplasia.  In the surrounding tissue, there are low numbers of lymphocytes and plasma cells.  There is mild epidermal orthokeratotic hyperkeratosis.

MORPHOLOGIC DIAGNOSIS:  Mammary gland:  Malignant myoepithelioma, greyhound, canine.

SYNONYM:  Spindle cell carcinoma

GENERAL DISCUSION:

• Mammary gland myoepitheliomas and malignant myoepitheliomas are rare neoplasms derived solely from myoepithelial cells of the mammary gland
• Myoepithelial cells are contractile cells with smooth muscle and epithelial components; they are normally located between the luminal epithelial cell layer and the basal lamina of various secretory structures (sweat glands, salivary glands, lacrimal glands, and mammary glands)
• Myoepithelial cells are divided into two subtypes depending on location (Zappulli 6-10)
  • Type 1 = Suprabasal: located on basement membrane; flattened/elongate or stellate resting suprabasal myoepithelial cells (type Ia); large round cells with scant possibly vacuolated cytoplasm are proliferative suprabasal myoepithelial cells (type Ib)
  • Type II = Interstitial: expanding into the interstitium; appear more spindle to stellate with more abundant cytoplasm and often on a myxoid matrix (type IIa), can appear as larger round to oval cells with scant cytoplasm and minimal matrix (type IIb)
• Canine mammary tumors frequently feature proliferation of both luminal epithelial cells and interstitial myoepithelial cells (Sammarco, Vet Pathol. 2020)

PATHOGENESIS:

• Canine mammary tumors are hormone-dependent; early ovariectomy has a major protective effect; male dogs can develop mammary tumors; rare and typically benign
• p53 defects have been associated with the development of canine mammary tumors
• p63 (a homologue of the p53 gene) is overexpressed in many human epithelial neoplasms; it has been suggested that p63 is an oncogene in this type of neoplasm
• Cyclooxygenase-2 (COX-2) expression is increased in canine mammary tumors; COX-2 promotes expression of the anti-apoptotic protein bcl-2, thereby inhibiting cellular senescence;  COX-2 also helps promote tumor invasiveness that is mediated through increased activity of matrix metalloproteinases 2 and 9
• Neoplastic myoepithelial cells in canine complex carcinomas have labelling similar to that of terminally-differentiated myoepithelial cells, while those of malignant myoepitheliomas were comparable with intermediary myoepithelial cells and had significantly higher Ki67 expression (Rasotto et al., J Comp Pathol. 2014)

TYPICAL CLINICAL FINDINGS:

• Mammary gland neoplasia occurs most commonly in older bitches that are sexually intact or ones that were spayed later in life
• Most dogs are clinically healthy when initially presented for evaluation of mammary tumors

TYPICAL GROSS FINDINGS:

• Mammary gland tumors may be small, large, ulcerated, fixed, well circumscribed, and involve one or multiple glands
• The caudal 4th and 5th mammary glands are more commonly involved than the more cranial glands

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Malignant myoepithelioma:

• Bundles and whorls of fusiform spindle to stellate, or round to oval cells without a lobular pattern
• Poorly demarcated cell borders
• Anisocytosis and anisokaryosis can be marked with variable mitotic figures
• Matrix may be myxoid; cytoplasm may have vacuoles containing the myxoid matrix
• Lack of glandular or tubular differentiation, however neoplasm may entrap residual normal tubules/ducts
• Grading: when using the histologic grading of canine mammary tumors published by Pena et al in Vet Pathol (2013), use a de facto score of 2 in tubule formation to best correlate grade and biologic behavior (Zappuli, Surgical Pathology of Tumors, 2019)

Myoepithelioma:

• Spindle to stellate cells in short bundles in a myxoid matrix
• Does not entrap tubules
• Similar basic features as malignant counterpart, but lacks cellular and nuclear pleiomorphis, there is minimal anisocytosis and anisokaryosis, and there is a low mitotic count (typically < 2 per 2.37 square millimeters)

ADDITIONAL DIAGNOSTIC TESTS:

• Immunohistochemistry:
• Myoepithelial cells: positive for alpha smooth muscle actin, calponin, p63, CK5, vimentin, CK14, ALP
  • p63: Sensitive and highly specific marker; nuclear immunoreactivity in myoepithelial cells in canine mammary tissue
  • Calponin: less specific; cytoplasmic immunoreactivity; stains both myoepithelial cells and myofibroblasts which may be present in infiltrative neoplasms (Sanches-Cespedes 47)
  • CK 5 is a myoepithelial- and squamous-cell differentiating marker in canine tumors
• Rule out carcinoma-and-malignant myoepithelioma by performing epithelial markers (broad spectrum cytokeratins, CK8, CK18, CK19)
• Alkaline phosphatase (ALP): Positive (fibrocytes are ALP negative)

