Pseudoprogression of lung large cell neuroendocrine carcinoma resembling pancreatic cancer during durvalumab therapy.

A 74-year-old Asian man was referred for numb and painful sensation in the right upper limb. He was diagnosed with lung large cell neuroendocrine carcinoma and started receiving durvalumab therapy as second-line treatment. Sixteen days after the first dose, laboratory examination revealed increased liver enzyme levels and a marked inflammatory response. Contrast-enhanced computed tomography revealed an unenhanced tumor measuring approximately 25 mm in the head of pancreas and dilation of the intra- and extra-hepatic bile ducts. Magnetic resonance cholangiopancreatography confirmed stricture in the lower common bile duct and main pancreatic duct. We suspected acute cholangitis caused by a pancreatic cancer and performed an endoscopic biliary drainage. Two weeks after the procedure, computed tomography revealed significant shrinkage of the tumor. The tumor gradually reduced and pseudoprogression in lung large cell neuroendocrine carcinoma was ultimately diagnosed. Acute cholangitis caused by pseudoprogression resembling pancreatic cancer during immune therapy has not yet been reported. We are mindful that pseudoprogression should be considered as a differential diagnosis if a rapidly developing pancreatic tumor is observed during immunotherapy.