Realization of dose verification tool for IMRT plan based on DPM

Objective: To build a Monte Carlo dose verification tool for IMRT Plan by implanting an irradiation source model into DPM code and to extend the ability of DPM to calculate any incident angles and irregular-inhomogeneous fields. Methods: The virtual source and the energy spectrum unfolded from the accelerator measurement data were used, in combination with optimized intensity maps, to calculate the dose distribution of the irradiation irregular-inhomogeneous field. The irradiation source model of accelerator was substituted by a grid-based surface source. The contour and the intensity distribution of the surface source were optimized by IMRT. The dose calculation was realized by combining the position of the emitter with the fluence map from the IMRT plan. The weight of the emitter was decided by the grid intensity. The direction of the emitter was decided by the combination of the virtual source and the emitting position. The weighted fraction of the emitter was also combined with the flux grid intensity based on the particle transport model of DPM code. Results: The accuracy of calculation was verified by comparing with the measured data. It was illustrated that the differences were acceptable ( < 2% inside the field, 2-3 mm in the penumbra). The dose calculation of irregular field by DPM simulation was also compared with that of FSPB (Finite Size Pencil Beam). The passing rate of gamma analysis was 95.1% for peripheral lung cancer. The regular field and the irregular rotational field were all within permissible range of error. The calculation time of regular fields were less than 2 h, and that of the test of peripheral lung cancer was 160 min. Conclusions: The adapted DPM code with its simple irradiation source model is faster than that with classical Monte Carlo procedure. Its computational accuracy and speed satisfy the clinical requirement, and it can be useful as a Monte Carlo dose verification tool for IMRT Plan. (authors)