Hormone receptor-positive adenoid cystic carcinoma of the breast with cross-modality imaging discrepancy: a case report and literature review

Primary adenoid cystic carcinoma of the breast (ACCB) is a rare salivary gland–type malignancy characterized by biphasic luminal epithelial and myoepithelial/basal differentiation. Although most ACCBs exhibit a triple-negative phenotype, hormone receptor–positive tumors occur, and evidence guiding endocrine therapy is limited. A 42-year-old woman presented with a gradually enlarging right breast mass. Ultrasonography and MRI were suspicious for malignancy (BI-RADS 4B and BI-RADS MRI category 5), whereas diagnostic mammography showed focal increased glandular density (BI-RADS 0) and benign punctate calcifications (BI-RADS 2). Core needle biopsy showed cribriform architecture, and immunohistochemistry demonstrated expression of CK7, CD117, and p63, supporting a diagnosis of ACCB. The biopsy showed estrogen receptor expression in 10%–20% of tumor cells, progesterone receptor expression in approximately 5%, and a HER2 score of 0. The patient underwent right mastectomy and sentinel lymph node biopsy. The resection specimen confirmed a 3.0 cm ACCB with predominantly cribriform architecture and additional solid growth, with ER and PR each expressed in approximately 30% of tumor cells, a HER2 IHC score of 0, and a negative sentinel lymph node [pT2 pN0(sn)]. She completed 5 years of adjuvant tamoxifen therapy. At the last follow-up, approximately 73 months after surgery, no local, regional, or distant recurrence was detected. This case highlights the value of integrating histomorphology and immunohistochemical findings when imaging findings differ across modalities and illustrates the immunophenotypic heterogeneity of ACCB. The recurrence-free outcome provides additional follow-up information for hormone receptor–positive ACCB but cannot establish benefit from endocrine therapy; individualized treatment and long-term surveillance remain warranted.