DIFFERENTIAL DIAGNOSIS: 

Benign mammary tumors:

• Complex adenoma: Mixed proliferation of secretory epithelial cells and expansion of myoepithelial cells into the interstitium in a myxoid matrix
• Fibroadenoma: Proliferation of tubules with single layers of luminal epithelial cells and suprabasal myoepithelial cells in abundant fibrous stroma
• Benign mixed tumor (R-N15A): Proliferation of glandular (luminal and myoepithelial) and mesenchymal elements (e.g. cartilage or cartilage and bone)

Malignant mammary tumors:

• Complex carcinoma: Composed of two cell populations; malignant epithelial component and benign myoepithelial component
• Carcinoma and malignant myoepithelioma: Composed of two cell populations; malignant features in both the myoepithelial and epithelial components
• Osteosarcoma: Mammary gland is the most common site of extraskeletal soft tissue osteosarcoma in dogs
• Carcinosarcoma: Also called malignant mixed tumor; malignant epithelial and mesenchymal components
• Fibrosarcoma: Uncommon; fusiform cells with distinct interwoven pattern; can arise from preexisting mammary neoplasm or from the interstitial stroma of the mammary gland

COMPARATIVE PATHOLOGY:

• Cats: Myoepitheliomas and malignant myoepitheliomas have not been described in cats; one recent report of four mammary neoplasms previously described as malignant pleomorphic tumors were diagnosed as carcinoma-and-malignant myoepitheliomas based on IHC analysis; behavior is potentially less aggressive than feline simple carcinomas (Sammarco, Vet Pathol. 2020)
• Primates: Report of one cynomolgus macaque with a malignant myoepithelioma with a squamous epithelial component in the mammary gland
• Mice: Myoepithelioma most frequently arises from the submaxillary and parotid salivary glands; occasionally associated with mammary, preputial, or Harderian glands; most common in BALB/c and BALB/cBy mice
  • Often associated with concomitant bone marrow/splenic myeloid hyperplasia, likely due to secretory produce from tumor; large tumors may metastasize to lung; often cystic due to necrosis

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016:3, 114.
2. Dore M, Lanthier I, Sirois J. Cyclooxygenase-2 expression in canine mammary tumors. Vet Pathol. 2003;40:207-212.
3. Foster, RA. Female Reproductive System and Mammae.  In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. Louis, MO: Elsevier; 2017: 1147-1193.
4. Goldschmidt M, Pena L, Rasotto R, Zappulli. Classification and grading of canine mammary tumors. Vet Pathol. 2011;48(1):117-131.
5. Goldschmidt MH, Pena L, Zappulli V. Tumors of the Mammary Gland. In: Meuten DJ, ed. Tumors in Domestic Animals, 5th ed. Ames, IA: John Wiley & Sons, Inc.; 2017:752.
6. Heller DA, Clifford CA, Goldschmidt MH, et al. Cyclooxygenase-2 expression is associated with histologic tumor type in canine mammary carcinoma. Vet Pathol. 2005;42:776-780.

7. Ramalho LNZ, Ribeiro-Silva A, Cassali GD, Zucoloto S. The expression of p63 and cytokeratin 5 in mixed tumors of the canine mammary gland provides new insights into the histogenesis of these neoplasms. Vet Pathol. 2006;43:424-429.
8. Rasotto R, Goldschmidt MH, Castagnaro M, Carnier P, Caliari D, Zappulli V. The dog as a natural animal model for study of the mammary myoepithelial basal cell lineage and its role in mammary carcinogenesis. J Comp Pathol. 2014;151:166-180.
9. Sammarco A, Finesso G, Zanetti R, et al. Biphasic Feline Mammary Carcinomas Including Carcinoma and Malignant Myoepithelioma. Vet Pathol. 2020; 57(3): 377-387.
10. Sanchez-Cespedes R, Millan Y, Guil-Luna S, et al. Myoepithelial cells in canine mammary tumors. Vet J. 2016; 207: 45-52.
11. Schlafer DH, Foster RA. Female genital system. In: Maxie MG, ed. Jubb, Kennedy, Palmer’s Pathology of Domestic Animals. Vol 3. Philadelphia, PA: Elsevier Saunders; 2016:460-464.
12. Yang X, Hori T, Fukuds A, et al. Malignant myoepithelioma with a squamous epithelial component in the mammary gland of a cynomolgus monkey. Tox Path. 2003; 31:549-553.
13. Zappuli, V, Pena L, Rasotta R, et al. Mammary Tumors. Surgical Pathology of Tumors of Domestic Animals. Vol 2. Kiupel M, ed. Chicago, IL: CL Davis Foundation; 2019.

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