Clinicopathological features of microinvasive lobular carcinoma

Microinvasive lobular carcinoma (MILC) is a rare form of minimally invasive carcinoma with incomplete pathological and clinical characterisation. We sought to identify the clinical and pathological features of MILC diagnosed at our institution between 2009 and 2023.

Patient demographic, pathological and clinical data were collected. Patients with a prior or synchronous history of invasive carcinoma were excluded. The study cohort included 30 female patients with a median age of 53 years. The most common imaging target was microcalcifications (63%). MILC was diagnosed on biopsy alone in three cases (10%), excision alone in 22 cases (73%) and on both biopsy and excision in five cases (17%). Most MILCs were unifocal (57%). MILCs were associated with classic lobular carcinoma in-situ (LCIS) in 90% of cases, florid LCIS in 27% and pleomorphic LCIS in 17%. None of 21 patients (70%) who underwent sentinel lymph node biopsy had lymph node involvement. Biomarker status was available in 16 (53%) MILC; 12 were oestrogen receptor+/progesterone receptor+/human epidermal growth factor receptor 2- (ER+/PR+/HER2-), two were ER+/PR-/HER2-, one was ER-/PR-/HER2+ and one was ER-/PR-/HER2 2+ but insufficient for fluorescence in-situ hybridisation (FISH) analysis. Median follow-up was 49 months. A patient with MILC at the excision margin of the index excision specimen did not undergo re-excision; 15 months later she developed an ipsilateral invasive carcinoma with ductal and lobular features and died of disease 73 months after initial diagnosis. All other 29 patients are alive with no evidence of disease.

MILC is a relatively indolent tumour arising in classic and variant LCIS, with a low risk of axillary or distant metastases.