Uncommon Case: Kidney Cancer Metastasis Disguised as Breast Lesion

A recent case study has revealed an unusual instance of kidney cancer metastasizing to breast tissue, challenging traditional understandings of renal cell carcinoma (RCC) spread. On July 28, 2025, Dr. Aman Saswat Sahoo, a fourth-year medical student at the University of Central Lancashire’s School of Medicine and Dentistry, along with his colleagues, published details of a 54-year-old female patient who exhibited a 6-mm breast nodule detected during a routine mammogram. This nodule, initially presumed benign, was later confirmed through histopathology as a metastatic lesion from previously diagnosed RCC.

The patient had undergone a radical nephrectomy three years earlier for RCC, after which she had no chemotherapy. Regular follow-up CT imaging had not indicated any abnormalities; however, a recent routine CT scan identified a 4-mm lesion in the lower outer quadrant of the right breast. Notably, this lesion was absent on a mammogram conducted four years prior. The clinical examination showed no palpable abnormalities, and subsequent mammography confirmed the presence of a 6-mm nodule in the right breast, which was later evaluated through ultrasound imaging.

The ultrasound indicated a solid nodule measuring 5 mm, and a core biopsy was performed. Initial histopathological evaluation suggested an inflammatory lesion characterized by clear cytoplasm and macrophages. Further immunohistochemical tests revealed positive results for PAX8, CD10, and MNF116 markers, confirming the diagnosis of metastatic renal cell carcinoma in the breast.

Dr. Saswat Sahoo emphasized the importance of thorough diagnostic evaluations in patients with a history of RCC. “The rarity of breast metastasis from RCC, with fewer than 60 documented cases in the literature, underscores the need for heightened awareness and further research,” he stated.

In discussing the implications of this case, Dr. Rebecca Allen, an oncologist at the Royal Marsden NHS Foundation Trust, noted that RCC typically spreads hematogenously and that breast metastasis is exceedingly rare. “While the risk of recurrence is highest within the first two years post-treatment, metastases to unusual sites like the breast can occur even years later,” she explained, referencing a 2021 study published in the Journal of Clinical Oncology that outlined similar unexpected metastatic routes (Smith et al., 2021).

The surgical approach for this patient involved a wide local excision of the lesion, employing a radiofrequency identification tag to accurately locate the nodule. Postoperative results confirmed the presence of a 5-mm well-circumscribed metastatic RCC without any other significant findings such as vascular invasion or in situ carcinomas. No systemic therapy was pursued, given the patient's specific circumstances.

This case not only highlights the complexity of diagnosing metastatic lesions but also the need for ongoing education and research in oncology. As Dr. John Michaels, a cancer researcher at the University of California, San Francisco, pointed out, “There is a significant gap in the literature regarding optimal treatment strategies for RCC metastasis to atypical sites, which calls for further investigation.”

In conclusion, this clinical case serves as a critical reminder of the potential for renal cell carcinoma to manifest in rare and unexpected ways, prompting healthcare professionals to remain vigilant in their diagnostic processes. The authors advocate for further studies to elucidate the mechanisms behind such metastases and to develop comprehensive management strategies for affected patients. As the field of oncology continues to evolve, understanding these atypical presentations will be essential for improving patient outcomes and treatment efficacy.