A case of mucinous carcinoma of the breast that demonstrated a good pathological response to neoadjuvant chemotherapy despite a poor clinical response.

A 30-year-old woman presented with a right breast tumor. Mucinous carcinoma was diagnosed by core needle biopsy (T2: 5 cm N1 M0). Despite receiving a neoadjuvant anthracycline and taxane regimen, the patient demonstrated no clinical response (NC). Based on the patient's strong preference, we performed breast-conserving surgery. On histological examination, we observed widespread mucus and a few viable malignant cells, a Grade 2 therapeutic response. Neither optimal management procedures nor guidelines for chemotherapy for primary mucinous carcinoma of the breast have been established. It is a reasonable assumption, however, that discordance between the clinical response and therapeutic response to neoadjuvant chemotherapy may occur in cases of mucinous carcinoma.