Adenoid Cystic Carcinoma

General

Adenoid cystic carcinoma can have a characteristic low-power appearance; however, the diagnosis depends on recognizing two lines of differentiation of the neoplastic cells.

• Definition: Adenoid cystic carcinoma is a type of invasive ductal carcinoma that features two types of neoplastic cells: luminal and basaloid/myoepithelial.

• Clinical Significance: Adenoid cystic carcinoma usually present as a single, small, palpable mass, which consist predominantly of invasive nests. If present, the noninvasive component represents only a minor component of the carcinoma. This type of carcinoma almost never metastasizes; consequently patients with adenoid cystic carcinoma experience a more favorable prognosis compared to those with conventional invasive ductal carcinomas. Like their salivary gland counterparts, mammary adenoid cystic carcinomas usually contain the MYB-NFIB fusion gene.

• Gross Findings: The carcinoma usually forms a well defined grey, tan, or pink nodule between one and three centimeters in dimension.

• Microscopic Findings: Geographic islands of neoplastic cells infiltrate the mammary parenchyma. Careful evaluation reveals two cell types: luminal epithelial and basaloid/myoepithelial cells. The typical adenoid cystic carcinoma does not stain for estrogen receptors.

• Differential Diagnosis: Cribriform carcinoma (Cribriform carcinoma is a rare form of low‑grade invasive ductal carcinoma in which the neoplastic cells grow in large nests and form glandular spaces identical those found in cribriform ductal carcinoma in-‑situ. The neoplastic cells display the features of luminal cells only and they express estrogen receptors.)

• Discussion:

The diagnosis of mammary adenoid cystic carcinoma rests on the presence of a single neoplastic population in which certain carcinoma cells show attributes of luminal cells and others display features of basaloid/myoepithelial cells.

The luminal cells appear polarized and radially arrayed around a lumen. They possess basally situated nuclei containing granular chromatin and small nucleoli and eosinophilic apical cytoplasm. The basaloid/myoepithelial cells do not demonstrate polarization. They have oval spindly nuclei and inconspicuous nucleoli. The nuclei appear circumferentially oriented with respect to spaces formed by the cells.

The proportion of luminal and basaloid/myoepithelial cells varies from case to case and from region to region within a single case. Both types of cells create spaces. The luminal cells form small compact tubules, which may appear empty or contain dense, eosinophilic secretion.

The basaloid/myoepithelial cells produce basement membrane material, which appears pale-blue or pink.

One can have difficulty distinguishing the two populations of cells in high-grade adenoid cystic carcinomas, but careful histological study and immunohistochemistry will usually allow one to distinguish them. (High-grade adenoid cystic carcinoma)

The luminal cells usually demonstrate more intense reactivity for one keratin molecule or another than the basaloid/myoepithelial cells do, and a stain for p63 usually highlights the basaloid/myoepithelial component.

A stain for keratin 5/6	A stain for p63

Flattening of peripheral basaloid/myoepithelial cells creates an appearance that one could confuse with an in-situ component; however, the infiltrative pattern of growth establishes the invasive nature of the nests.

	A stain for p63	A stain for smooth muscle myosin

The carcinoma cells in adenoid cystic carcinoma stain for CD117 (c-Kit) and typically do not stain for ER, PR, and HER2. (A stain for CD117 (c-Kit))

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Breast Pathology