Delayed Tumor Shrinkage Beyond Progressive Disease in Small‐Cell Lung Cancer Receiving Immunochemotherapy: A Case Report

ABSTRACT Small‐cell lung cancer (SCLC) is an aggressive malignancy with a poor prognosis. Although SCLC initially responds rapidly and drastically to platinum‐based chemotherapy, it quickly develops resistance and recurs shortly thereafter. Recently, immunochemotherapy has achieved better survival outcomes than traditional chemotherapy alone, as demonstrated in pivotal trials such as IMpower133 and CASPIAN. However, delayed or atypical tumor responses under immunotherapy remain poorly understood, especially in SCLC. Herein, we present a case of extensive‐disease SCLC that showed a delayed tumor shrinkage following transient progression and prolonged elevation of tumor volume and ProGRP levels during immunochemotherapy. This case highlights the potential for delayed response beyond progressive disease even in SCLC, emphasizing the importance of careful evaluation before changing treatment in selected patients receiving immune checkpoint inhibitors.