A Case Report of a Rare ER+, PR- Pure Metaplastic Breast Squamous Cell Carcinoma with HER2 Overexpression

BACKGROUND Breast squamous cell carcinoma (SCC) is a subtype of metaplastic breast carcinoma (MBC), which is a rare malignancy and accounts for 0.1% of all invasive breast carcinomas. Guidelines on definitive management and treatment of breast SCC are not well established, given its rarity and diverse immunohistochemistry (IHC) profile, and lack of clinical data. Most cases of breast SCC are triple-negative breast cancer - negative for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). This case report outlines the clinicopathological profile of a pure breast SCC case with a rare IHC profile; HER2 and ER positive. CASE REPORT A 41-year-old woman presented with a right breast mass that had been growing for 2 months. Biopsy confirmed breast SCC, a rare malignancy with IHC profile as follows: HER2 overexpression, ER positive, and PR negative. She underwent neoadjuvant chemotherapy for 3 months followed by right mastectomy with axillary clearance, adjuvant radiotherapy, and oral tamoxifen therapy. Unfortunately, she did not receive anti-HER2 therapy. She developed early locoregional recurrence at 2 months postoperatively, which was treated with excision of the right chest wall and transverse rectus abdominis musculocutaneous (TRAM) flap. She developed liver and lung metastasis and succumbed to her disease at 15 months post-diagnosis. CONCLUSIONS Breast SCC is a rare and aggressive tumor with heterogeneous clinicopathological features. Available guidelines do not outline the definitive treatment for breast SCC, given its rarity and heterogenous IHC profile, leading to a general lack of clinical data. Hence, due to the challenges in managing this rare condition, treatment modalities need to be individualized.