Background Small cell carcinoma (SCC) is a neuroendocrine tumor that usually originates in the lungs but can also arise from extrapulmonary sites. Extrapulmonary small cell carcinomas (EPSCCs) are aggressive and rare, with limited data guiding their management. This case report contributes to the literature by presenting the diagnosis and treatment of primary peritoneal SCC. Case presentation This case describes a 69-year-old man who presented with abdominal distension and pain, raising concerns for metastasis. He had a history of prostate cancer treated with radical prostatectomy and pelvic lymph node dissection. Biopsy and imaging revealed metastatic SCC involving the peritoneum and omentum. A diagnosis of primary peritoneal SCC or SCC of unknown primary origin was made due to the absence of a detectable primary tumor in typical sites. The patient underwent three separate courses of carboplatin/etoposide chemotherapy (6 cycles each), with each course resulting in significant disease regression and symptom relief. Recurrence was managed effectively with repeated chemotherapy cycles, but long-term follow-up showed the need for continued treatment to maintain disease control and quality of life. Conclusion This case underscores the importance of a multidisciplinary approach and patient-centered care in managing rare cancers like primary peritoneal SCC. Further research is essential to clarify its molecular characteristics and improve therapeutic options. Graphical abstract

Introduction: Multiparametric MRI (mpMRI) interpreted using Prostate Imaging Reporting and Data System (PI-RADS) v2.1 is central to prostate cancer (PCa) assessment, but inter-reader variability remains a concern. This study evaluated inter-reader agreement, diagnostic performance, the utility of prostate-specific antigen density (PSA-D), and imaging predictors of biopsy-detected PCa. Methods: We conducted a retrospective study of 102 patients who underwent prostate mpMRI followed by transrectal ultrasound (TRUS)-guided biopsy. Three experienced radiologists independently scored examinations using PI-RADS v2.1. Agreement was assessed using intraclass correlation coefficients (ICC) and weighted κ statistics. Diagnostic performance was evaluated at a PI-RADS ≥3 threshold. Patients were stratified by PSA-D, and logistic regression identified imaging predictors of biopsy outcomes. Reader confidence was scored on a 10-point scale. Results: PCa was detected in 60/102 patients (58.8%) and csPCa in 46/102 (45.1%). Overall agreement was moderate to good (ICC, 0.749; κ, 0.748), with higher agreement in the transitional compared to the peripheral zone (ICC, 0.822 vs. 0.662). Averaged PI-RADS scores yielded 100% sensitivity for both PCa and csPCa, with specificities of 47.6% and 35.7%, respectively. PSA-D stratification showed higher PPVs at PSA-D ≥0.15 ng/mL², while sensitivity and NPV were 100% in both strata; specificity remained relatively low. Reader confidence correlated with PI-RADS category and interpretation accuracy. In multivariable patient-level analyses, DWI hyperintensity was independently associated with biopsy-detected PCa and csPCa. Conclusions: PI-RADS v2.1 showed moderate to good inter-reader agreement and high sensitivity but modest specificity for PCa and csPCa. Agreement was higher for transitional-zone lesions, and DWI hyperintensity was independently associated with PCa and csPCa. PSA-D subgroup estimates were imprecise, and its incremental value requires confirmation in larger prospective cohorts.