Emergence of lung cancer with a low PD-L1 expression level after the administration of immune check point inhibitor for lung adenocarcinoma with a high PD-L1 expression level: A case report.

BACKGROUND: Checkpoint therapy against PD-1 has proven effective and positive results have been observed in several types of cancer, including lung cancer, renal cancer, lymphoma and melanoma. However, the effects of long-term ICI treatment remain insufficient and the development of resistance is an issue that remains to be solved. CASE PRESENTATION: A 70-year-old man was diagnosed with lung adenocarcinoma (stage IVB, T4N3M1c) with a high programmed death ligand-1 (PDL1) expression level (tumor proportion score [TPS]: 80% score at the time of the diagnosis, before treatment). At 16.5 months after the start of pembrolizumab, following the administration of 22 cycles of pembrolizumab, a new lesion appeared. Biopsy by video-assisted thoracic surgery (VATS) was performed for this lesion and a pathological diagnosis of lung adenocarcinoma with a low PD-L1 expression level. After the operation, pembrolizumab treatment was continued. The patient currently remains alive without disease progression at 20 months after the initial therapy. CONCLUSIONS: Our case highlights the importance of biopsy by VATS during immune checkpoint inhibitor (ICI) treatment when deciding the treatment strategy for newly confirmed tumors.