A Case of Benign Breast Inclusions in an Axillary Sentinel Lymph Node Mimicking a Low-Grade Carcinoma

Abstract Benign inclusions in lymph nodes are a rare phenomenon that has been described throughout the body. These foci of nonneoplastic, ectopic tissue in the axillary lymph nodes can mimic low-grade metastatic breast carcinomas and result in a diagnosis of false-positive nodes that would lead to unnecessary treatment. We present a challenging case of a patient with a left breast grade 1 invasive ductal carcinoma undergoing an intraoperative axillary sentinel lymph node frozen-section consultation that contained numerous small tubules in the subcapsular space. The lymph node was interpreted as positive for carcinoma, and the patient underwent an axillary lymph node dissection. On permanent sections, myoepithelial cells and surrounding basement membranes were identified around some of the tubules, and a large, squamous inclusion cyst became visible. Immunohistochemical stains were performed that showed that p63 and smooth muscle myosin heavy chain highlighted myoepithelial cells around the tubules, whereas cytokeratin 5/6 and estrogen receptor stains showed a mosaic pattern of positivity. Combined, these findings supported a revised diagnosis of benign glandular and benign squamous inclusions. This case underscores the importance of thoroughly examining the location and histologic and cytologic features of a low-grade–appearing epithelium before determining lymph node positivity and, if it is unclear, deferring the diagnosis to permanent sections where immunohistochemical stains can be performed. Knowledge of this uncommon finding is important to avoid false-positive results, unnecessary treatments, and their associated comorbidities.