Abstract
Adenoid cystic carcinoma (ACC) of the breast is an exceedingly rare subtype of triple-negative breast cancer, accounting for less than 0.1% of all breast malignancies. In contrast, invasive lobular carcinoma (ILC) is a more common special histologic type, comprising 5-15% of breast cancers and typically characterized by estrogen receptor positivity and complete loss of epithelial cadherin (E-cadherin) expression. The coexistence of these two biologically and morphologically distinct neoplasms within the same breast lesion is exceptionally rare and, to our knowledge, has not been previously documented. A 62-year-old Caucasian female patient presented with nonspecific stinging discomfort in the right breast. Diagnostic imaging revealed a 1.8 cm irregular, spiculated mass. A core needle biopsy demonstrated invasive mammary carcinoma that was estrogen receptor-positive, progesterone receptor-positive, and human epidermal growth factor receptor 2 (HER2)-negative. Following right breast mastectomy and sentinel lymph node biopsy, final pathology revealed a biphasic neoplasm composed of both ACC and lobular carcinoma (LC) components. The ACC component exhibited classic cribriform and tubular architecture, a triple-negative immunophenotype, strong transformation-related protein 63 (p63) and cluster of differentiation 117 (CD117; c-Kit) positivity, and a low Ki-67 proliferation index (9.8%). The LC component lacked a classic single-file growth pattern but showed dyscohesive cell clusters with complete loss of E-cadherin expression, strong estrogen receptor positivity (94.18%), and a markedly elevated Ki-67 index (42.9%). All seven sentinel lymph nodes were negative for metastatic carcinoma. This rare case of synchronous ACC and ILC underscores the diagnostic complexity of composite breast tumors. Accurate classification requires careful correlation of histomorphology with immunohistochemical findings, particularly when one or both components exhibit non-classical architectural patterns. Given the contrasting biological behavior of triple-negative ACC and hormone receptor-positive ILC, an individualized, multidisciplinary management strategy is essential. Increased awareness of such mixed neoplasms may enhance diagnostic precision and improve patient care.