A progesterone-receptor-positive huge retroperitoneal tumour mimics metastasis in a breast cancer patient: sarcomatoid renal cell carcinoma.

We report a rare case of breast cancer concomitant with progesterone-receptor-positive renal cell carcinoma. A 48-year-old woman was diagnosed as having infiltrating ductal carcinoma of the breast and underwent modified radical mastectomy. A synchronous retroperitoneal tumour was detected by sonography of the abdomen in a routine cancer staging. Initially, the tumour was diagnosed as a synchronous retroperitoneal metastasis by needle biopsy; further tests revealed that it was progesterone receptor-positive. The retroperitoneal tumour showed poor response to full courses of adjuvant chemotherapy for breast cancer. Subsequently, the patient underwent a radical operation that included nephrectomy. The final pathology confirmed a sarcomatoid renal cell carcinoma. The post-operative course was uneventful. The patient had no recurrence at the 1-year follow-up. In this report, accurate diagnosis and adequate treatment were discussed. An intra-abdominal tumour with progesterone receptor- (PR) positive features is usually considered to be metastatic in breast cancer patients. For breast cancer patients with a PR-positive retroperitoneal tumour, renal cell carcinoma should be differentiated from a metastatic lesion of breast cancer, even if PR-expression is rare in renal cell carcinoma. To the best of our knowledge, this is the first case of PR-positive expression in breast cancer concomitant with renal carcinoma. In clinical settings, it is challenging for the surgeon to make an accurate diagnosis and to provide prompt treatment in such cases.