The advantages of collimator optimization for intensity modulated radiation therapy

Abstract

Purpose: The goal of this study was to improve dosimetry for pelvic, head and neck and other cancers with aspherical planning target volumes (PTV) using collimator optimization for intensity modulated radiation therapy (IMRT). Methods: A retroactive study on the effects of collimator optimization of 20 patients was done by comparing collimator angles from optimized plans in Eclipse version 11.0. Keeping all other parameters equal, plans were created with four collimator techniques: CA 0 , all fields have collimators set to 0°, CA­ E , using the Eclipse collimator optimization, CA A­, minimizing the area of the jaws around the PTV, and CA X , minimizing the x-jaw gap. The minimum area and the minimum x-jaw angles were found by evaluating each field beam’s eye view of the PTV with ImageJ and finding the desired parameters with a custom script. The evaluation of the plans included the monitor units (MU), the maximum dose of the plan, the maximum dose to organs at risk (OAR), the conformity index (CI) and the number of split fields. Results : Compared to the CA 0 plans, the monitor units decreased on average by 6% for the CA X with a p-value of 0.01 from an ANOVA test. The average maximum dose stayed within 1.1% between all four methods with the lowest being CA X . The maximum dose to the most at risk organ was best spared by the CA A , which decreased by 0.62% from the CA 0 . Minimizing the x-jaws significantly reduced the number of split field from 61 to 37. Conclusion: In every field tested the CA X optimization produced as good or superior results than the other three techniques. For aspherical PTVs, CA X on average reduced the number of split fields, the maximum dose, minimized the dose to the surrounding OAR, and reduced the MU all while achieving the same control of the PTV.