Thermal limits on MV x‐ray production by bremsstrahlung targets in the context of novel linear accelerators

Abstract

Purpose To study the impact of target geometrical and linac operational parameters, such as target material and thickness, electron beam size, repetition rate, and mean current on the ability of the radiotherapy treatment head to deliver high-dose-rate x-ray irradiation in the context of novel linear accelerators capable of higher repetition rates/duty cycle than conventional clinical linacs. Methods The depth dose in a water phantom without a flattening filter and heat deposition in an x-ray target by 10 MeV pulsed electron beams were calculated using the Monte-Carlo code MCNPX, and the transient temperature behavior of the target was simulated by ANSYS. Several parameters that affect both the dose distribution and temperature behavior were investigated. The target was tungsten with a thickness ranging from 0 to 3 mm and a copper heat remover layer. An electron beam with full width at half maximum (FWHM) between 0–3 mm and mean current of 0.05–2 mA was used as the primary beam at repetition rates of 100, 200, 400, and 800 Hz. Results For a 10 MeV electron beam with FWHM of 1 mm, pulse length of 5 μs, by using a thin tungsten target with thickness of 0.2 mm instead of 1 mm, and by employing a high repetition rate of 800 Hz instead of 100 Hz, the maximum dose rate delivered can increase 2 times from 0.57 Gy/s to 1.16 Gy/s. In this simple model, the limiting factor on dose rate is the copper heat remover’s softening temperature, which was considered to be 500 °C in our study. Conclusions A high dose rate can be obtained by employing thin targets together with high repetition rate electron beams enabled by novel linac designs, whereas the benefit of thin targets is marginal at conventional repetition rates. Next generation linacs used to increase dose rate need different target designs compared to conventional linacs.

Bayesian optimization to design a novel x‐ray shaping device

“…In this context, it is important to note that there is currently no term in the objective function accounting for thermal stress on the target, and the target designs presented here would probably differ if this were considered. Modeling of thermal fatigue in bremsstrahlung targets is complex 3 and beyond the scope of this work. However, these effects should be incorporated into dedicated target and scanning magnet design efforts in future work, especially in the context of improved focusing of the electron beam to enable higher dose rates.…”

“…First, delivery speed with an MLC-based system is limited by both the speed of mechanical leaf motion, and limitations on the ability to cool bremsstrahlung x-ray targets, which are already operated very near the point of thermal fatigue. 3 These limitations mean that conventional x-ray systems are unable to take advantage of recent fundamental advances in electron accelerator technology which promise order of magnitude increases in electron beam intensity. 4 With SPHINX, mechanical motion is eliminated, and the electron beam energy is distributed over a much larger bremsstrahlung target.…”

An automated optimization strategy to design collimator geometry for small field radiation therapy systems

“…As can be seen from figure 1, the optimized SDC distances we obtained could be compatible with the CyberKnife system without interfering with other components in the head such as the primary collimator and electron filter, and therefore could be a practical solution. While dose rate might also be increased by increasing the electron beam current (beam loading and/or duty cycle) within the limits of radio frequency power and target heating (Wang et al 2017), the geometrical optimization would provide a substantial additional factor. This would allow the option of faster painting of larger volumes with the larger beams to reduce treatment times, while using smaller beams to achieve sharper gradients where necessary.…”

FLASH radiotherapy with photon beams

“…Wang et al used heat transport modeling and Monte Carlo simulations to optimize tungsten target design to achieve UHDR irradiations with a 10 MV X-ray beam generated by a 10 MeV 0.1-0.2 mA electron beam of various temporal structures delivered by a novel electron linac. 55 The highest dose rate of 1.16 Gy/s at 100 cm source-to-surface distance (SSD) was predicted for a 0.2 mm thick tungsten target with a copper heat remover irradiated by a 1 mm electron beam with 5 μs pulses delivered at a high 800 Hz repetition rate. In order to increase this dose rate to >40 Gy/s, the irradiation SSD would have to be decreased from 100 to 17 cm, which would increase uncertainties in delivery and inhomogeneity in the dose deposition.…”