Renal Cell Carcinoma in Pregnancy With Breast Metastasis: A Case Report and Literature Review.

Renal cell carcinoma (RCC) during pregnancy is exceedingly rare and presents significant diagnostic and therapeutic challenges. We report the case of a 45-year-old gravida 3 woman in whom a right renal mass was incidentally identified during the second trimester. At 34 weeks of gestation, she underwent cesarean delivery followed by right radical nephrectomy. Histopathology revealed a 12 cm clear cell RCC with renal sinus invasion. Postpartum staging demonstrated multiple pulmonary metastases, and systemic therapy was initiated with cabozantinib plus nivolumab. Despite temporary stabilization, disease progression occurred, and subsequent lines of therapy included axitinib, everolimus, and sunitinib. During follow-up, a newly developed breast mass was confirmed as metastatic RCC, representing an exceptionally rare metastatic site. Comprehensive molecular profiling revealed pathogenic variants in NF1 and FBXW7, alterations that are associated with aggressive tumor biology and resistance to systemic therapy. Despite multidisciplinary management, the patient experienced rapid disease progression and died 13 months after diagnosis. This case underscores several important clinical considerations: the difficulty of establishing a timely diagnosis of RCC in pregnancy due to overlapping maternal symptoms, the need to balance maternal and fetal safety when planning surgical intervention, and the limitations of systemic therapy in the peripartum period. Furthermore, the presence of aggressive molecular alterations may contribute to resistance and poor outcomes. The rare occurrence of breast metastasis highlights the potential for unusual dissemination patterns in RCC. Reporting such uncommon cases contributes to improved understanding of disease behavior, helps guide clinical decision-making, and emphasizes the importance of individualized, multidisciplinary management in this challenging setting.