Although GEPNETs are slow-growing tumors, most patients are diagnosed with metastatic disease, and therefore it is important that the management of each patient be discussed in a multidisciplinary setting to optimize the treatment strategy. Patients should be considered for clinical trials and refractory cases referred to a specialty center.

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e16019 Background: Small cell carcinoma of bladder (SCCB) is a rare but aggressive variant of bladder neoplasm. There is limited insight for risk prognostication and treatment guidance in this entity. Immune checkpoint inhibitors (ICI), anti-PD-1 or anti-PD-L1 antibodies, have been approved for treating urothelial carcinoma, while the evidence of their efficacy in SCCB is lacking. PD-L1 expression in tumor tissue of urothelial cancer has been postulated to correlate with response to ICI but with controversy. We have studied the expression of PD-L1 in SCCB and its association with patient survival. Methods: Nineteen cases of SCCB diagnosed between 2011 and 2017 in a single center were identified. Formalin-fixed paraffin-embedded tumor samples were stained for PD-L1 (Ventana PD-L1 SP142). Cases showing positive stain in 5% or more of tumor cells and tumor stromal mononuclear cells (TSMC) were considered positive. Results: Among 19 cases of SCCB, 4 (21%) stained positive for PD-L1. All 4 cases had strong PD-L1 staining ( > 30%) seen in the TSMC but barely in tumor cells (focal < 5% cells in 2/4 cases). Except for one patient who died from surgery, all remaining 3 patients with positive PD-L1 staining are still alive. Twelve out of 19 SCCB patients developed metastatic disease; 4 of them were treated with ICI. The only responder of the 4 patients had strong PD-L1 expression in TSMC cells. The overall survival for patients with positive PD-L1 staining was 41 months versus 14 months for those with negative staining (p = 0.09). Age, pathologic stage and treatments were similar between the two groups. Conclusions: In our study, PD-L1 expression was seen in 21% of tumor samples from patients with SCCB, mostly in TSMC, but minimal in tumor cells. The strong expression of PD-L1 in TSMC correlates with a trend of improved survival and potential response to ICI in SCCB. PD-L1 expression in TSMC, rather than tumor cells, could be used as a marker for prognosis in SCCB.