A rare case of metaplastic carcinoma of the breast that could be diagnosed with preoperative core needle biopsy and the improbability of skin invasion

Abstract Background Metaplastic carcinoma of the breast (MCB) is a rare type of breast cancer that accounts for 1% of all cases of invasive breast cancer. This MCB is categorized into special types of breast cancers. MCB is characterized by the presence of multiple components and transitional cells between two tissue types, because the tumor cells differentiate in different directions and degrees into a variety of cells other than glandular cells. However, MCB is often misdiagnosed preoperatively as ordinary breast cancer based on several tissue imaging findings. The diagnosis of MCB is highly difficult with cytology and even with core needle biopsy (CNB). The diagnostic rate of MCB with CNB is as low as 40%, with the remaining 60% diagnosed as ordinary mammary duct carcinoma. MCB is unlikely to respond to anticancer drugs, surgery as the initial treatment is prioritized over preoperative anticancer drug therapy. Therefore, accurate preoperative diagnosis is important. To date, no report has described the likelihood of skin invasion in a patient with MCB. Case presentation This case report describes a 67-year-old woman with an unremarkable past or family history, who visited the author's clinic after noticing a swelling of her right nipple and a lump located around her right nipple 2 months previously. CNB was performed. Results The pathological findings of the CNB specimen indicated metaplastic carcinoma. Metaplastic carcinoma with sarcomatoid changes, i.e., spindle cell carcinoma or matrix-producing carcinoma, was suspected. There was no skin invasion despite the presence of both a large mass and large swelling of the nipple in this patient. Conclusions For MCB, preoperative anticancer drug therapy could be avoided and surgery may be considered the initial treatment. Despite the presence of a large mass, MCB may be less likely to cause skin invasion than ordinary breast cancer of the same size.