Monte Carlo modeling of a Novalis Tx Varian 6 MV with HD-120 multileaf collimator.

A Monte Carlo model of the Novalis Tx linear accelerator equipped with high-definition multileaf collimator (HD-120 HD-MLC) was commissioned using ionization chamber measurements in water. All measurements in water were performed using a liquid filled ionization chamber. Film measurements were made using EDR2 film in solid water. Open rectangular fields defined by the jaws or the HD-MLC were used for comparison against measurements. Furthermore, inter- and intraleaf leakage calculated by the Monte Carlo model was compared against film measurements. The statistical uncertainty of the Monte Carlo calculations was less than 1% for all simulations. Results for all regular field sizes show an excellent agreement with commissioning data (percent depth-dose curves and profiles), well within 1% of difference in the relative dose and 1 mm distance to agreement. The computed leakage through HD-MLCs shows good agreement with film measurements. The Monte Carlo model developed in this study accurately represents the new Novalis Tx Varian linac with HD-MLC and can be used for reliable patient dose calculations.

Clinical experience with machine log file software for volumetric-modulated arc therapy techniques.

Mobius FX, an add-on software module from Mobius Medical Systems™ for intensity-modulated radiation therapy (IMRT) quality assurance (QA), uses linac treatment logs to efficiently calculate and verify the 3D dose delivered to patients. An advantage of the Mobius FX module is that it does not require device positioning. In this study, we compared the Mobius FX with another QA option, ArcCheck, as well as with the treatment planning system (TPS) using 30 volumetric-modulated arc therapy (VMAT) plans planned and delivered on a Varian TrueBeam linac. The plans, which involved 6 and 10 MV and consisted of 2 to 3 arcs per plan, were selected to provide a clinically relevant sample. The average gamma value for all plans between Mobius FX and the TPS was 99.96% for the criterion of 3%-3 mm and 98.80% for the criterion of 2%-2 mm. Very similar results were found when comparing Mobius FX and the TPS dose calculations with those acquired by traditional methods (i.e., ArcCheck). As the gamma criterion of the analysis was narrowed, discrepancies between Mobius FX and traditional methods appeared. Profile analysis showed the production of comparable results when using the Mobius FX method or traditional QA methods. In conclusion, the Mobius FX method for pretreatment of patient-specific QA is capable of producing results similar to those obtained by traditional methods.