Histologic transformation of lung cancer during pembrolizumab therapy: A case report

Immune checkpoint inhibitors that block the programmed death 1/programmed death ligand 1 pathways are widely used to treat advanced lung cancers. There are seldom cases of histologic transformation reported after treatment with immunotherapy. Here, we report the case of a 69‐year‐old man with stage IV lung squamous cell carcinoma. He received pembrolizumab monotherapy and had a partial response. After 22 cycles of pembrolizumab, chest computed tomography (CT) showed a left hilar tumor, bilateral pleural effusion and lymphadenopathy. The cytology of pleural effusion and bronchoscopic biopsy of an intraluminal lesion revealed small cell lung cancer. After two cycles of chemotherapy (etoposide/carboplatin), CT scan revealed shrinkage of lesions. This is the first case of lung squamous cell carcinoma with histologic transformation after treatment with pembrolizumab alone.