ESTRO 2024 - Abstract Book

S3209

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

To predict the proton radiation range accurately during treatment planning, device-specific empirical and stoichiometric Hounsfield Look-Up Tables (HLUT) were created. This involved using tissue-equivalent materials in head and body phantoms for each imaging protocol. To characterize the proton beam, lateral and depth dose profiles were measured at four positions along the beam's path at 90 MeV energy, employing EBT3 film stacks, a microdiamond detector and a Giraffe multilayer ionization chamber for each position. For the irradiation with photons, depth dose profiles and lateral profiles were obtained as well using EBT3 dosimetry films for seven collimators at the SmART+ IB’s isocenter operating at 200 kV, 20 mA, with a 0.5 mm copper filter.

Results:

The analysis of the CBCT homogeneity reveals a negligible range uncertainty of approximately 0.1 mm for the head phantom, and the 60 kV, 0.5 mA imaging protocol is chosen for future experiments due to its balance between low imaging dose (<26 mGy) and high image quality.

The QA procedure provides accurate alignment with a deviation of less than 0.35 mm and takes approximately 45 minutes before each beamtime.

The HLUTs for all four imaging protocols show consistent trends in low CT numbers but diverges for high CT number tissues. The impact of the transport box and mouse bed on the HLUTs is recognized due to the altered CT numbers. To address this, the empirical HLUT tailored for the 60 kV, 0.5 mA protocol has been selected for mouse experiments, and its validity has been confirmed with an uncertainty of ∆r80 ≤ 0.51 mm. A comprehensive 3D analysis of both proton and photon beams yield expected FWHM and penumbra values. Using various collimators, the dose rates range from 0.79-2.75 Gy/min at a 10 mm depth for photons and reach 17.72 Gy/min at the Bragg peak for protons. Both models are implemented in the treatment planning software µRayStation and were subsequently validated.