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

Background: 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. Case Presentation: 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. Conclusion: 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.