Rapid Spontaneous Tumor Regression Mimicking Pseudoprogression in Pembrolizumab-treated Non–small Cell Lung Cancer

Atypical tumor responses can occur during immune checkpoint inhibitor therapy. A 71-year-old man receiving pembrolizumab treatment for stage IV non-small cell lung cancer was found on 18F-FDG PET/CT to have developed new lesions in the stomach, left adrenal gland, and lungs. Because a gastric biopsy confirmed metastatic disease, treatment was discontinued. Remarkably, all lesions resolved after 4 weeks without any treatment. Pembrolizumab therapy was restarted, and follow-up CT showed further reduction in the lung cancer without new lesions. This case of spontaneous regression of gastric metastasis mimicking a pseudoprogression response to pembrolizumab treatment highlights the possibility of unusual presentations during immunotherapy.