ESTRO 2023 - Abstract Book

S1573

Digital Posters

ESTRO 2023

The beam transport efficiency measured with a Faraday cup was about 80%, enabling >500 nA at the isocenter in UHDR mode. By comparison, the clinical beam optic has an efficiency of ~ 8%. The beam output was measured with a plane parallel ionization chamber (IC) (PPC05, IBA dosimetry). (Both literature data and onsite comparisons with a Faraday cup indicated no significant recombination effects) The IC measurements showed a non-linear MU vs. dose relation for varying beam currents, likely due to recombination effects in the IC of the nozzle beam monitoring system. For the same MU, the dose changed by ~ 15% when the current was increased from 300 nA to 700 nA at the cyclo. Small variations in beam current (< 5%) are associated with negligible dose variations (<1%), and the dose values are consistent given the same beam current and MU (range of variations < 1%). The results for the beam output are summarized in figure 2.

For preclinical irradiations, two field sizes were defined (4x4 cm^2 and 1.5x6 cm^2); both have a 135 mm RS to increase the spot and maximize the field dose rate, and use an aperture downstream of RS to sharpen the lateral penumbra. For both fields, a combination of beam current, spot positions, and MU per spot was found that ensures an adequate dose homogeneity and beam penumbra, a field dose rate of at least 70 Gy/s for the UHDR mode, and 1 Gy/s for the conventional mode, for a dose of 19 Gy RBE.

Conclusion The initial characterization indicates that our system is adequate for UHDR proton preclinical irradiations.

PO-1839 Radon-transformation and filtered back-projection - a way to fast Ad-hoc VMAT-adaptation?

K. Bratengeier 1 , K. Kirschbaum 1 , B. Hahn 2 , P. Paulus 3