IGRT kV-CBCT dose calculations using Virtual Source Models and validated in phantoms using OSL

Guillaume Boissonnat (1) , Helena Chesneau (1) , Juan-Carlos García Hernández (1) , Thomas Dautremer (1) , Eric Barat (1) , Delphine Lazaro (1)

1 LM2S (CEA, LIST) - Laboratoire Modélisation et Simulation de Systèmes (CEA, LIST)

Purpose or Objective: With the growing use of X-ray imaging equipment in Image-Guided RadioTherapy (IGRT), the need to evaluate the dose-to-organs delivered by kV-CBCT imaging acquisition increases. This study aims to propose accurate Monte Carlo (MC) calculations of the patient dose-to-organs delivered by two commercially available kV-CBCT systems: the XVI from Elekta’s VERSA HD accelerator and the OBI from Varian’s TrueBeam system. Simulations are to be validated using in phantom OSL measurements. Material and Methods: For both kV-CBCT systems, the kV irradiation head geometry was implemented in the MC simulation code Penelope. As a first step, the resulting photon distributions were expressed as Virtual Source Models (VSM) for every standard irradiation condition (kVp, filtration, collimation); it was then validated and adjusted using in water-phantom measurements performed with a calibrated Farmer-type ionization chamber. In a second step, the validated VSMs were used to simulate the dose delivered by both the XVI and OBI systems in anthropomorphic phantoms, using standard clinical imaging protocols. Simulated dose-to-organs were then confronted to dose measurements performed using OSL inserted into the same phantoms, following a dosimetric protocol for OSLs previously established [1]. In addition, VSM results were confronted to their direct MC counterparts in order to evaluate the benefit of using such technique. Results: The current study highlights the possibility to reproduce OSL dose-to-organ measurements using VSM-driven Monte Carlo simulation with an overall agreement better than 20 %. In addition, the use of VSM in the MC simulation enables to speed-up the calculation time by a factor better than two (for the same statistical uncertainty) compared to direct MC simulation. Nevertheless, if direct and VSM calculations are in agreement inside the irradiation field, outside, VSM results tend to be significantly lower (10-30%). Conclusion: The use of a VSM was demonstrated to simplify and fasten MC simulations for personalized kV-CBCT MC dose estimation. In addition, OSLs enable to perform the low dose measurement in the kV range needed for in phantom X-ray imaging equipment dose QA. This study is to be completed in the near future by the addition of other standard X-ray imaging equipment dedicated to IGRT.

Mots clés: radiotherapy; Image-Guided RadioTherapy (IGRT); X-ray imaging; Medical imaging; photon distribution; dosimetry; dose measurement; dose to organ; irradiation; Monte Carlo; Particle transport; PENELOPE code; anthropomorphic phantom; simulation; modelling; ionizing radiation; X-rays

Domaines: Médecine nucléaire; Physique Médicale [physics.med-ph]; Modélisation et simulation; Applications [stat.AP]

Guillaume Boissonnat, Helena Chesneau, Juan-Carlos García Hernández, Thomas Dautremer, Eric Barat, et al.. IGRT kV-CBCT dose calculations using Virtual Source Models and validated in phantoms using OSL. ESTRO 36, May 2017, Vienna, Austria. Radiotherapy and Oncology, 123 (Supplement 1), S777; poster EP-1455, 2017, ESTRO 36, May 5-9, 2017, Vienna, Austria. ⟨10.1016/S0167-8140(17)31890-X⟩. ⟨hal-02268688⟩