Enhanced setup and positioning accuracy in nasopharyngeal carcinoma radiotherapy: A customized cushion/mask/bite-block immobilization approach

Abstract Objective To determine whether a customized cushion/mask/bite-block (CMB) immobilization strategy was superior to the standard head and neck immobilization system in terms of enhancing setup and positioning accuracy across diverse anatomical planes during IMRT for nasopharyngeal carcinoma (NPC). Methods This study analyzed CBCT images of 22 patients undergoing IMRT for NPC, assessing setup errors at distinct anatomical levels (sphenoid, C1, C4, C7, mandible) and calculating corresponding CTV-PTV margin expansions. Intra-fractional positional uncertainties were also evaluated. Differences in setup errors, required PTV margins and intra-fractional positional accuracy were compared between the two groups. Results The CMB group showed significant improvement in inter-fractional setup error within the ROI for PTV compared to the standard headrest group. Compared with the standard headrest group, the CMB group significantly reduced systematic and random errors, especially in the X (left-right) and Y (superior-inferior) directions. The respective systematic and random errors (mm) were as follows: X (0.73 ± 0.98 vs. 1.16 ± 0.99), Y (1.12 ± 0.86 vs. 1.70 ± 0.96), and Z (0.86 ± 1.12 vs. 0.85 ± 1.14). Furthermore, the CMB group significantly improved the positional repeatability of various local ROIs (sphenoid, C1, C4, C7, and mandible). Additionally, the CMB group had reduced CTV-PTV margin expansions in the three directions. Conclusion Compared to the standard headrest group, the CMB immobilization protocol significantly improved setup and positioning accuracy for IMRT in NPC patients, reducing the need for PTV margin expansion across various anatomical levels. This novel immobilization approach was deemed effective, simple, and robust during IMRT for NPC. Advances in knowledge This study provides valuable clinical insights into improving setup and positioning accuracy at various anatomical levels during IMRT for NPC.