Uncommon Presentation of Primary Bladder Signet Ring Cell Carcinoma With Peritoneal Carcinomatosis: A Rare Case Report

Primary signet ring cell carcinoma (SRCC) of the bladder is an exceptionally rare and aggressive malignancy, accounting for only 0.12%–0.6% of all bladder cancers. This case report describes a 54‐year‐old female who presented with urinary incontinence and abdominal pain, initially misdiagnosed as a urinary tract infection. Imaging revealed suspicious bladder findings, and subsequent cystoscopy with transurethral resection identified SRCC, later confirmed by immunohistochemistry (PD‐L1 positive, CDX‐2/ER negative). Despite peritoneal carcinomatosis, the patient responded to cisplatin/gemcitabine chemotherapy and immunotherapy, demonstrating tumor shrinkage on follow‐up imaging. This case highlights the diagnostic challenges of SRCC due to its nonspecific symptoms and potential histological overlap with other metastatic gastrointestinal tumors. Early recognition and a multidisciplinary approach are critical for improving patient outcomes.