A theoretical approach for non-equilibrium radiation dosimetry.

This study presents a theoretical approach to the dosimetry for small and non-equilibrium radiation fields. We applied the newly developed VMCBC algorithm to the dosimetry for megavoltage photon beams using Monte Carlo techniques. The approach assumes that a Monte Carlo simulated beam can be calibrated per incident particle at the target in an x-ray tube or in an accelerator head. Since the geometry of the accelerator head and beam defining systems can be modeled in detail, the output of a radiation beam can be accurately related to the number of incident particles through particle transport calculations. The proposed methodology is benchmarked and validated using existing radiosurgery beam commissioning data, which were experimentally measured for narrow beams defined by conical collimators with diameters ranging from 7.5 mm to 30 mm. The Monte Carlo predicted beam outputs agree with the measurement values within the uncertainty of the experiments. The Monte Carlo approach developed and introduced in this study allows the user to perform absolute radiation dosimetry in addition to relative dose distributions at locations where charged-particle equilibrium (CPE) does not exist, such as radiation dose from a narrow stereotactic radiosurgery beam, and where experimental measurements are difficult. The BEAMnrc/DOSXYZnrc code was employed in the Monte Carlo simulations.