Monte Carlo calculation of absorbed doses due to imaging sessions delivered to patients during Tomotherapy Image-Guided Radio Therapy courses
Résumé
Introduction: The delivery of complex plans is usually included in an image-guided radiotherapy (IGRT) process that is based on a daily in-room imaging session in order to adjust the tumor position. IGRT strategies lead to deliver additional absorbed doses to treated patients due to these imaging sessions. The aim of this study was to calculate the tridimensional dose distribution due to Mega Voltage Computed Tomography (MVCT) imaging sessions with Monte Carlo (MC) simulations for Tomotherapy (Accuray) courses.
Material and methods: MC modelling of the MVCT with the code GATE/GEANT4 was extensively evaluated by comparing measurements and simulations for percent depth dose and off-axis profiles in water and water-equivalent material for static and rotational irradiations, using two fan beam widths. In the isocenter plane, beam dimensions were 4x400mm2 and 50x400mm2. In addition, measured and MC computed absorbed doses were compared for a helical irradiation delivered to the STereotactic End-to-End Validation anthropomorphic phantom (STEEV, CIRS). Depending on the irradiation settings and the material, dose measurements were performed with an ionization chamber, a diamond detector, radiochromic films or optically stimulated luminescent dosimeters (OSL).
Results: For depth dose profiles acquired with static irradiations, simulations and measurements agreed within 3% for depths varying from 0 to 12cm for both beam widths, but reached 6% at a 20cm depth for the 4mm wide beam. For off-axis profiles acquired with static irradiations, simulations and measurements agreed within 3% in high dose region the longitudinal axis. Discrepancies were slightly higher in the lateral axis. Simulated and measured profiles obtained for a 360° irradiation were also in good agreement. Absorbed doses measured with 27 OSL dosimeters in the STEEV phantom for a 12cm long helical irradiation varied from 13 to 16mGy according to the position. MC calculated doses in the same conditions differed by less than 10%.
Conclusion: The MC modelling of the Tomotherapy MVCT allows to calculate tridimensional absorbed dose distributions in patients. This model can now be used to quantify the absorbed doses due to imaging sessions during Tomotherapy courses and to evaluate the dosimetric impact of different IGRT strategies.