Case Report: Metastatic succinate dehydrogenase-deficient gastrointestinal stromal tumor treated with chemotherapy and immune checkpoint inhibitor.

We report a young adult with a recurrent metastatic succinate dehydrogenase (SDH)-deficient gastrointestinal stromal tumor (GIST) achieving a pathological complete response with a chemoimmunotherapy regimen. The patient underwent a gastroscopy, which revealed a large-ulcerated tumor on the greater gastric curvature. Histological analysis showed a poorly differentiated SDHB-deficient GIST with PDL1 expression at a 100% rate and a RET gene fusion. Rapid metastatic relapse after subtotal gastrectomy was resistant to imatinib. The patient received four cycles of cisplatin, etoposide, and pembrolizumab followed by pembrolizumab maintenance with a complete response. Late relapse 3 years later was treated with the same regimen and achieved complete pathological response. Two years later, an isolated focus was removed, showing the recurrent SDH-deficient GIST. Pembrolizumab is still in maintenance with no relapse to date. The role of chemoimmunotherapy as part of treatment in recurrent metastatic SDH-deficient, PDL1-positive GIST patients is worth further investigation.