A Case of Pathological Complete Response after “Conversion Surgery” for Initially Unresectable Locally Advanced Intrahepatic Cholangiocarcinoma

INTRODUCTION: Intrahepatic cholangiocarcinoma (iCCA) is the second most common liver cancer and has a poor prognosis. Given the recent advancements in drug therapy, the topic of so-called "conversion surgery" in biliary tract cancer, including iCCA, is evolving; however, only a few cases have been reported. CASE PRESENTATION: A 50-year-old female was referred to our hospital for a liver tumor identified on abdominal ultrasonography. She was diagnosed with iCCA based on tumor biopsy. Due to extensive vascular and bile duct invasion, iCCA was initially considered unresectable. After 8 cycles of gemcitabine, cisplatin plus S-1 (GCS) therapy, CT revealed a partial response. Considering that microsatellite instability-high (MSI-H) was detected, we switched the regimen from GCS to pembrolizumab. However, after 1 cycle of pembrolizumab therapy, immune checkpoint inhibitor (ICI)-induced hepatitis was suspected; therefore, pembrolizumab therapy was suspended. GCS therapy was restarted, and after another 3 cycles, the iCCA was deemed resectable; therefore, conversion surgery was performed. Postoperative histopathological examination revealed a pathological complete response (pCR), and the patient remained alive more than 5 years postoperatively without recurrence or metastasis. CONCLUSIONS: We experienced a case of pCR induced by GCS chemotherapy and pembrolizumab monotherapy. Although the direct contribution of pembrolizumab remains unclear, a possible synergistic effect with GCS chemotherapy was suggested, particularly in MSI-H tumors.