Abstract Renal Cell Carcinoma (RCC) has a metastasis rate of approximately 33%, but metastasis to the breast is extremely rare1. The common sites of metastasis include lung, bone, lymph nodes, and liver2. Here we present a case of RCC that metastasized to the breast three months after radical nephrectomy. In September 2019, a 65 year old female was diagnosed with cT2bN0M0 RCC. She was treated with Cabozantinib due to her past history of heart failure. Five months after the initial diagnosis she underwent left radical nephrectomy and left adrenal gland resection due to metastasis. This surgery resulted in negative margins and no lymph node involvement. Three months later a 1.6cm breast mass was discovered and a biopsy confirmed RCC. After it was confirmed that she had no other sites of metastasis and after multidisciplinary discussion, a lumpectomy was performed. All margins were negative apart from a focally positive margin, however fascia was removed without any evidence of muscular invasion. Although it was discussed at a multidisciplinary conference, adjuvant radiation was not started due to a lack of data to support its use in RCC. Four months later a surveillance CT revealed recurrence with right adrenal gland metastases and multiple subcentimeter nodules in the left breast. She was then started on Axitinib with pembrolizumab. After nine cycles of this immunotherapy regimen a CT found equivocal standardized uptake value (SUV) on the right adrenal metastasis and a 6th rib lytic lesion, but the breast mass did not have an elevated SUV. She underwent repeat breast biopsy which showed RCC, and she continued on her immunotherapy regimen for 2 more cycles due to the presence of other metastatic disease. She underwent repeat breast imaging which showed known masses, 2 of which had increased in size (one from 11 mm to 23 mm and one from 7 mm to 10 mm) and osseous destruction of the sixth rib which was causing increased rib pain. She underwent palliative left mastectomy and radiation therapy to the affected rib and chest wall. The final pathology showed 3 foci of metastatic clear cell carcinoma (1.8cm, 2cm, and 0.3cm) with negative margins in the breast and the patient received adjuvant radiation therapy of the left chest wall and was restarted on pembrolizumab. A later CT showed soft tissue invasion on the left chest wall. The chest wall lesion demonstrated metastatic RCC, but the pembrolizumab was stopped due to lack of response and arthritis. Three months later she presented to the ED after a fall and was found to have metastases in the left anterior frontal lobe. After brain lesion resection she was started on Everolimus. Five months later there were no new lesions seen on imaging, and the brain lesion had decreased in size. This case demonstrates a rare breast metastasis of RCC to add to the small body of case reports that describe this phenomenon. Documenting these cases is important in order to direct treatment options for future cases of RCC metastasis to the breast. Although resection of oligometastatic disease has been described, data is insufficient to indicate that it has an effect on survival. 1. Flanigan, R. C., Campbell, S. C., Clark, J. I. 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-06-10.
Moulton et al. (Tue,) studied this question.