Analysis of setup uncertainties and determine the variation of the clinical target volume (CTV) to planning target volume (PTV) margin for various tumor sites treated with three-dimensional IGRT couch using KV-CBCT

摘要

Objective: The aim of this study was to determine the variation of patient setup uncertainties and clinical target volume (CTV) to planning target volume (PTV) margins for different tumor sites based on clinical data with three-dimensional IGRT couch using pre-treatment verification by KV-CBCT. Methods: In this study the tumor sites were divided in four categories: (1) brain (50 patients), (2) head and neck (HN) (226 patients), (3) thorax (64 patients), and (4) pelvis (82 patients). The brain and head and neck patients were immobilized using thermoplastic three and five clamp mask and individual head-rest in the supine position. Using vac-loc had done the immobilization of thorax patients, and thermoplastic mask was used with feet-fix in the supine position for pelvis patients. All brain and HN patients underwent first 3 days of pretreatment verification by KV-CBCT imaging and weekly one for remaining fractions, whereas for thorax and pelvis cases, patients underwent daily pretreatment verification by KV-CBCT images for the determination of overall distributions of setup error in the directions of anteroposterior, mediolateral, and craniocaudal, and CTV to PTV margins were analyzed with the help of van Herk’s, Stroom’s, and ICRU formulas. Results: The maximum percentage displacement within ±?3 mm for brain, HN, thorax, and pelvis cases were 96.5, 93.3, 90.9, and 88.3 in the direction of anterioposterior respectively. The maximum CTV to PTV margin, calculated from van Herk’s formula for the brain, HN, thorax, and pelvis were 4.939 mm, 4.62 mm, 7.16 mm, and 5.763 mm in the craniocaudal axis respectively. The comparison of CTV to PTV margin in three consecutive years on 2016, 2017, and 2018 showed that for brain and head and neck cases, the margin initially had decreased by 1.03 mm and 1.11 mm, and then it had increased by 0.75 mm and 0.37 mm on 2018 with respect to 2017. But for thorax and pelvis cases, margins were gradually decreased on three consecutive years. Conclusions: For adequate target coverage, van Herk-calculated CTV to PTV margin are used in our institution. To reduce the setup uncertainties and CTV to PTV margin frequently, CBCT is essential. In the yearly comparison of CTV to PTV margin, we observed that for brain and HN cases were not in descending order due to the differently deformation of head rests which were used for molding and treatment separately.