Room: AAPM ePoster Library
Purpose: Stray radiation dose to healthy tissue contributes to the risk of toxicities such as second cancers. Previous attempts to model out-of-field dose have focused on beam energies below 10 MV or have neglected the stray dose from photoneutrons. The purpose of this work was to develop a fast, accurate analytical model of stray photon and photoneutron dose from megavoltage x-ray beams.
Methods: We systematically measured in-field and out-of-field doses in water from photon beams with beam energies from 6 to 25 MV and field sizes ranging from 2×2 to 20×20 cm². Ion chamber measurements of dose were made for two linacs (Elekta Precise) at the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany. Detailed MCNPX Monte Carlo models of the treatment machines and vaults were used to simulate the photon and photoneutron spectra in air and at each measurement position in the water phantom. This data was used to train an analytical model of stray dose from leakage and scattered photons for all beam energies and from photoneutrons for beam energies = 10-MV.
Results: The energy spectra of photoneutrons in air predicted by the Monte Carlo simulation agreed well with measured photoneutron spectra. The average relative percent difference between Monte Carlo and analytical model calculations of absorbed dose from photoneutrons was 9.2% across all beam energies, field sizes, and points considered. Similarly, the average percent difference between measurement and analytical model calculations of total dose was 15%. The average CPU time to calculate total absorbed dose to 106 points with the analytical model was under 4 minutes.
Conclusion: These results demonstrate that it is feasible to quickly and accurately predict whole-body photoneutron exposures with physics-based analytical models. Such models could be incorporated into treatment planning systems to estimate and minimize the risk of radiogenic late effects such as carcinogenesis and cataractogenesis.