Clinical microbeam radiation therapy with a compact source: Specifications of the line-focus X-ray tube.

Background and purpose: Microbeam radiotherapy (MRT) is a preclinical concept in radiation oncology with arrays of alternating micrometer-wide high-dose peaks and low-dose valleys. Experiments demonstrated a superior normal tissue sparing at similar tumor control rates with MRT compared to conventional radiotherapy. Possible clinical applications are currently limited to large third-generation synchrotrons. Here, we investigated the line-focus X-ray tube as an alternative microbeam source. Materials and methods: We developed a concept for a high-voltage supply and an electron source. In Monte Carlo simulations, we assessed the influence of X-ray spectrum, focal spot size, electron incidence angle, and photon emission angle on the microbeam dose distribution. We further assessed the dose distribution of microbeam arc therapy and suggested to interpret this complex dose distribution by equivalent uniform dose. Results: An adapted modular multi-level converter can supply high-voltage powers in the megawatt range for a few seconds. The electron source with a thermionic cathode and a quadrupole can generate an eccentric, high-power electron beam of several 100 keV energy. Highest dose rates and peak-to-valley dose ratios (PVDRs) were achieved for an electron beam impinging perpendicular onto the target surface and a focal spot smaller than the microbeam cross-section. The line-focus X-ray tube simulations demonstrated PVDRs above 20. Conclusion: The line-focus X-ray tube is a suitable compact source for clinical MRT. We demonstrated its technical feasibility based on state-of-the-art high-voltage and electron-beam technology. Microbeam arc therapy is an effective concept to increase the target-to-entrance dose ratio of orthovoltage microbeams

THE LINE FOCUS X-RAY TUBE: AN X-RAY SOURCE FOR FLASH AND SPATIALLY FRACTIONATED RADIATION THERAPY

Background and Aims: FLASH and spatially fractionated radiationtherapy (SFRT) demonstrated reduced side effects at equal tumourcontrol compared to conventional radiotherapy. Currently onlylarge synchrotrons may facilitate clinical x-ray FLASH or SFRTtreatments. We are constructing a prototype of an innovative, table-top x-ray source that will allow FLASH and SFRT treatments. Thesource is based on the line focus x-ray tube (LFxT) concept and willeventually deliver dose rates of up to 200 Gy/s.Methods: We designed a thermionic electron gun that generatesa low-emittance, high-current electron beam at 300 keV. Twoquadrupole magnets focus the electrons onto a 50 micrometer wide focalspot on a tungsten-molybdenum target that spins at 250 Hz. Weassessed the radiation field, temperature and mechanical stressconditions with finite-element and Monte Carlo simulations. Theseparation tube equipped motor drive and liquid metal bearings fitfor operation in ultra-high vacuum. We developed a high-voltagesupply based on modular multi-level converter (MMC) technologyfor increased power in a future clinical source.Results: Finite element simulations showed an operation of theLFxT in the heat capacity limit permitting substantially enhanceddose rates at small focal spot sizes. Thermal and mechanical stressesare tolerated by the target design. An MMC based DCDC converterwith 320 battery-powered units can store enough energy to supplythe source with 2 MW electrical power in a duty cycle of 2%

Heat management of a compact x-ray source for microbeam radiotherapy and FLASH treatments.

BACKGROUND: Microbeam and x-ray FLASH radiation therapy are innovative concepts that promise reduced normal tissue toxicity in radiation oncology without compromising tumor control. However, currently only large third-generation synchrotrons deliver acceptable x-ray beam qualities and there is a need for compact, hospital-based radiation sources to facilitate clinical translation of these novel treatment strategies. PURPOSE: We are currently setting up the first prototype of a line-focus x-ray tube (LFxT), a promising technology that may deliver ultra-high dose rates (UHDR) of more than 100Gy/s from a table-top source. The operation of the source in the heat capacity limit allows very high dose rates with micrometer-sized focal spot widths. Here, we investigate concepts of effective heat management for the LFxT, a prerequisite for the performance of the source. METHODS: For different focal spot widths, we investigated the temperature increase numerically with Monte Carlo simulations and finite element analysis (FEA). We benchmarked the temperature and thermal stresses at the focal spot against a commercial x-ray tube with similar power characteristics. We assessed thermal loads at the vacuum chamber housing caused by scattering electrons in Monte Carlo simulations and FEA. Further, we discuss active cooling strategies and present a design of the rotating target. RESULTS: Conventional focal spot widths led to a temperature increase dominated by heat conduction, while very narrow focal spots led to a temperature increase dominated by the heat capacity of the target material. Due to operation in the heat capacity limit, the temperature increase at the focal spot was lower than for the investigated commercial x-ray tube. Hence, the thermal stress at the focal spot of the LFxT was considered uncritical. The target shaft and the vacuum chamber housing require active cooling to withstand the high heat loads. CONCLUSIONS: The heat capacity limit allows very high power densities at the focal spot of the LFxT and thus facilitates very high dose rates. Numerical simulations demonstrated that the heat load imparted by scattering electrons requires active cooling