A case report and literature review of negative post-resection 18F-FDG PET/CT findings in incidental primary signet-ring cell carcinoma of the urinary bladder treated with laparoscopic radical cystectomy

Primary signet-ring cell carcinoma of the urinary bladder (SRCCU) is an extremely rare and aggressive malignancy. We report a 69-year-old man who was incidentally diagnosed with primary SRCCU following transurethral resection of the prostate (TURP) and transurethral resection of bladder tumor (TURBT) at an outside hospital. After referral to our hospital, gastrointestinal endoscopy and systemic imaging evaluation revealed no extravesical primary lesion. Multiparametric pelvic MRI showed no bladder lesion or pelvic lymphadenopathy, and 18 F-fluorodeoxyglucose positron emission tomography/computed tomography ( 18 F-FDG PET/CT) revealed no abnormal FDG-avid lesions. Given the aggressive biological behavior of SRCCU, laparoscopic radical cystectomy (LRC) combined with pelvic lymph node dissection (PLND) was performed after multidisciplinary team evaluation. Final pathology indicated SRCCU invading the deep muscularis propria and involving the prostatic stroma and right seminal vesicle, with perineural invasion and one metastatic lymph node in the external iliac-obturator region. The pathological stage was pT4aN1 cM0. The patient received six cycles of adjuvant paclitaxel monotherapy, and no evidence of recurrence was observed during approximately 15 months of follow-up after LRC. This case suggests that negative findings on multiparametric pelvic MRI and 18 F-FDG PET/CT after TURP/TURBT may not exclude infiltrative residual disease, adjacent-organ involvement, or regional lymph node metastasis in SRCCU. In carefully selected patients, radical cystectomy combined with PLND may facilitate definitive pathological staging, while its therapeutic role requires further investigation.