What happened, in summary

A 65-year-old woman sought care for a painful right breast lump with discharge that had been present for about a year. Imaging showed a very large breast mass, and chest imaging also raised concern for possible spread to the lungs. A biopsy initially suggested invasive ductal carcinoma with mucin.

She underwent a modified radical mastectomy. Examination of the surgical specimen showed an 18 cm tumor involving the breast and overlying skin. One of 15 axillary lymph nodes contained cancer. Further pathology and imaging supported a rare diagnosis of primary mucinous cystadenocarcinoma of the breast rather than a cancer that had spread to the breast from another organ.

The tumor was estrogen- and progesterone-receptor negative but HER2-positive. This HER2 result was confirmed with additional testing.

After surgery, she received systemic chemotherapy and was still on treatment 6 months later. Her treatment course remained ongoing at the latest documented point.