Microinvasive lobular carcinoma arising in a fibroadenoma

A 51-year-old woman had a 35 mm circumscribed calcified lesion identified on screening mammography, designated R4. Excision showed a fibroadenoma with multiple foci of lobular neoplasia (atypical lobular hyperplasia and classical lobular carcinoma in situ [LCIS]). A focus of microinvasive lobular carcinoma (MILC) was also present, confirmed on immunohistochemistry. The MILC cells were ER positive, Her-2-negative, and e-cadherin negative. Microinvasive carcinoma, defined as "invasive carcinoma with no focus measuring >1 mm" (TNM UICC 7th edition) is usually encountered in ductal carcinoma in situ but may occur with classical, florid, or pleomorphic LCIS. In one series MILC constituted 0.4% of all invasive lobular carcinomas and was present in 0.4% of all LCIS. MILC is a histologically subtle lesion, the identification of which lends further weight to the concept of lobular neoplasia as a precursor lesion. MILC has been observed in hamartoma but, to our knowledge, has not previously been reported in fibroadenoma.