Comparison of physical dose distributions of a carbon ion radiotherapy treatment plan obtained from matRad treatment planning system and PHITS Monte Carlo simulation
Keywords:
Carbon ion radiotherapy, treatment planning, Monte Carlo simulation, matRad, PHITSAbstract
Background: In carbon ion radiotherapy (CIRT) treatment planning, dose prescription is done in term of relative biological effectiveness (RBE)-weighted dose in the unit of Gy(RBE). There are two major biophysical models used in CIRT: the microdosimetric kinetic model (MKM) and the local effect model (LEM). For the same prescribed dose, both RBE models can lead to different physical dose distributions in the patient. Therefore, it is not possible to directly compare clinical data using different RBE models. In the past, the conversion factors between MKM and LEM prescribed doses were evaluated using two treatment plans calculated based on both models. However, direct conversion from a treatment plan to the other based on the underlying biophysical model has not been performed. The latter approach requires the information of the physical dose distribution from a treatment plan and the resulting voxel-by-voxel energy spectra of primary and secondary charged particles, which can be obtained by Monte Carlo simulation.
Objectives: To compare physical dose distributions obtained by Particle and Heavy Ion Transport code System (PHITS) Monte Carlo simulation and matRad treatment planning system, to be used for the conversion of CIRT prescribed doses between the LEM and the MKM.
Material and methods: A virtual water phantom with the size of 50 x 35 x 50 cm3 was generated. The target was defined as a spherical volume with a radius of 3 cm, centered at 7 cm water-equivalent depth from the entrance surface of the phantom. Treatment planning of the target was performed with an open-source treatment planning system matRad with the prescribed dose of 4.3 Gy(RBE). The parameters from matRad including number, position, energy and width of carbon ion beam spots were used to define sources in PHITS. Physical dose distributions from matRad and PHITS were compared using the gamma analysis method.
Results: The differences between the Bragg peak positions from matRad and PHITS for monoenergetic carbon ion beams were approximately 1 mm. By shifting the water phantom 1 mm upstream of the beam entrance in the Monte Carlo simulation, the gamma passing rates at 3%/3mm between both sets of physical dose distributions were averagely 99% and 90% in the target region and all regions, respectively.
Conclusion: The combination of matRad and PHITS can be applied to evaluate the conversion factors between LEM and MKM treatment plans, especially in the target region.
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