(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: 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 & Neck (226 patients), 3) Thorax (64 patients), and 4) pelvic (82 patients). All brain and H&N patients underwent first three days pretreatment verification by KV-CBCT imaging and weekly once for remaining fractions, whereas for thorax and pelvic 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 also CTV to PTV margins were analyzed with the help of van Herk’s, strooms and ICRU formulas.
Results: The maximum percentage displacement within ± 3mm for brain, H&N, thorax and pelvic case were 96.5, 93.3, 90.9 and 88.3 in the direction of anterioposterior respectively. The overall distributions of translational setup error in anteroposterior, mediolateral and craniocaudal axis for all treatment sites were analyzed. The overall distributions for brain and H&N were narrow, whereas the distributions for the thorax and pelvic lesions were comparatively broad. The 3D vector distances ≥7mm were rare for brain and H&N lesions at 7.64% and 8.2% respectively. The maximum margin, calculated from van Herk’s formula for brain, H&N, thorax and pelvic were 4.939mm, 4.62mm, 7.16mm and 5.763mm in the craniocaudal axis respectively.
Conclusion: For better 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 because setup errors vary according to each immobilization systems, patients and daily setup.
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