Important points to consider when evaluating the response to drug therapy for lung cancer

Drug therapy for primary lung cancer, including molecular-targeted drugs and immune-check point inhibitors, has markedly progressed in the precision medicine era. The Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 have been used to determine the presence of progressive disease (PD); however, there are some drawbacks to performing such an evaluation based on the greatest diameter of the tumor alone. We herein review the limitations of the response evaluation using RECIST v1.1 and recent advances made in the early detection of the therapeutic effect and prediction of the prognosis using functional imaging modalities, such as fluorodeoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT), dual-energy CT, perfusion CT, and diffusion-weighted magnetic resonance imaging. In addition, we describe the atypical response patterns after immunotherapy using anti-PD-1/PD-L1 agents.