Positioning accuracy and dosimetric evaluation of customized immobilization for FBCT guided VMAT in nasopharyngeal carcinoma

This study aims to expand and enhance traditional fixation techniques by comparing positioning errors and dosimetric variations in the target area and organs at risk (OARs) between conventional and customized immobilization strategies during radiotherapy for nasopharyngeal carcinoma (NPC). Additionally, we evaluate the benefits and necessity of continuous improvements in immobilization devices. Specify beam technique (VMAT) and sample size (n = 30; 15 per group): the traditional group (standard styrofoam, a thermoplastic mask (TM) with a universal oral stent (OS)) and the customized group (modified styrofoam, TM with a self-developed customized OS). Using diagnostic fan-beam computed tomography (FBCT) scans, we quantitatively assessed positioning errors in the overall target area as well as the oropharynx and neck regions with the two fixation methods. Additionally, we evaluated dosimetric changes in target areas and principal OARs during three stages of NPC radiotherapy (initial plan, week 4, and week 6) to ascertain dosimetric benefits. Compared to the traditional group, the customized group exhibited notably smaller positional errors throughout individual treatment sessions. The modifications implemented in the customized group had a more substantial impact and weight on reducing errors in the neck region. The customized group showed improved target dose coverage, such as the dose covering 95% of the target volume (D95), dose covering 98% of the target volume (D98) of primary planning gross tumor volume (PGTVp), the D95 and D98 of planning clinical target volume 1 (PCTV1), etc. Additionally, the customized group demonstrated a lower radiation dose to brainstem, temporomandibular joint, parotid-R, thyroid gland and other OARs. The customized immobilization strategy significantly enhances the reproducibility of patient positioning during NPC radiotherapy. A notable advantage of customized solutions may allow margin reduction and selective OAR sparing. Moreover, the "customized" clinical application should be utilized individually, taking into account multiple factors.