Technical note: A small animal irradiation platform for investigating the dependence of the FLASH effect on electron beam parameters

A systematic review of electron FLASH dosimetry and beam control mechanisms utilized with modified non-clinical LINACs

BACKGROUND

FLASH has been shown to spare normal tissue toxicity while maintaining tumor control. However, existing irradiation platforms and dosimetry are not compatible. Consequently, an abundance of FLASH delivery devices and new dosimetry across all modalities has been created. Many review articles concluded that dosimetry is modality-dependent. Focusing on electrons, researchers have modified clinical LINACs to enable FLASH dose rates. Modified LINACs caused the development of unique control systems that have yet to be characterized. Improvement could be made when considering the organization of reviews.

PURPOSE

To systematically perform a literature survey on electron FLASH dosimetry and beam control mechanisms with modified LINACs, detail where articles originated, and organize the results.

METHODS

A literature survey was performed from two websites using specified keywords and sifted results to find articles that fit the criteria. The results were organized in tables and summaries effectively by matching up dosimeters with their measurement goal, referring to their specific models, outlining the irradiation conditions they were tested in, and detailing their calibration procedure. Furthermore, included was the unique topic of control mechanisms.

RESULTS

Twenty-eight matches were found. Various dosimeters were examined to measure absorbed dose, beam characteristics (BC), dose per pulse (DPP), and pulse counting (PC). Specific detectors and the irradiation conditions are organized and presented in a table. Each model's pros and cons are presented in another table for further consideration. A third table is provided to detail beam control methods.

CONCLUSIONS

Dosimetry is majorly film-based for absorbed dose and beam characteristic measurements. Many candidates for dosimeters for the use of DPP and PC have been tested, but they have yet to be tested without limitations. Beam control mechanisms primarily consist of unacceptable delivery errors. Many suggestions for improvement were given, mainly consisting of finding new dosimeters and modulating the dose DPP.

What's in a Proton FLASH Beam? Characterizing Ultra-High Dose Rate Protons Using a Commercial Plastic Scintillator

While biological studies of the FLASH effect in proton beams have mainly been performed in the plateau region at maximum beam energy and current, this type of delivery has limited clinical applications. Naturally, it is anticipated that plans to treat patients clinically with FLASH-radiotherapy (FLASH-RT) will capitalize on the Bragg peak. However, as the proton spot widens with depth, the time required to deliver the entire dose to any single point increases. This decreases the dose rate, making the ultra-high dose rates required to trigger the FLASH effect harder to achieve over large areas. Importantly, the dose rate is difficult to measure directly. Time and dose linearity of a fast-resolving commercial plastic scintillation detector were characterized against an ionization chamber. The percent depth dose of a 250 MeV proton beam scanned across a small area (3.5 × 3.5 cm2) was measured at depths of 3-40 cm in solid water. The plastic scintillation detector was used to evaluate the instantaneous and voxel-averaged dose rates as a function of depth for conventional (2 nA nozzle current) and ultra-high dose rate (100 nA) beams. The response of the plastic scintillation detector was shown to be linear with time (±2.5 ms) and absorbed dose (±2%). The scintillator and ionization chamber measurements agreed well as a function of depth (and therefore energy) within 2% for depths <34 cm. Beyond 34 cm, expected quenching effects were observed in the plastic scintillation detector. The voxel-averaged dose rate varied from 52.7 Gy/s at the entrance to 29.3 Gy/s at mid-depth, to 70.4 Gy/s near the Bragg peak, while the maximum instantaneous dose rate decreased from 472 Gy/s near the entrance to 236 Gy/s at the Bragg peak. The plastic scintillation detector has proven useful for investigators to evaluate the complex relationship between dose rate and pencil-beam scanning ultra-high dose rate beam characteristics. There is a loss of dose rate near the Bragg peak due to spot widening, which may acutely impact our ability to exploit the FLASH effect for sparing normal tissues upstream of the intended treatment area. A thorough preclinical investigation of whether the FLASH effect is maintained near the Bragg peak is necessary before this technique can begin translation to the clinic.

A high-throughput focused collimator for OAR-sparing preclinical proton FLASH studies: commissioning and validation

Objective. To fabricate and validate a novel focused collimator designed to spare normal tissue in a murine hemithoracic irradiation model using 250 MeV protons delivered at ultra-high dose rates (UHDRs) for preclinical FLASH radiation therapy (FLASH-RT) studies.Approach. A brass collimator was developed to shape 250 MeV UHDR protons from our Varian ProBeam. Six 13 mm apertures, of equivalent size to kV x-ray fields historically used to perform hemithorax irradiations, were precisely machined to match beam divergence, allowing concurrent hemithoracic irradiation of six mice while sparing the contralateral lung and abdominal organs. The collimated field profiles were characterized by film dosimetry, and a radiation survey of neutron activation was performed to ensure the safety of staff positioning animals.Main results. The brass collimator produced 1.2 mm penumbrae radiation fields comparable to kV x-rays used in preclinical studies. The penumbrae in the six apertures are similar, with full-width half-maxima of 13.3 mm and 13.5 mm for the central and peripheral apertures, respectively. The collimator delivered a similar dose at an average rate of 52 Gy s-1for all apertures. While neutron activation produces a high (0.2 mSv h-1) initial ambient equivalent dose rate, a parallel work-flow in which imaging and setup are performed without the collimator ensures safety to staff.Significance. Scanned protons have the greatest potential for future translation of FLASH-RT in clinical treatments due to their ability to treat deep-seated tumors with high conformality. However, the Gaussian distribution of dose in proton spots produces wider lateral penumbrae compared to other modalities. This presents a challenge in small animal pre-clinical studies, where millimeter-scale penumbrae are required to precisely target the intended volume. Offering high-throughput irradiation of mice with sharp penumbrae, our novel collimator-based platform serves as an important benchmark for enabling large-scale, cost-effective radiobiological studies of the FLASH effect in murine models.