A Case Report on Pericardial Effusion Associated With Selpercatinib in the First Patient With Rearranged During Transfection (RET) Fusion-Positive Lung Cancer

Selpercatinib was approved in 2021 for the treatment of rearranged during transfection (RET) fusion-positive lung cancer. The most frequent adverse events associated with selpercatinib are hepatic dysfunction, hypersensitivity, hypertension, QT interval prolongation, and peripheral edema. However, pericardial effusion has not been reported as an adverse event in the package insert or background clinical studies. A 70-year-old woman with RET fusion gene-positive lung cancer cT2bN2M1c (OSS, BRA) stage IVB developed pericardial effusion about 18 months after starting selpercatinib treatment. The patient wished to continue selpercatinib treatment, so the dose was gradually and carefully reduced to 40 mg/day. This dose reduction resulted in a decrease in pericardial effusion while maintaining tumor regression. RET fusion-positive lung cancer is rare, accounting for only 1%-2% of all lung cancers. Since selpercatinib was approved relatively recently in 2021, the long-term adverse effects of its administration remain unclear. Long-term treatment with selpercatinib may potentially be associated with fluid accumulation, including pericardial effusion, although further studies are needed to clarify this relationship.