In fact, immunotherapy using CD8+ cytotoxic T lymphocytes (CTLs) engineered to express both herpes simplex virus type-1 thymidine kinase (HSV1-TK) and interleukin-13 zetakine CAR has been explored to treat high-grade gliomas, and PET imaging with a 9-[4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine ([18F]FHBG) can be used to track HSV1-tk reporter gene expression in CAR-engineered T lymphocytes [34]. Here, TKT is linked to central nervous system cancer.