2248 - Feasibility of Photon Based Ultra-High Dose Rate Therapy - Using kVp X-Ray Tubes

Purpose/Objective(s): To assess the possibility using adapted X-ray source to probe mechanisms of FLASH radiation in a pre-clinical environment. Currently, FLASH-radiation has only been tested using electron accelerators and in proton facilities. The first option can only been used in small animals or for superficial reasons, while the latter requires expensive equipment competing for time in a clinical environment. Recently, a number of groups proposed to use adapted X-Ray tubes, which can reach continuous FLASH dose rates, this is in contrast with the pulsed architecture of charged particle accelerators. In this paper we present the impact of the calculated electron dose deposition spectrum on the FLASH effect using a radiobiological model comparing a linear accelerator based device with the X-ray source results.

Materials/Methods:

A model based on extreme changes in oxygen equilibrium was used to calculate complex DNA-damage. The variable parameters in the model are dose delivered, initial oxygenation of the sample, and oxygen distribution among cells. The latter expressed as an oxygen histogram as function of dose delivered (i.e. how many cells under a given oxygen condition received which dose). The X-ray source spectrum was provided by another group (University of Victoria, BC, Canada) who have already shown FLASH effects. This was used as a starting point for a Monte Carlo simulation using MCNP6 (Oak Ridge Nat. Labs.) . The outcome of the simulation provides a dose deposition spectrum (i.e. the dose deposited by electrons of a given energy). For the electron accelerator a 9MeV beam was used. In the latter case the contribution of low energy electrons (<10 keV) is negligible. For both conditions we calculated a the theoretical FLASH sparing effects at different doses delivered and various initial partial oxygen pressures.

Results: The dose deposition spectrum for the X-ray source shows that the majority of dose is delivered by electrons with energies below 10keV with a maximum around 1keV. This is, according to the model used an energy region where FLASH effects are diminishing. Results are provide in the following table. The columns show the sparing factor, while the rows show different oxygenation levels. This for both electron-based and X-Ray radiations.

Conclusion: The model shows that there is no difference between X-ray based FLASH treatments and electron based. According to this model the contribution of high-LET electrons is too low to make a substantial difference. The reduction of the sparing factor is consistent for all dose- and oxygenation levels. But being of the order of a few promille they are not noticeable in radio-biological terms.

Abstract 2248 - Table 1

Dose ? pO2 (Torr)?	10 - E9	10 -X	20-E0	20-X	50-E9	50-X	153-E9	153-X
1Gy	0.74581	0.74478	0.98217	0.98091	0.99849	0.99832	0.99986	0.99984
2Gy	0.55540	0.55516	0.75608	0.75480	0.99576	0.99531	0.99971	0.99967
5Gy	0.44115	0.44139	0.51702	0.51649	0.76792	0.76679	0.99904	0.99892
10Gy	0.40307	0.40346	0.43733	0.43705	0.56065	0.55986	0.99194	0.99120