Development and modification of a virtual source model for Monte Carlo based IMRT verification

A Monte Carlo based phase space source model has been developed and modified to allow for IMRT dose verification. The model allows for the simulation of arbitrary intensity distributions without the inefficient step of calculating particle transport through the field defining collimators. The components of the linear accelerator treatment head were simulated using the code MCNP4B, providing the photon fluence in matrix form, at a plane above the micro-multileaf collimator. This map samples the starting particle's position, energy and direction. For IMRT treatments, the leaf sequence is obtained from a translation algorithm, which incorporates leaf leakage and transmission. A second program reads the sequence file and constructs the corresponding intensity map for the Monte Carlo simulation, accounting for leakage. The sum of all segments provides a treatment-specific intensity map, which is multiplied by the discrete phase space fluence map, producing the cumulative fluence grid for the leaf sequence. The modified phase space source was benchmarked against standard depth dose and profile ion chamber measurements for several field sizes. The agreement between the calculation and measurement for all benchmark fields is well within Task Group 53 criteria. The modified source model was also compared with film measurement in various theoretical and clinical examples, for both single field conformal and IMRT treatments. Small differences are seen in regions with highly irregular leaf positions, but in general there is good agreement. The unique features of this modified phase space model make it a valuable and accurate tool for IMRT dose verification.