Breast cancer spread into a separate kidney cancer

What happened, in summary

A 64-year-old Japanese woman had been living with breast cancer for many years. Her left breast invasive ductal carcinoma was first diagnosed in 2007. The original tumor was estrogen- and progesterone-receptor positive, with an equivocal HER2 result. In 2011, the cancer spread to bone and she received several lines of hormone-based treatment.

By 2025, she was receiving fulvestrant with capivasertib, and her metastatic breast cancer was thought to be confined to bone. Routine imaging then found a 23 mm mass in the right kidney. The mass looked like a new kidney cancer rather than breast-cancer progression, so it was removed with a partial nephrectomy.

Pathology showed that most of the kidney mass was a separate clear-cell kidney cancer. Unexpectedly, however, part of the mass contained metastatic breast carcinoma growing within the kidney cancer. The breast-cancer component still showed weak estrogen-receptor expression.

This finding meant that her breast cancer had developed a new visceral metastasis and was considered to have progressed. A change in systemic treatment was planned after the pathology result.