Neutron dose in and out of 18 MV photon fields

Simulating the head of a TrueBeam linear particle accelerator and calculating the photoneutron spectrum on the central axis of a 10-MV photon using particle and heavy-ion transport system code

Photon energy is higher than the (γ,n) threshold, allowing it to interact with the nuclei of materials with high z properties and liberate fast neutrons. This represents a potentially harmful source of radiation for humans and the environment. This study validated the Monte Carlo simulation, using the particle and heavy-ion transport code system (PHITS) on a TrueBeam 10-MV linear particle accelerator's head shielding model and then used this PHITS code to simulate a photo-neutron spectrum for the transport of the beam. The results showed that, when comparing the simulated to measured PDD and crosslines, 100% of the γ-indexes were <1 (γ3%/3mm) for both simulations, for both phase-space data source and a mono energy source. Neutron spectra were recorded in all parts of the TrueBeam's head, as well as photon neutron spectra at three points on the beamline.

A REVIEW OF ANALYTICAL MODELS OF STRAY RADIATION EXPOSURES FROM PHOTON- AND PROTON-BEAM RADIOTHERAPIES

External-beam radiation therapy is safe, effective and widely used to treat cancer. With 5-year cancer survival for adults above 70%, increasingly research is focusing on quantifying and reducing treatment-related morbidity. Reducing exposures to healthy tissues is one strategy, which can be accomplished with advanced-technology radiotherapies, such as intensity-modulated photon therapy and proton therapy. Both of these modalities provide good conformation of the therapeutic dose to the tumor volume, but they also deliver stray radiation to the whole body that increases the risk of radiogenic second cancers. To minimize these risks, one needs to create and compare candidate treatment plans that explicitly take into account these risks. Currently, clinical practice does not include routine calculation of stray radiation exposure and, consequently, the assessment of corresponding risks is difficult. In this article, we review recent progress toward stray dose algorithms that are suitable for large-scale clinical use. In particular, we emphasize the current state of physics-based dose algorithms for intensity-modulated photon radiotherapy and proton therapy.