The “perfect storm” in bladder oncology: a case of diverticular carcinoma with plasmacytoid features and its clinical ramifications

BACKGROUND: Carcinoma arising within a bladder diverticulum is a rare and diagnostically challenging entity. Its management is further complicated by aggressive histological variants such as plasmacytoid urothelial carcinoma (PUC). The anatomical lack of muscularis propria in diverticula predisposes to early extra‑diverticular spread and impedes accurate clinical staging. The coexistence of a diverticular location and plasmacytoid histology creates a high‑risk clinical scenario with limited evidence to guide therapy. CASE PRESENTATION: A 74‑year‑old male with a history of high‑grade urothelial carcinoma, previously treated with transurethral resection and two cycles of neoadjuvant chemotherapy (gemcitabine plus cisplatin), was found on imaging to have a persistent tumor within a right‑sided bladder diverticulum. He underwent robot‑assisted laparoscopic radical cystectomy. Histopathological examination confirmed high‑grade PUC with 40% squamous differentiation. The tumor invaded peridiverticular adipose tissue (pT3a) and exhibited extensive lymphovascular invasion (> 10 vessels). Immunohistochemistry was positive for GATA3 (weak), p63, CD138 (partial), and CK7 (weak positivity), with PD‑L1 expression (Combined Positive Score = 5), Focal weak membranous E-cadherin staining in approximately 30% of tumor cells, HER2 equivocal (2+). Surgical margins and lymph nodes were negative. The patient was disease‑free at the 3‑month postoperative follow‑up. However, 5 months after surgery, pelvic MRI revealed metastatic lesions, consistent with stage IV disease (rT0N+M1). He subsequently received palliative radiotherapy to the pelvis (200 cGy×25f) and combined targeted therapy (disitamab vedotin 120 mg) plus immunotherapy (toripalimab 240 mg). At the last follow‑up in May 2026 (approximately 7 months postoperatively), he was clinically stable without severe symptoms and continued outpatient radiotherapy. CONCLUSION: This case highlights the diagnostic and therapeutic complexities of managing high‑grade PUC within a bladder diverticulum. It underscores the essential role of high‑quality cross‑sectional imaging for assessing extra‑diverticular extension, the need for meticulous pathological evaluation to identify this aggressive variant, and the centrality of radical surgery. The presence of lymphovascular invasion and PD‑L1 expression supports the consideration of multimodal strategies and close surveillance. Given the rarity of this dual‑pathology entity, collaborative reporting is crucial to optimize management and improve outcomes.