Seminal Vesicle Mass Fistulising to the Rectum: A Rare Urological Presentation of Lung Cancer Metastasis

Metastatic involvement of the male genitourinary tract by lung cancer is exceedingly rare. We report a 56-year-old man with metastatic lung adenocarcinoma (initial stage T3N2M1b) under pembrolizumab, who presented with severe pelvic pain. Pelvic magnetic resonance imaging and computed tomography demonstrated a large mass with an imaging epicentre favouring the left seminal vesicle, involving the prostate and fistulising to the distal rectum, without pelvic ascites or peritoneal disease. A total PSA of 0.81 ng/mL and a previous negative prostate biopsy made a primary prostatic malignancy less likely. Biopsy of the rectal component revealed a poorly differentiated carcinoma with an immunophenotype (CK7+, TTF-1+, p40-, CDX2-, NKX3.1-, PAX8-) consistent with metastatic adenocarcinoma of pulmonary origin. The patient underwent palliative pelvic radiotherapy, with improvement of pelvic pain; he subsequently developed pneumaturia and faecaluria and died eight months later from disease progression. Seminal vesicle metastasis from lung carcinoma has been reported previously; to our knowledge, however, this is the first report presenting with rectal fistulisation. This case highlights a diagnostically challenging presentation and the need to consider metastatic disease when evaluating atypical seminal vesicle masses in oncological patients.