Solid basaloid adenoid cystic carcinoma of the breast: A high-grade triple negative breast carcinoma which rarely responds to neoadjuvant chemotherapy.

Solid basaloid adenoid cystic carcinoma of the breast (SB-AdCC) is an exceedingly rare but important entity that warrants clear separation from classic AdCC (C-AdCC) for optimal treatment. This case series retrospectively reviewed the diagnosis and treatment of 20 breast AdCCs. While four breast pathologists reached consensus on the diagnosis of all C-AdCCs, there was considerable disagreement when diagnosing pure SB-AdCCs. The morphology and immunohistochemical profiles of SB-AdCC closely resemble those of basaloid triple-negative breast carcinoma (TNBC). Molecular profiling of SB-AdCC revealed frequent mutations in the Notch pathway and alterations in chromatin modifiers, such as CREBBP and KMT2D. MYB-NFIB fusion was rare and detected in only 2 of 9 (22.2 %) SB-AdCCs. Axillary lymph node metastasis was present in 2 of 10 patients with SB-AdCC at the time of surgery. During a median follow-up of 27 months, one patient with SB-AdCC developed axillary recurrence. Moreover, 6 of 12 (50.0 %) patients with SB-AdCC developed distant metastases. Of the three patients who underwent neoadjuvant chemotherapy for SB-AdCC, one achieved near-complete pathological response, while the remaining two had minimal response. In conclusion, this series underscores the aggressive clinical course of SB-AdCC, similar to conventional TNBC. Pathologically, SB-AdCC also closely mimics conventional TNBC. It is imperative for pathologists to clearly indicate C-AdCC or SB-AdCC or give a percentage of each component to ensure appropriate treatment strategies and avoid both over-treatment and under-treatment.