Fiber-based In-vivo Realtime Dosimetry for Pulsed Radiotherapy – FIDELIO

State-of-the-art real-time dosimeters used for quality assurance or in-vivo dosimetry (IVD) measurements in radiotherapy (RT) have become ill-adapted to modern treatment practices which use highly conformational and stereotactic irradiations, or ‘pencil-beam scanning’ irradiations in the case of high-energy protontherapy (HEPT). The typical size of their sensitive volume (a few millimeters) does not allow an accurate resolution of the spot profiles and high dose gradients inherent in these techniques. They are also mainly based on radio-electric conversion which is not suited to MRI-guided RT nor to the high dose rates involved in the HEPT proton pulses or in the forthcoming flash-RT (however considered the major breakthrough of the next few years in RT).
The FIDELIO project aims at remedying these weaknesses by developing new radioluminescent silica-based fiber dosimeters. Radioluminescence (RL) is immune to electromagnetic fields and its efficiency is not limited, but enhanced, at high dose rates. The sensitive volume will be the fiber core itself. This brings two decisive advantages against existing or under development fiber-based dosimetric systems which use thick plastic fibers (1 mm in diameter) often combined with non-‘tissue equivalent’ inorganic scintillators. First, the RL yield of sensitized silica glasses can be high enough to reduce the core diameter down to a few tens of micrometers. This will offer high spatial resolution but also “invisibility” to treatment beams (for IVD) since the standard outer diameter of silica fibers is 125 micrometers. Second, silica glasses are nearly tissue equivalent with respect to both photons and protons at clinical energies and this markedly reduces the energy dependence of their response at a given dose in water. Two kinds of probes will be developed according to particular requirements specifications: one optimized for small photon fields and flash-RT, the other for HEPT pencil beams. At the end of the project, optimized probes will come with a full demonstrator breadboard including a remote Cerenkov-corrected RL detection and rad-hard transport fibers, and with related data sheets.
The FIDELIO consortium comprises the most legitimate medical, academic and industry players at national level to cover the full range of the expertise needed to meet the set objectives. Beyond management and dissemination/valorization workpackages (WP1 and 6), the scientific development is organized in three WPs focused on the experiment development at the material (WP2), component (WP3) and system level (WP4), and one WP dedicated to modeling, including Monte-Carlo simulations (WP5). The general idea is to design and fabricate RL probes in WP2 that will be submitted to a pre-clinical screening through laboratory performance tests in WP3 (RL sensitivity and its dependence on temperature, particle energy, cumulated dose…). Those passing these tests will be mounted on the RL system and will enter the clinical assessment of WP4 (Output Factor, Percentage Depth Dose, profiles and angular dependence measurements) under treatment beams at CyberKnife (6 MV X rays) and ProteusOne synchrocyclotron (protons up to 226 MeV). These characterizations will be confronted to reference data, notably provided by Monte Carlo simulations of WP5. Performances in WP3 or WP4 will be analyzed thanks to the models of WP5 to improve the design/composition in tasks of WP2. This important feedback will allow an optimization process to converge towards best photon and proton dosimeters according to respective requirements. Optimized probes will be finally calibrated against reference dosimeters or absolute dosimetric data (protons) before being submitted to reproducibility tests and error assessment.
Dissemination of the project results will comply with the Open Science policy of the ANR.