First Monte Carlo beam model for ultra-high dose rate radiotherapy with a compact electron LINAC

Abstract

Background: FLASH radiotherapy based on ultra-high dose rate (UHDR) is actively being studied by the radiotherapy community. Dedicated UHDR electron devices are currently a mainstay for FLASH studies.

Purpose: To present the first Monte Carlo (MC) electron beam model for the UHDR capable Mobetron (FLASH-IQ) as a dose calculation and treatment planning platform for preclinical research and FLASH-radiotherapy (RT) clinical trials.

Methods: The initial beamline geometry of the Mobetron was provided by the manufacturer, with the first-principal implementation realized in the Geant4-based GAMOS MC toolkit. The geometry and electron source characteristics, such as energy spectrum and beamline parameters, were tuned to match the central-axis percentage depth dose (PDD) and lateral profiles for the pristine beam measured during machine commissioning. The thickness of the small foil in secondary scatter affected the beam model dominantly and was fine tuned to achieve the best agreement with commissioning data. Validation of the MC beam modeling was performed by comparing the calculated PDDs and profiles with EBT-XD radiochromic film measurements for various combinations of applicators and inserts.

Results: The nominal 9 MeV electron FLASH beams were best represented by a Gaussian energy spectrum with mean energy of 9.9 MeV and variance (σ) of 0.2 MeV. Good agreement between the MC beam model and commissioning data were demonstrated with maximal discrepancy < 3% for PDDs and profiles. Hundred percent gamma pass rate was achieved for all PDDs and profiles with the criteria of 2 mm/3%. With the criteria of 2 mm/2%, maximum, minimum and mean gamma pass rates were (100.0%, 93.8%, 98.7%) for PDDs and (100.0%, 96.7%, 99.4%) for profiles, respectively.

Conclusions: A validated MC beam model for the UHDR capable Mobetron is presented for the first time. The MC model can be utilized for direct dose calculation or to generate beam modeling input required for treatment planning systems for FLASH-RT planning. The beam model presented in this work should facilitate translational and clinical FLASH-RT for trials conducted on the Mobetron FLASH-IQ platform.

Keywords: FLASH; Monte Carlo; electron; radiotherapy; ultra‐high dose rate.

Figure 1.

(a) Various setups including the so-called ‘Pristine’, ‘No applicator’, ‘6 cm applicator’ and ‘10 cm applicator’; enlarge views of the applicators and inserts are included in the supporting material, Figure S-1. (b) shows the components inside the red dashed box in detail with labels to mark the positions of beam source, primary scatted and secondary scatter. (c) shows the setup for beam data measurement with 3D-printed water tank (in orange) under UHDR mode. (d) shows the geometry of secondary scatter of Mobetron which contains a large foil and a small foil. (e) is the 3D-printed front pointer used to position the 3D-printed water tank with 2 cm air gap. (f) shows the geometry of 3D-printed water tank. (g) shows the configuration of Mobetron in GAMOS.

Figure 2.

Dependence of the thickness of small foil in the secondary scatter on the MC PDD (a) and Profile (b) in 1cm depth for pristine beam. NT is the nominal thickness i.e.0.254 mm. Discrepancy and γ indices of 2mm/2% between film measurements and MC calculation with small foil thickness of NT+0.1mm are also included. (c)-(e) shows the comparison between film data and MC calculations for pristine beam profiles in depths of 2cm, 3cm and 4cm.

Figure 3.

PDDs (a)-(c) and profiles (d)-(f) verification of Mobetron UHDR beam model for I6, A6I6 and A10I10 which corresponds to the largest field size of each setup in Table 1. γ indices were calculated with the criterion of 2mm/2%

Figure 4.

Absolute doses calculated with the UHDR MC beam model (red boxes) and those measured with film (black dots) at 1cm depth for all setups in Table 1.

Figure 5.

Dose distribution at orthogonal views (a) and the cumulative volume histogram for a whole-brain irradiation of a mouse with I2.5 setup from MC beam model simulation. The dashed white lines in the orthogonal views indicate the slice location for the other two perspectives.

Figure 6.

Comparison of PDDs measured with vertical film in the 3D-printed water tank and diode detector in the IBA Blue Phantom under CONV mode (a). Comparison of Profiles measured with vertical film in the 3D-printed water tank and parallel film inserted in solid water slabs under UHDR mode (b).

Figure 7.

Dependence of mean energy (a1) -(a2), beam energy divergency $\sigma$ (b1)-(b2), thickness of the primary scatter (c1)-(c2), diameter (d1)-(d2) and thickness (e1)-(e2) of the large foil in the secondary scatter on the Mobetron UHDR beam model.