ESTRO 2024 - Abstract Book

S3239

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

1. Gebauer, B., Pawelke, J., Hoffmann, A. & Lühr, A. Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in-beam MR scanner. Med Phys (2023) doi:10.1002/MP.16448. 2. Arjomandy, B. et al. AAPM task group 224: Comprehensive proton therapy machine quality assurance. Med Phys 46, e678–e705 (2019). 3. Lin, H. et al. Applications of various range shifters for proton pencil beam scanning radiotherapy. Radiation Oncology 16, (2021). 4. Gueli, A. M., Cavalli, N., De Vincolis, R., Raffaele, L. & Troja, S. O. Background fog subtraction methods in Gafchromic® dosimetry. Radiat Meas 72, 44–52 (2015).

1195

Proffered Paper

Simultaneous measurement of instantaneous dose rate and O 2 dynamics during proton PBS irradiation

Eleni Kanouta 1 , Line Kristensen 1,2 , Jacob G Johansen 1 , Brita S Sørensen 1,2 , Per R Poulsen 1

1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark. 2 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus, Denmark

Purpose/Objective:

FLASH radiotherapy utilizes ultra-high dose rates to reduce radiation-induced toxicities. While the underlying mechanism of FLASH is not fully understood, the partial oxygen pressure (pO 2 ) might be essential for understanding the radioprotection of the normal tissue. During proton pencil beam scanning (PBS) irradiations, the changes in pO 2 at a point will depend on the spatial and temporal structure of the beam. In this study, the change in pO 2 in a water solution was measured during proton PBS irradiations and correlated to simultaneous dose rate variations in the solution assessed by point dose measurements with high temporal resolution.

Material/Methods:

The change in pO 2 -level was tested in four irradiations. All irradiations were performed with variations of a proton PBS scanning pattern previously used for FLASH studies in mice. The scanning pattern consisted of 5 x 7 spots arranged with 5mm spacing and horizontal scanning direction (Figure 1, top). The irradiation was delivered with a 244MeV transmission beam and a field dose rate of 2.7Gy/s (maximum instantaneous dose rate of 40Gy/s). The pattern variations consisted of changing the scanning pattern to vertical scanning and increasing the spot distances to 7mm. The fields were delivered to the setup shown in Figure 1. A sealed cylindrical container (5mm diameter, 10mm height) with a 1µM/ml solution of the Oxyphor PtG4 1 probe in saline water was placed in the beam in the isocenter plane. A calibrated scintillator detector 2 , placed on the exit side of the cylindrical container, directly measured the instantaneous dose rate in a single point with 50kHz sampling rate.