Underestimation of dose delivery in preclinical irradiation due to scattering conditions

The aim of this study was to evaluate, by comparing simulation results with measurement results, the impact of the lack of scattering volume in experimental conditions of preclinical irradiation. First, a Monte Carlo model of a small animal irradiator, the Faxitron CP-160, was developed with GATE (Geant4 Application for Tomography Emission). To validate the model, simulated data were compared to depth dose and off-axis ratio profiles measured with a plane-parallel ionization chamber and Gafchromic(®) EBT films, respectively, in a solid water phantom. The AAPM TG-61 protocol was applied to measure the dose rate at the surface of a semi-infinite reference phantom. Then, the model was used to determine the dose distributions in three different phantom settings: a semi-infinite water phantom, a 2.8-cm-thick water phantom and a 2.8-cm-diameter cylindrical water phantom. The dose distributions measured and simulated with Monte Carlo methods in a semi-infinite water phantom were similar (<2%), thus validating our Monte Carlo model. The highest dose underestimation was observed between the reference and the cylindrical phantom (more than 15% difference for the entrance dose) and was due to the lack of lateral scatter and backscatter. The use of standard backscatter factors and AAPM TG-61 protocol may result in a significant underestimation of the dose absorbed by small irradiated phantoms, such as mice or cells, in preclinical studies.

For preclinical radiotherapy studies, radiobiologists were used to determine the irradiation time depending only on the source surface distance. This work aimed to demonstrate that scatter conditions have a large impact on dose rate. Measurements and Monte Carlo simulations were used.