Beam Modeling in Commercial Treatment Planning System for IMRT and VMAT performance with an Elekta MLCI2 Multileaf Collimator.

Abstract

Linear accelerator multileaf collimator requires to be tested with best possible quality assurance tools and accordingly treatment planning system input with the data for appropriate modelling of MLC. Dose calculation is affected due to MLC modelling, especially when using the high standard techniques of treatment like IMRT or VMAT. An MLCI 2 multileaf collimator is verified by 2D detector matrix (IBA dosimetry, Germany) using the quality assurance kit Express QA test package & clinical cases verification. The standard plan in QA mode is made in TPS and delivered under medical linear accelerator like pre-treatment verification.The measured and calculated fluence is compared and accordingly the Gamma analysis done. Express QA tests & clinical cases fields showed a great agreement with TPS calculations with 3% DD and 3 mm DTA Gamma criteria. The open field 10 x 10 cm² and 20 x 20 cm² found to be passed with 100% results for 3% &3mm criteria. 3ABUT test helped in setting the leaf offset value from default 0.0mm to 0.15mm. FourL test provides adjustment in leaf transmission value and leaf groove width from 0.012 to 0.0073 and 1.0mm to 0.7mm respectively. The H&N and Prostate clinical cases passed with more than 95% for set criteria (3%DD&3mm).The absolute point dose measurement agreement was found to be more than 97%.This study confirmed that the appropriate MLC check before starting IMRT and VMAT in clinic and even after any repair is required thorough quality assurance check using Express QA and TG 119 package. Small changes in the MLC parameters like leaf offset, groove width and transmission n the TPS model can cause large changes in the calculated dose.At least annually Express QA test is recommended to be performed by every user to confirm the status of changed MLC parameters in due course of time.