ESTRO 2022 - Abstract Book

S163

Abstract book

ESTRO 2022

B.S. Sorensen

Denmark Abstract not available

SP-0201 Commissioning an experimental beam line for in vivo FLASH experiments

M. Kim 1

1 University of Pennsylvania, Radiation Oncology, Philadelphia, USA

Abstract Text To investigate the normal tissue sparing effect of FLASH radiotherapy a research proton beam line was commissioned to deliver ultra-high dose rates for pre-clinical studies. Many groups have been investigating the use of electrons for ultra- high dose rate studies; however, protons provide unique advantages due to their physical properties. A double scattered system was devised with appropriate collimation for use in small animal studies. Absolute dose was measured using a NIST- traceable calibrated Advanced Markus chamber and validated against absolute integral charge measurements using a Faraday cup. Initial studies included mouse whole abdomen irradiation with both FLASH and standard dose rates to compare fibrosis and loss of proliferating cells in intestinal crypts with a shoot-through proton beam of 230 MeV. Later studies included the use of a ridge filter to determine if the FLASH effect was seen in the spread-out Bragg peak (SOBP). Proton FLASH studies show increased normal tissue sparing in in vivo models using both the shoot-through and SOBP techniques as well as equivalent tumor control between the two dose rates. Real-time dose rates were determined using a NaI detector to measure prompt gamma rays. Beam control systems were validated to have accurate control of beam flux on a millisecond time scale. After initial commissioning activities and verification of dosimetry, the research beam line was also utilized for the enrollment of canine patients for a large animal clinical trial.

SP-0202 Translating FLASH into the clinic: Beam delivery and dosimetry

R. Moeckli 1

1 Institue of Radiation Physics, CHUV, Lausanne, Switzerland

Abstract Text It has been observed that a biological effect called FLASH effect, which spares normal tissues while maintaining the same tumor control, appears when the dose is delivered at an ultra-high dose rate (UHDR). The FLASH effect has been observed with typical irradiations of about 10-20 Gy in less than 100ms in different animal species (fish eggs, mice, cats and pigs). A first patient was treated in 2019 and two clinical protocols have since been established. Most experiments were performed with UHDR electron beams, but also with photons and protons. The redundant observations of FLASH effect on animals, make it relevant to consider a clinical transfer under specific conditions. Nevertheless, important questions remain. There is still no metrological traceability of UHDR beams and redundant dosimetry must be used to characterize the reference dose. The dosimetric instruments needed for (absolute and) relative dosimetry are not yet optimized for UHDR even if some promising detectors are emerging. The detectors used today for UHDR measurements are those that were used for conventional beams. This leads to many practical problems during commissioning and quality assurance measurements, such as the difficulty of getting a direct reading from the detector or the difficulty related to radiation protection because the beam simply cannot be left on for a few seconds or minutes, as is the case during conventional commissioning. Another concern is the monitoring and safety of the beam. In the case of UHDR irradiations, monitoring is not only related to the beam fluence, but also to the beam structure and in particular to the number of pulses that will be delivered. As the durations are related to µ s for the pulse length and hundreds of ms for the total delivery, the corresponding flux makes the traditional transmission chambers obsolete due to saturation effect. Therefore, another device is needed that can control the beam (in terms of pulse structure and pulse counting), but also, and more problematically, adapt the pulse configuration to delivery variations and ensure patient safety. When talking about a dose delivered in 10 pulses, an error of one pulse results in a 10% difference! In addition, the current description of safety features needed for treatment machines in the usual guidelines may also be outdated or may not allow for a specific detector that could be used for these tasks. Clinical transfer requires additional features for the beams, namely that they must be large and energetic enough to treat the usual deep-seated tumors. The question of which type of beam, between very high energy electrons (VHEE), protons or photons, should be used remains open. Finally, the biological cause of the FLASH effect is not yet understood and the physical parameters of the beams that trigger the FLASH effect are not fully defined. In summary, FLASH RT is very promising for cancer treatment in radiotherapy, but important questions remain to be answered before a wide diffusion of this treatment technique becomes possible in clinical practice.

SP-0203 Translating FLASH into the clinic: Treatment planning and preparing for clinical trials