A Case of Pure Mucinous Breast Carcinoma in a 25-Year-Old Female Who Showed Complete Pathological Response to Neoadjuvant Chemotherapy despite Poor Clinical Response.

INTRODUCTION: Mucinous breast carcinoma is a rare histologic subtype of primary breast cancers accounting for 1-6%. It is a rare histological variant in young patients and usually presents without lymph node involvement, and its pathological response to neoadjuvant chemotherapy is rarely reported. CASE PRESENTATION: Pure mucinous breast carcinoma in a 25-year-old female was treated with neoadjuvant chemotherapy every 3 weeks for 8 cycles. After the fifth cycle, the mass size showed no change. We performed modified radical mastectomy in the left breast and axillary lymph node clearance. However, the pathological report showed a complete elimination of both the breast tumor and axillary lymph nodes, which were filled with mucus but did not contain malignant cells. DISCUSSION: Chemotherapy was profoundly effective against the tumor cells, but ineffective against large amounts of extracellular mucus. Even though the cancer cells were sensitive to chemotherapy, the volume of mucinous cancer couldnot be reduced. CONCLUSION: In summary, the evaluation criteria of tumor response to chemotherapy based on maximum diameter only should be considered insufficient for mucinous carcinoma.