Studies like that by Abdelaziz et al. [39] within our review demonstrate that adequate 25(OH)D levels (>30 ng/mL) enhance radiotherapy efficacy, reducing the severity of oral mucositis (p < 0.001) by upregulating DNA repair genes (e.g., GADD45) and dampening inflammation via NF-κB suppression. The gene discussed is NFKB1; the disease is oral mucositis.