Analysis of which local set-up errors can be covered by a 5-mm margin for cone beam CT-guided radiotherapy for nasopharyngeal carcinoma

OBJECTIVE: To analyse which local set-up errors can be covered by a 5-mm margin for cone beam computed tomography (CBCT)-guided radiotherapy in nasopharyngeal carcinoma (NPC). METHODS: 11 regions of interest (ROIs) were registered for 24 NPC patients, with a total of 323 CBCT scans. According to the registration results, clinical target volume–planning target volume (CTV–PTV)/organs at risk-planning risk volume (OAR-PRV) margin analysis; Pearson correlation analysis; Bland–Altman plots; and a receiver operating characteristic (ROC) analysis were used to investigate which local set-up errors of substructure can be represented by the PTV(ROI). RESULTS: The clinical target volume-PTV/OAR-planning risk volume margins were less than 5 mm for C1(ROI)-C4(ROI), mandible (M(ROI)), and sphenoid sinus (S(ROI)) with respect to PTV(ROI). C1(ROI)-C4(ROI), M(ROI), and S(ROI) exhibited significant correlations and consistencies in the mediolateral, superior–inferior, and anteroposterior (AP) directions and significant receiver operating characteristic analysis results in the anteroposterior direction. CONCLUSION: Only the upper local set-up error of C1(ROI)-C4(ROI), M(ROI), and S(ROI) can be covered by a 5-mm margin for CBCT-guided NPC radiotherapy with a large ROI. Using these ROIs as an integral reference ROI is better than individual bony landmark. ADVANCES IN KNOWLEDGE: This report is helpful to CBCT registration for NPC radiotherapy in clinical practice.