# Solid–Basaloid Adenoid Cystic Carcinoma of the Ipsilateral Breast Remnant in Postoperative Luminal HER2-Type Breast Cancer: A Case Report

## TL;DR

A rare case of solid–basaloid adenoid cystic carcinoma in the breast of a patient previously treated for breast cancer is reported.

## Contribution

This paper presents a rare case of SB-AdCC in a postoperative breast remnant, highlighting its aggressive nature and triple-negative classification.

## Key findings

- The patient had a high Ki-67 index of 80%, indicating high-grade malignancy.
- The tumor was diagnosed as triple-negative and SB-AdCC following histological examination.
- The patient showed no signs of metastasis one year after surgery and chemotherapy.

## Abstract

Solid–basaloid adenoid cystic carcinoma (SB-AdCC) is a rare and aggressive variant of AdCC of the breast. Moreover, it is an extremely rare subtype of triple-negative breast cancer, accounting for <1% of all breast cancers. We report a case of SB-AdCC.

Ten years ago, an 82-year-old woman underwent a partial mastectomy for the luminal HER2 (human epidermal growth factor receptor type 2)-type invasive ductal carcinoma of the left breast cancer. Ten years later, she noticed another lump in her left breast. A needle biopsy was performed, and an unclassified carcinoma was detected. Histological examination revealed that the carcinoma was of the triple-negative type. A mastectomy of the left breast and sentinel lymph node biopsy were performed. The postoperative pathological diagnosis was an SB-AdCC of the breast. The Ki-67 index was 80%, corresponding to a high-grade malignancy. She received postoperative chemotherapy with tegafur–gimeracil–oteracil potassium. At 1 year postoperatively, she is recovering well without signs of metastasis.

We reported a case of SB-AdCC.