Extrapelvic Urinary Extravasation Caused by Retroperitoneal Metastasis of Pulmonary Squamous Cell Carcinoma: A Case Report

A 52-year-old man with pulmonary squamous cell carcinoma, clinically staged as cTxN3M1c (stage IVB), received four cycles of first-line chemotherapy with cisplatin plus TS-1 followed by seven cycles of second-line nivolumab. Subsequent imaging revealed progressive enlargement of cervical lymph nodes, left adrenal metastasis, and retroperitoneal metastases. Over several days, he developed left flank and back pain and presented to the emergency department. An emergent contrast-enhanced abdominal computed tomography (CT) demonstrated contrast extravasation from the upper left ureter, left ureteral obstruction caused by retroperitoneal metastasis, and mild hydronephrosis. He was diagnosed with extrapelvic urinary extravasation and underwent placement of a polymeric double-J ureteral stent. His pain resolved promptly thereafter, allowing initiation of third-line chemotherapy. Extrapelvic urinary extravasation due to retroperitoneal dissemination of pulmonary squamous cell carcinoma is extremely rare, and no prior cases were identified in our literature search. Nevertheless, recognizing this condition as a potential differential diagnosis for flank or back pain during disease relapse may facilitate early intervention, support continuation of systemic therapy, and ultimately improve quality of life.