Development and validation of an X-ray model for an Elekta Precise multileaf collimator to be used in Monte Carlo dose calculations

[摘要] English: Linear accelerators (Linacs) produce megavoltage (MV) energy photon and electron beams to irradiate tumour volumes in patients. More complex field shapes can be setup quickly with multileaf collimators (MLC's), thus more advanced treatments like intensity-modulated radiation therapy (IMRT) are possible. This is one of the reasons why treatment planning models should be accurately commissioned and accurate dose calculation algorithms employed. Monte Carlo (MC) based dose calculations are very suitable to solve this issue. The aim of this project was to continue the development of an X-ray source model for MC dose calculations for an ElektaTM Precise MLC Linac.Methods & MaterialsAn in-house developed graphical user interface (GUI) was used to calculate exit fluence based on a mathematical model and energy spectra derived from the Schiff formula. This produced an input source file for source number 4 in the DOSXYZnrc code. DOSXYZnrc was used to calculate X-ray dose distributions in water and RW3 solid water phantoms. These dose distributions were compared to actual measured film or water tank dose data. A gamma index was calculated to compare the MC and measured dose. The criteria used for the γ-index was 2 % dose / 2 mm distance-to-agreement. Dose distribution data for square, rectangular and off-set fields were compared.Results & DiscussionPrior to source commissioning a GAFCHROMIC® EBT2 film dosimetry system, that entails using a film scanner, was setup. With the use of the EBT2 film, scanner properties like scanner uniformity, film orientation, film scanning side and repeatability were investigated. Film orientation produced the largest discrepancy of 3.5 % between portrait and landscape orientation. The remaining properties were within 1 % variation. A range of fields for 6, 8 and 15 MV beams were modelled, simulated and compared to corresponding measured water tank data. Parameters in the MC source model were adjusted until the gamma-index criteria were met for each comparison. These source parameter values were retained for further more complex field simulation and evaluation against measurements. Rectangular, small and medium sized off-set fields met the gamma-index criteria. For off-set fields greater than 15×15 cm2, the model failed the criteria at some dose points at the field edges. The jaws and MLC transmission parameters required adjustments for the irregular MLC field shapes comparisons.ConclusionThe source model performed well and can be employed for dose verification ranging from simple regular fields to conformal treatments. In order to use the model for IMRT treatment verification, the model needs to be validated clinically. The only requirement is that Linac scatter factors must be measured separately to calculate the correct amount of monitor units necessary for patient treatment. Additional scatter sources can be implemented in the model to increase the accuracy at the field edges of the off-axis cases, which will require verification.

[发布机构] University of the Free State