Ultra‐high dose rate radiation production and delivery systems intended for FLASH

Abstract

Higher dose rates, a trend for radiotherapy machines, can be beneficial in shortening treatment times for radiosurgery and mitigating the effects of motion. Recently, even higher doses (e.g., 100 times greater) have become targeted because of their potential to generate the FLASH effect (FE). We refer to these physical dose rates as ultra‐high (UHDR). The complete relationship between UHDR and the FE is unknown. But UHDR systems are needed to explore the relationship further and to deliver clinical UHDR treatments, where indicated. Despite the challenging set of unknowns, the authors seek to make reasonable assumptions to probe how existing and developing technology can address the UHDR conditions needed to provide beam generation capable of producing the FE in preclinical and clinical applications. As a preface, this paper discusses the known and unknown relationships between UHDR and the FE. Based on these, different accelerator and ionizing radiation types are then discussed regarding the relevant UHDR needs. The details of UHDR beam production are discussed for existing and potential future systems such as linacs, cyclotrons, synchrotrons, synchrocyclotrons, and laser accelerators. In addition, various UHDR delivery mechanisms are discussed, along with required developments in beam diagnostics and dose control systems.

FLASH radiotherapy treatment planning and models for electron beams

“…Furthermore, several aspects related to delivery technology render VHEE beams an attractive candidate modality for FLASH RT. With current technology, small-sized VHEE beams can be readily produced and scanned at UHDR, and VHEE accelerators and gantries are more compact and cheaper than current proton beam technology [5,23,62,63]. While to date there are no clinical VHEE RT devices, interest in creating such devices has seen a resurgence [5,23,62,63], taking on the challenges of designing and building clinical UHDR VHEE RT devices for FLASH RT [64][65][66][67][68].…”

“…With current technology, small-sized VHEE beams can be readily produced and scanned at UHDR, and VHEE accelerators and gantries are more compact and cheaper than current proton beam technology [5,23,62,63]. While to date there are no clinical VHEE RT devices, interest in creating such devices has seen a resurgence [5,23,62,63], taking on the challenges of designing and building clinical UHDR VHEE RT devices for FLASH RT [64][65][66][67][68]. In the absence of existing VHEE RT devices, UHDR VHEE treatment planning and beam modelling was so far focused on predicting VHEE dose distributions and temporal beam delivery characteristics to assist the design and optimization of future VHEE devices and to compare them with standard-of-care RT.…”

“…The investigated delivery concepts for UHDR VHEE RT reach from 3D conformal delivery using a few fixed-beam portals [65,69] to intensity modulated delivery of 0.1-5 mm beamlets from 13 or more fixed-beam portals [64,[70][71][72], see Table 1 b). Technological aspects of UHDR VHEE RT delivery were recently reviewed elsewhere [23,62]. Principal trade-offs to be assessed and optimised by UHDR VHEE treatment planning are dosimetric target coverage and conformity, and temporal dose delivery aspects that optimise the FLASH effect.…”

“…In particular, large doses above 5-10 Gy needed to be delivered within some 100 ms in a given tissue region to trigger and optimise the FLASH effect [73,74]. Current C-arm gantry concepts with rotation speeds of the scale of a minute may no longer be applicable and fixed beam lines or motionless or fast-rotating gantries will become mandatory [62,64,65,75]. However, despite substantial prevailing uncertainties in the knowledge and modelling of the FLASH effect, treatment planning studies are already now useful in evaluating feasibility of UHDR VHEE device configurations.…”

“…All curves are for parallel 10x10 cm 2 fields, unless specified to be focused [24,98,131]. [64,70], (f) 3D-CRT treatments using a few VHEE beams and fixed beam lines can be achieved in short time scales compatible with the FLASH effect [23,62,65,66], (g) Assuming a protection of all organs-at-risk and healthy tissues by a dose-modifying factor of 1, 0.9, 0.8, 0.7, and 0.6, (h) Using partially 2D anatomies, (i) For PHASER project [64], (j) For "scaled-down" PHASER project. (k) Dose engine was not reported.…”