A Case of Primary Lung Adenocarcinoma With Recurrent Brain Metastasis due to Transformation to Small Cell Carcinoma During Adjuvant Atezolizumab Therapy.

Histologic transformation from non-small cell to small cell lung cancer (SCLC) is a resistance mechanism to immune checkpoint inhibitors. We report herein a case of lung adenocarcinoma who developed liver and brain metastases during adjuvant atezolizumab therapy. The patient underwent a craniotomy to resect a brain metastasis, which was pathologically diagnosed as SCLC. He subsequently received platinum-based chemotherapy with durvalumab, resulting in sustained regression of the liver metastases. This case demonstrates a metastatic brain tumor-acquired resistance to atezolizumab through histologic transformation from adenocarcinoma to SCLC. Therefore, rebiopsy is needed if recurrent disease appears during immune checkpoint inhibitor treatment in patients with non-small cell lung cancer.