Megavolt bremsstrahlung measurements from linear induction accelerators demonstrate possible use as a FLASH radiotherapy source to reduce acute toxicity

Abstract

Recent studies indicate better efficacy and healthy tissue sparing with high dose-rate FLASH radiotherapy (FLASH-RT) cancer treatment. This technique delivers a prompt high radiation dose rather than fractional doses over time. While some suggest thresholds of > 40 Gy s−1 with a maximal effect at > 100 Gy s−1, accumulated evidence shows that instantaneous dose-rate and irradiation time are critical. Mechanisms are still debated, but toxicity is minimized while inducing apoptosis in malignant tissue. Delivery technologies to date show that a capability gap exists with clinic scale, broad area, deep penetrating, high dose rate systems. Based on these trends, if FLASH-RT is adopted, it may become a dominant approach except in the least technologically advanced countries. The linear induction accelerator (LIA) developed for high instantaneous and high average dose-rate, species independent charged particle acceleration, has yet to be considered for this application. We review the status of LIA technology, explore the physics of bremsstrahlung-converter-target interactions and our work on stabilizing the electron beam. While the gradient of the LIA is low, we present our preliminary work to improve the gradient by an order of magnitude, presenting a point design for a multibeam FLASH-RT system using a single accelerator for application to conformal FLASH-RT.

Development of a compact linear accelerator to generate ultrahigh dose rate high‐energy X‐rays for FLASH radiotherapy applications

“…The maximum dose rate measured with EBT-3 film achieved 79.8 Gy/s at an SSD of 50 cm and 46.6 Gy/s at an SSD of 67.9 cm both at a 2.1 cm depth of water phantoms. The dose rates in this work are compared with other published studies of high energy UHDR X-rays 19,20,22 shown in Table 4. The mean dose rates of this work and PARTER 19 are measured data, and the mean dose rates of TRIUMF 20 and the induction linac in LLNL 22 are designed values.…”

“…where Ḋ is the maximum X-ray dose rate, E is the beam energy, I is the beam current, 𝛼 is an exponent factor of energy E, and d is the SSD of the radiation system with units of m. The dose rate Ḋ follows the inverse square law of the distance d. 𝛼 is related to the material and the thickness of the conversion target with a typical value ranging from 2.6 to 2.7. 22 According to Equation (1), a small increase in E can result in a significant increase in the X-ray dose rate.…”

“…For 10 MeV electron beams, a 2.6 mA mean current and 26 kW mean power are necessary to achieve 40 Gy/s mean dose rate X-rays at a 70 cm SSD according to the previous work. 22 In this work, a UHDR high-energy X-ray radiation platform was developed for FLASH-RT applications based on an S-band (2.856 GHz) BTW linac. The vertical platform setup is shown in Figure 1.…”

“…In addition, the Lawrence Livermore National Laboratory (LLNL) in the United States considered producing UHDR X-ray irradiation by linear induction accelerators, which use an induced electric field generated by magnetic cores to accelerate electron beams. 22 In that study, X-rays with a high instantaneous dose rate were achieved using a 17 MeV linear induction accelerator with a low repetition rate, and a stable high repetition rate was obtained using a 6.5 MeV linear induction accelerator, respectively. A conceptual design that combined high-energy X-ray beams with a high instantaneous dose rate and high repetition rate was also proposed in that work.…”

“…A conceptual design that combined high-energy X-ray beams with a high instantaneous dose rate and high repetition rate was also proposed in that work. 22 In contrast to superconducting, Rhodotron and induction accelerating techniques, room-temperature radiofrequency (RF) linear accelerators (linacs) are widely used in medical and industrial applications due to their compact size, simple operation and maintenance, and low cost. However, to the best of our knowledge, there are no experimental reports on UHDR high-energy X-rays for FLASH radiotherapy produced by roomtemperature RF linacs because of several considerable technical challenges.…”