ESTRO 2024 - Abstract Book

S3389

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

ions of an energy of 150 MeV/u and 430 MeV/u. Due to saturation effects of the monitoring chambers, a fixed parametrization of the extraction system was used to extract a specific number of particles to achieve FLASH dose rates. In order to specify the dose rates, dose measurements with EBT3 films and the PTW PinPoint ionization chamber 31023 were performed. The measurements of carbon ions of 430 MeV/u were performed at two different depths within the depth dose distribution to generate different dose rates. The films were calibrated in all investigated depth to account for LET dependence. Furthermore, saturation correction for the PTW PinPoint ionization chamber 31023 was analyzed by varying the applied voltage and verifying the actual applied particle number and position relative to the PinPoint chamber via EBT3 films.

Results:

In table 1, an overview of dose rates that can be achieved at MIT applying proton and carbon ions is presented. The maximum dose rate that can be applied depends on the particle type and energy since these factors limit the maximum filling of the synchrotron. The highest dose rate of 230 Gy/s is observed for carbon ions with an energy of 150 MeV. This dose rate could further be increased by measuring at the Bragg peak (BP) region. However, due to the feasibility of the set-up, measuring in the entrance channel (EC) leads to a more accurate and reproducible workaround. In figure 1, exemplary results of a film measurement applying FLASH dose rates with carbon ions of 430 MeV/u and the highest synchrotron filling placed at the entrance channel are shown. The fraction of the spot where a dose rate of 40 Gy/s is achieved is a cirlce of diameter 5 mm. In figure 2, the corresponding x- and y-profiles are shown in comparison to exemplary, equivalent profiles of conventional dose rates. Compared to these profiles, FLASH profiles are slightly shifted and not symmetrical to the isocenter since the beam extraction is not controlled. This is also the reason for the variability of the number of extracted particles of around 10% for the same filling status of the synchrotron underlining the need for a controlled beam extraction. For the PTW PinPoint ionization chamber 31023, for proton and carbon ions saturation effects of approximately 1% were observed.

Table 1: Overview of dose rates that can be applied at MIT.

particle

energy/ MeV/u

extraction time/ ms

filling

status

measuring point

dose/ Gy

dose rate/ Gy/s

synchrotron*/ number of particles**/ x108

C12 C12 C12 C12

430 430 430 150 221

150 150 150 150 100 100

I13: 5.1 – 5.5 I13: 5.1 – 5.5 I10: 2.3 – 2.7 I13: 7.8 – 8.3 I13: 54 – 98 I13: 150 – 180

EC BP BP EC EC EC

7.5 ± 0.1

50.3 ± 0.6

17.1 ± 0.5

115.5 ± 3.3

7.0 ± 0.2

46.7 ± 1.2

34.7 ± 0.4

231.3 ± 2.6

Proton Proton

9.6 ± 0.9

96 ± 9

81

17.2 ± 0.9

175 ± 9

*I13 corresponds to the highest synchrotron filling status, and I10 to a less filled one.

** The number of extracted particles does not belong to the number of particles applied in the treatment room due to a beam leakage of around 25% along the high energy beam transfer line.