Beam Modeling in Commercial Treatment Planning System for IMRT and VMAT performance with an Elekta MLCI ۲ Multileaf Collimator

Introduction: Linear accelerator multileaf collimator (MLC) requires to be tested with best possible quality assurance tools and accordingly treatment planning system input with the data for appropriate modeling of MLC. Dose calculation is affected due to MLC modeling, especially when using the high standard treatment techniques like intensity-modulated radiation therapy (IMRT) or volumetric modulation arc therapy (VMAT). Material and Methods: An MLCI ۲ (Elekta Inc.) multileaf collimator is verified by ۲D 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 a medical linear accelerator like pre-treatment verification. The measured and calculated fluence is compared and accordingly the Gamma analysis is done. Results: Express QA tests & clinical cases fields showed a great agreement with TPS calculations with ۳% Dose Distribution ( DD ) and ۳ mm Distance to Aggrement ( DTA) Gamma criteria. The open field ۱۰ x ۱۰ cm۲ and ۲۰ x ۲۰ cm۲ found to be passed with ۱۰۰% results for ۳% &۳mm criteria. ۳ABUT test helped in setting the leaf offset value from default ۰.۰mm to ۰.۱۵mm. FourL test provides adjustment in leaf transmission value and leaf groove width from ۰.۰۱۲ to ۰.۰۰۷۳ and ۱.۰mm to ۰.۷mm respectively. H&N and Prostate clinical cases passed with more than ۹۵% for set criteria (۳%DD&۳mm). The absolute point dose measurement agreement was found to be more than ۹۷%. Conclusion: This study confirmed that the appropriate MLC check before starting IMRT and VMAT in a clinic and even after any repair is required thorough quality assurance check using Express QA and TG ۱۱۹ 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 every user to confirm the status of changed MLC parameters in due course of time.

Keywords:

Radiotherapy , Beam Commissioning Monaco TPS , MatriXX Evolution