What happened, in summary

A 43-year-old woman with a family history of breast cancer had a slowly growing lump in her left armpit for 2 years. The 1 × 1 cm swelling became uncomfortable. Examination found no breast mass or enlarged lymph nodes. Ultrasound described the lesion as a 13 × 6 mm infected sebaceous cyst, and mammography showed dense breast tissue with mild skin thickening in the axilla.

The lump was removed under local anesthesia. Pathology unexpectedly showed grade 2 invasive ductal carcinoma growing within ectopic breast tissue in the axilla. The tumor measured 9 mm, was surrounded by normal breast tissue, and showed no lymphovascular invasion. It was estrogen-receptor positive, progesterone-receptor positive at 34%–66%, and HER2 negative.

Breast MRI did not identify a primary tumor in the usual breast tissue. She underwent deeper re-excision because one margin was close, together with sentinel-node biopsy. The additional tissue and all 5 lymph nodes were cancer-free. She then received radiotherapy to the left breast, axilla, and supraclavicular area. Tamoxifen was started but stopped after a few months at her request because of toxicity.