Monte Carlo dose verification for intensity-modulated arc therapy

Intensity-modulated are therapy (IMAT), a technique which combines beam rot ation and dynamic multileaf collimation, has been implemented in our clinic . Dosimetric errors can be created by the inability of the planning system to accurately account for the effects of tissue inhomogeneities and physica l characteristics of the multileaf collimator (MLC). The objective of this study is to explore the use of Monte Carlo (MC) simulation for IMAT dose ve rification. The BEAM/DOSXYZ Monte Carlo system was implemented to perform d ose verification for the IMAT treatment. The implementation includes the si mulation of the linac head/MLC (Elekta SL20), the conversion of patient CT images and beam arrangement for 3D dose calculation, the calculation of gan try rotation and leaf motion by a series of static beams and the developmen t of software to automate the entire MC process. The MC calculations were v erified by measurements for conventional beam settings. The agreement was w ithin 2%. The IMAT dose distributions generated by a commercial forward pla nning system (RenderPlan, Elekta) were compared with those calculated by th e MC package. For the cases studied, discrepancies of over 10% were found b etween the MC and the RenderPlan dose calculations. These discrepancies wer e due in part to the inaccurate dose calculation of the RenderPlan system. The computation time for the IMAT MC calculation was in the range of 20-80 min on 15 Pentium-III computers. The MC method was also useful in verifying the beam apertures used in the IMAT treatments.