Clinical and pathological analysis of invasive mucinous micropapillary carcinoma of the breast: a series of case reports

The aim of the study was to analyse the clinicopathological features of invasive mucinous micropapillary carcinoma (MUMPC), a rare breast cancer variant. We retrospectively reviewed 7 MUMPC cases diagnosed in 2017–2024, evaluating clinical, imaging, histopathological, and immunohistochemical data (including epithelial membrane antigen, hormone receptors, human epidermal growth factor receptor 2, and Kiel 67 antigen). The cohort comprised 7 female patients aged 40–77 years (mean, 53.8 years). Tumour size range was 0.9–6.0 cm (mean, 3.4 cm). Histologically, all tumours exhibited classic features: abundant extracellular mucin containing tumour cells arranged in micropapillary, glandular, and cluster patterns, with characteristic retraction-like spaces. Regarding nuclear grade, 5 cases showed low-to-intermediate-grade nuclei, while 2 cases exhibited high-grade nuclei. Immuno- phenotypically, 6 cases demonstrated strong positivity for oestrogen and progesterone receptors (50–95% of cells). The human epidermal growth factor receptor 2 status was negative (score 0) in 4 cases, equivocal (2+) in 2 cases (both confirmed to have amplification by fluorescence in situ hybridisation), and positive (3+) in 1 case. Two patients had lymph node metastasis; 5 patients remained recurrence-free at 60 months of follow-up, 1 patient died at 17 months, and 1 patient was newly diagnosed. Mucinous micropapillary carcinoma displays distinct clinicopathological and immunophenotypic features, suggesting potentially more aggressive behaviour than pure mucinous carcinoma. Accurate diagnosis and individualised management are essential.