Microinvasive (T1mic) Lobular Carcinoma of the Breast: Clinicopathologic Profile of 16 Cases

Introduction: The entity of microinvasive lobular carcinoma (MILC, < 0.1 cm) of the breast remains hitherto poorly characterized. Methods: Clinical and pathologic material of cases in which MILC was either suspected or diagnosed over an 18-year period (1991 to 2009) was studied. Only cases in which MILC could be indisputably established, by immunohistochemical stains in equivocal cases, were included. Results: Sixteen (0.02%) cases were confirmed to be MILC among 75,250 breast cases during the study period. Mean age of patients was 52 years (range, 41 to 65 y). Patients presented with either radiographic (13 cases) or palpable (3 cases) abnormality. MILC was unilateral in all cases (right: 8, left: 8). Mean number of microinvasive foci was 1.5 (range, 1 to 5). MILCs were associated with lobular carcinoma in situ (LCIS) predominantly of the classical type in 11 cases, of the florid type in 4 cases, and of the pleomorphic type (with high-grade nuclei) in 1 case. Slight enhancement of stromal cellularity was the only histologic hint of MILC at low-power microscopy. Six MILC cases, in which results could be obtained, were positive for both estrogen receptor and progesterone receptor, and all 6 were negative for HER2/neu. The final surgical procedure was excisional biopsy (8 cases), ipsilateral mastectomy (6 cases), and bilateral mastectomy (2 cases). Synchronous invasive ductal carcinoma (2 cases, 1 of which was contralateral), intraductal carcinoma (2 cases), and LCIS (16 cases) were identified therein. Axillary lymph node biopsies (including 9 sentinel lymph node samplings) were performed in 13 cases, with negative results in each. All patients were alive without evidence of recurrence or of metastases in a mean follow-up of 24 months (range, 1 to 72 mo). Conclusions: On the basis of this study, MILC is a rare disease that is typically associated with classical, florid, or pleomorphic forms of LCIS. MILC cases in this series showed low morbidity and no metastatic potential, at least in the short term.