Incapacitating migratory polyarthritis and renal tumor in a young patient

Introduction: Paraneoplastic syndromes (PNS) are clinical manifestations caused by systemic effects of malignancy rather than direct tumor invasion or metastasis. Renal cell carcinoma (RCC) is frequently associated with PNS, which may precede tumor diagnosis and represent an important diagnostic clue. Rheumatologic manifestations such as migratory polyarthritis are uncommon but well-described paraneoplastic phenomena. Case Report: We report the case of a 43-year-old previously healthy male who presented with a one-month history of rapidly progressive, incapacitating migratory polyarthritis predominantly involving the lower-limb large joints, accompanied by intermittent febrile episodes. Extensive infectious, autoimmune, and rheumatologic evaluations were negative, including antinuclear antibodies, rheumatoid factor, antiphospholipid antibodies, and blood and urine cultures. During the etiological work-up, contrast-enhanced abdominal computed tomography revealed a solid heterogeneous mass measuring approximately 4.7 cm in the upper and middle pole of the right kidney. The patient underwent laparoscopic transperitoneal partial nephrectomy with selective arterial clamping. Pathological examination demonstrated clear cell renal cell carcinoma, eosinophilic variant, ISUP grade 2, staged as pT1b, with negative surgical margins. Conclusion: This case highlights migratory polyarthritis as a rare but significant paraneoplastic manifestation of RCC and underscores the importance of considering occult malignancy in patients with atypical, severe inflammatory arthritis of unclear etiology.