CHI3L1 and small cell lung carcinoma: We have previously found elevated levels of biomarkers reflecting neuroaxonal injury (neurofilament light chain [NfL] and tau), inflammatory signaling (YKL‐40 [also known as chitinase‐3‐like 1], interleukin [IL]‐15), astrogliosis (glial fibrillary acidic protein [GFAP]), and synapse integrity (GAP‐43 [growth‐associated protein 43]) in the cerebrospinal fluid (CSF) after prophylactic cranial irradiation (PCI) in patients with small cell lung cancer.