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Feasibility of Monitor Unit Calculation Using a Commercial Monte Carlo Algorithm for Electrons

S Mossahebi

S Mossahebi*, H Xu , M Guerrero , B Yi , K Prado , P Aryal , W D'Souza , S Chen , Univ. of Maryland School Of Medicine, Baltimore, MD


SU-I-GPD-T-64 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Manual Monitor Unit (MU) calculation is the standard practice for electron therapy which is based on machine commissioning data that were collected using flat water phantoms. Electron dose calculation accuracy of treatment planning systems has been improved significantly with Monte Carlo (MC) algorithm. This study evaluates the feasibility of MU calculation using RayStation MC algorithm.

Methods: PDDs and profiles for all electron energies were measured in a water phantom with an ion-chamber and compared to PDDs and profiles calculated using RayStation v5.0 (RaySearch Laboratories, Stockholm, Sweden) MC Algorithm. Planar dose at depth of maximum dose was measured for all electron energies with MapCheck2 (Sun Nuclear corporation, Melbourne, Florida) at 100/105/110cm SSD and compared to RayStation. The gamma analysis (2mm/2%) was performed for PDD/profile/planar dose comparison. All cone factors and 25 different cutout factors were measured for all energies and compared to RayStation predictions. The algorithm was also validated using 81 clinical electron patient plans. Three MU calculation methods were used for each patient: manual calculation based on AAPM TG-71, RayStation calculation on patient CT images, and RayStation calculation on flat water phantom. The MUs were compared.

Results: All PDD, profile and planar dose comparisons yield to >95% gamma passing rate. For all electron energies, the agreements between RayStation and measurements cone and cutout factors were within 1% and 3%, respectively. For 81 clinical patients, the manual calculations had an agreement within 3% with RayStation calculations based on water phantom; however, the agreement between manual calculations and CT images varies depending on patients. Among these patients, 42 patients had agreements within 3%, but 39 patients who had irregular surface showed difference greater than 3%.

Conclusion: RayStation electron MC algorithm can accurately predict the dose; the MU calculations using RayStation can replace the manual calculations.

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