XRCC2 and radiation pneumonitis: Since single nucleotide polymorphisms (SNPs) may modify gene function or can be used as genetic markers to detect nearby disease-causing variants through association or linkage studies, we sought to evaluate the association of six potentially functional SNPs (i.e., RAD51 −135G>C/rs1801320, −172G>T/rs1801321, XRCC2 4234G>C/rs3218384, R188H/rs3218536 G>A, XRCC3 T241M/rs861539, and NBN E185Q/rs1805794) of genes involved in HR pathway with radiation pneumonitis (RP) and overall survival (OS) of NSCLC patients treated with definitive radio(chemo)therapy in the present study.