Answer: Invasive lobular carcinoma, alveolar type

Histology: The tumor is composed of irregularly shaped nests that infiltrate the fat. The pattern of growth is not lobulocentric like a carcinoma in situ. The tumor cells are uniform in size, shape and nuclear features. They are somewhat discohesive at high power, which suggests the possibility of invasive lobular carcinoma. The tumor cells are strongly and diffusely positive for estrogen and progesterone receptors, and lack expression of E-cadherin.

Discussion: The absence of immunoreactivity for E-cadherin helps to define a lesion as lobular. E-cadherin is expressed in virtually all ductal carcinoma in situ lesions and invasive ductal carcinomas, but expression is lost in lobular carcinoma in situ and invasive lobular carcinoma, which correlates with their dyscohesive nature. The cytology of this tumor is typical of invasive lobular carcinoma, but its architecture is not. Alveolar and solid invasive lobular carcinomas are variants of invasive lobular carcinoma for which the architecture differs from the single-file, targetoid pattern of classic invasive lobular carcinoma. Pleomorphic invasive lobular carcinoma is a variant that differs in its cytology (severe nuclear atypia, histiocytoid appearance), but has the typical architecture. These variants of invasive lobular carcinoma lack the favorable prognosis of the paucicellular, classic invasive lobular carcinoma.