A Case of Granulocyte Colony-Stimulating Factor-Producing Lung Adenocarcinoma with KRAS G12V Mutation Responding to Pembrolizumab

We report a rare case of granulocyte colony-stimulating factor (G-CSF)-producing lung adenocarcinoma with a high PD-L1 expression and a KRAS G12V mutation that responded to pembrolizumab. A 72-year-old man presented with leukocytosis, elevated serum G-CSF levels, a diffuse bone marrow fluorodeoxyglucose uptake, and a rapidly progressive lung mass. A biopsy revealed TTF-1 positive adenocarcinoma with a PD-L1 tumor proportion score of 100 % and a KRAS G12V mutation. Pembrolizumab induced marked tumor regression and normalization of the G-CSF levels. However, immune-related pneumonitis resulted in treatment discontinuation. Pembrolizumab may be effective in selected patients with G-CSF-producing lung cancer despite its typically aggressive clinical behavior.