Occult Breast Carcinoma: Pathologic Features of an Uncommon Clinical Presentation in a Large Cohort.

Occult breast carcinoma (OBC) refers to the clinical presentation of breast carcinoma occurring in axillary lymph node(s) without a detectable primary breast cancer. Prior studies of OBC have focused on treatment regimens. We sought to study the clinical, morphologic, and immunohistochemical features of OBCs. We retrospectively identified cases of OBC treated at our center between 1996 and 2021. All patients included in the study had biopsy-proven axillary metastatic breast carcinoma and underwent MRI after a noncontributory mammogram/ultrasound. Patients with a prior history of breast carcinoma were excluded. The study included 68 patients with a median age of 56 years (range: 31 to 84 y). The morphology in 55 cases (81%) was poorly-differentiated carcinoma, no special type (ductal). The remaining tumors showed lobular, micropapillary, apocrine, clear cell, and signet ring cell morphology. Thirty-nine (57.4%) OBC were hormone receptor positive, 19 (33.3%) were HER2 positive and 13 (22.8%) tumors were triple negative. Fifty (74%) patients had a breast sampling procedure while 18 (26%) did not. Thirty-four (50%) patients underwent neoadjuvant chemotherapy. Fifty-nine (87%) patients underwent axillary lymph node dissection while 9 (13%) had sentinel lymph node biopsy only. Nineteen (56%) patients achieved a complete pathologic response in the axilla. Fourteen (21%) patients developed a recurrence: 5 in the ipsilateral breast or axilla, 1 in the contralateral axilla and mediastinum, and 8 in distant metastatic sites. The median time to recurrence was 51.9 months. The final pathologic lymph node stage was the only feature found to be significantly associated with the development of recurrence.