An 80-year-old man who was diagnosed with postoperative recurrence of squamous cell lung carcinoma with high programmed death-ligand 1 expression started receiving pembrolizumab therapy. However, after the first administration, he developed fever, dyspnea, and eosinophilia, and computed tomography findings revealed an increase in recurrent lesions and pleural effusion. We considered that his symptoms constituted immune-related adverse events (irAEs) and initiated steroid therapy. Two months later, we observed that the recurrent lesions had shrunk and the pleural effusion had disappeared. We determined that the increase in the recurrent lesions was due to pseudoprogression and resumed pembrolizumab administration. Subsequently, we noted regrowth of the recurrent lesions and suspected true progression; however, the lesions shrank after continuing treatment.
When discontinuing immune checkpoint inhibitor treatment owing to irAEs, lesion pseudoprogression may occur twice, and this should be kept in mind during evaluation.