Calculation of beam quality correction factor for relative positions of SOBP and ionization chamber using Monte Carlo simulations

In proton radiotherapy, the dosimetry protocol TRS-398 does not provide the beam quality correction factors kQ,Q₀ for all areas of the spread out Bragg peak (SOBP). Monte Carlo simulations using the TOPAS simulation toolkit were performed to calculate the beam quality correction factors at various depths of the SOBP to observe any variations. The SOBP of the generated proton beam had a range of 15 cm and a width of 15 cm. The beam quality correction factors kQ,Q₀ were calculated not only at the reference depth of 7.5 g/cm² recommended by TRS-398 but also at depths of 4 g/cm² and 13 g/cm². The comparison of the simulation results for the absorbed dose with actual measurements showed a slight difference at the surface above the water phantom, but the width of the SOBP was well matched with a difference of less than 1%. The kQ,Q₀ factor calculated at the reference depth of 7.5 g/cm² was 1.045, which is within the error range of the value of 1.030 provided by the TRS-398 protocol. The kQ,Q₀ factors calculated at the depths of 4 g/cm² and 13 g/cm² were 1.041 and 1.048, respectively. While all the calculated values were within the error range of the value suggested by TRS-398, the observed increase in the kQ,Q₀ factor with increasing depth suggests that a position-dependent beam quality correction factor determined through precise measurements may be required to calculate the correct dose.