What happened, in summary

A 59-year-old woman was being treated for a known cancer in the right breast. Her initial mammogram and subsequent imaging had not shown suspicious findings in the left breast. After neoadjuvant treatment for the right-sided cancer, bilateral mammography still described the left breast as normal. At her request, she underwent bilateral total mastectomy with immediate reconstruction. Examination of the left-breast surgical specimen unexpectedly found a separate grade 2 invasive ductal carcinoma with lobular features and a small focus of low-grade ductal carcinoma in situ. The left tumor was ER positive, PR positive and HER2 negative, with a low Ki67 level. This biology was the opposite of the known right-sided HER2-positive, hormone-receptor-negative cancer. Because the left cancer was discovered only after the breast had already been removed, the mastectomy was the confirmed treatment for this left-sided primary. Hormonal therapy and left-axillary ultrasound and MRI follow-up were planned after surgery.