Therapeutic response to neoadjuvant chemotherapy in HER2-positive pure mucinous breast cancer: a case report

Pure mucinous breast carcinoma (PMBC) is an uncommon histologic subtype that typically exhibits favorable biology and infrequent HER2 overexpression. HER2-positive PMBC constitutes an exceptionally uncommon and biologically distinct subset, for which evidence regarding the response to neoadjuvant systemic therapy remains scarce.

A 49-year-old woman was diagnosed with cT2N0M0 HER2-positive pure mucinous breast carcinoma. She received six cycles of neoadjuvant docetaxel, carboplatin, trastuzumab, and pertuzumab. Post-treatment imaging demonstrated only modest tumor size reduction. Despite the limited radiographic response, breast-conserving surgery with sentinel lymph node biopsy revealed minimal residual invasive carcinoma measuring less than 1 mm, with the lesion predominantly composed of acellular extracellular mucin and negative lymph nodes, consistent with a near pathologic complete response. We aimed to report the therapeutic response in this rare subtype and to highlight potential discrepancies between radiologic and pathologic findings following neoadjuvant treatment.

This case highlights a clinically significant discordance between radiologic findings and pathologic response in HER2-positive PMBC following neoadjuvant therapy. Persistent radiologic abnormalities may represent residual mucin rather than viable tumor cells, underscoring a potential limitation of imaging-based response evaluation. This report provides insight into response interpretation in this rare entity and supports the need for cautious clinical decision-making.