Commissioning an ultra-high-dose-rate electron linac with end-to-end tests

Abstract

Objective. The FLASH effect can potentially be used to improve the therapeutic ratio of radiotherapy (RT) through delivery of Ultra-high-dose-rate (UHDR) irradiation. Research is actively being conducted to translate UHDR-RT and for this purpose the Mobetron is capable of producing electron beams at both UHDR and conventional dose rates for FLASH research and translation. This work presents commissioning of an UHDR Mobetron with end-to-end tests developed for preclinical research.Approach. UHDR electron beams were commissioned with an efficient approach utilizing a 3D-printed water tank and film to fully characterize beam characteristics and dependences on field size, pulse width (PW) and pulse repetition frequency (PRF). This commissioning data was used to implement a beam model using the GAMOS Monte Carlo toolkit for the preclinical research. Then, the workflow for preclinical FLASH irradiation was validated with end-to-end tests delivered to a 3D-printed mouse phantom with internal inhomogeneities.Main results.PDDs, profiles and output factors acquired with radiochromic films were precisely measured, with a PRF that showed little effect on the UHDR beam energy and spatial characteristics. Increasing PW reduced theD_maxand R₅₀by 2.08 mmµs^-1and 1.28 mmµs^-1respectively. An end-to-end test of the preclinical research workflow showed that both profiles in head-foot and lateral directions were in good agreement with the MC calculations for the heterogeneous 3D printed mouse phantom with Gamma index above 93% for 2 mm/2% criteria, and 99% for 3 mm/3%.Significance. The UHDR Mobetron is a versatile tool for FLASH preclinical research and this comprehensive beam model and workflow was validated to meet the requirements for conducting translational FLASH research.

Keywords: FLASH; Mobetron; commissioning; electron; end-to-end test; ultra-high dose rate.

Figure 1.

Definitions of geometries for the UHDR Mobetron commissioning (a) and the corresponding schematic diagram (b). A photo for the for 10 cm applicator setup is shown in with the water tank below (c).

Figure 2.

Workflow, tools and data for preclinical research with the UHDR Mobetron. The diagram can be read as a timeline from left to right.

Figure 3.

UHDR Mobetron commissioning data including PDDs, profiles and output factors for ‘Pristine’ (a1)–(a3), ‘No applicator’ (b1)–(b3), ‘6 cm applicator’ (c1)–(c3), and ‘10 cm applicator’ (d1)–(d3) setups. Zoomed-in views of (b1), (c1) and (d1) can be found in figure S3 in the supplementary material.

Figure 4.

The dependence of PDD curves on PW for A10I10 setup (a). The R50 for PW-2 μs, 3 μs and 4 μs are 2.5%, 5.1% and 9.0% shallower than that of PW-1.2 μs. The linear regression fitting of the dependence of Dmax (b) and R50 (c) on PW.

Figure 5.

Dose distribution at orthogonal views (a) and the cumulative volume histogram for a total body irradiation of a mouse phantom with A10I10 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.

The comparison between in-vivo film measurements and MC calculated dose profiles in head-foot (a) and left-right (b) directions. The lower panels show the % residual between measurements and MC calculation.