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.