Occult Metastatic Breast Cancer First Appears as a Sternal Mass

What happened, in summary

A 62-year-old woman noticed a small, hard mass over her lower sternum. Initial chest imaging showed a localized soft-tissue lesion, while mammography and breast examination were normal. Ten months later, excision of the sternal mass showed metastatic adenocarcinoma. Additional staining strongly supported a breast origin, with estrogen-receptor and progesterone-receptor positivity and HER2 negativity.

Repeated biopsies of a suspicious right-breast area showed only benign sclerotic tissue, so no conventional breast primary was identified. Multidisciplinary review concluded that the sternal lesion represented metastatic breast adenocarcinoma and that the breast cancer should therefore be treated as Stage IV. Because there was no active breast lesion requiring local treatment, mastectomy and breast radiotherapy were not pursued. She started tamoxifen.

The same evaluation also found a separate rectal cancer, which was treated as another primary. More than 3 years after the sternal mass was first detected, imaging showed no active breast or anterior chest-wall disease, and the cancers were described as appearing in remission while she continued endocrine therapy and surveillance.