ABSTRACT
Microbeam radiotherapy (MRT) is a novel concept in radiation oncology with arrays of alternating micrometer-wide high-dose peaks and low-dose valleys. Preclinical experiments have shown a lower normal tissue toxicity for MRT with similar tumor control rates compared to conventional radiotherapy. A promising candidate for the demanded compact radiation source is the line-focus x-ray tube. Here, we present the setup of a prototype for an electron accelerator being able to provide a suitable x-ray beam for the tube. Several beam dynamic calculations and simulations were performed concerning particle tracking, thermal and electrostatic properties of the electron source, resulting in a proper beamline, including the cathode, the pierce electrode (PE) and the focusing magnets. These parts are discussed separately. The simulations showed that a rectangular cathode with a small width of 0.4mm is mandatory. To quickly shut down the electron beam, an additional voltage of -600V must be applied to the PE. Moreover, the electric field inside the vacuum chamber stays below 10MVm-1 to minimize the risk of field emission. The thermal simulation validates a small displacement of 0.1mm of the heated cathode with respect to the PE, which must be considered during manufacturing of the cathode-PE assembly. The simulations lead to an adequate choice of cathode, electrodes and beamline to achieve the required focal spot of 0.05×20mm2 with a beam current of 0.3A and an electron energy of 300keV. With this setup first MRT experiments with high dose rates up to 10Gys-1 can be executed.
Subject(s)
Neoplasms , Radiation Oncology , Humans , X-Rays , Electrons , Radiotherapy Planning, Computer-Assisted/methods , Etoposide , Monte Carlo MethodABSTRACT
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 (UHDRs) of more than 100 Gy/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.
Subject(s)
Radiation Oncology , X-Ray Therapy , Hot Temperature , Monte Carlo Method , X-RaysABSTRACT
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.
ABSTRACT
This article reports stable photoluminescence and high-contrast optically detected electron spin resonance (ODESR) from single nitrogen-vacancy (NV) defect centers created within ultrasmall, disperse nanodiamonds of radius less than 4 nm. Unexpectedly, the efficiency for the production of NV fluorescent defects by electron irradiation is found to be independent of the size of the nanocrystals. Fluorescence lifetime imaging shows lifetimes with a mean value of around 17 ns, only slightly longer than the bulk value of the defects. After proper surface cleaning, the dephasing times of the electron spin resonance in the nanocrystals approach values of some microseconds, which is typical for the type Ib diamond from which the nanoparticle is made. We conclude that despite the tiny size of these nanodiamonds the photoactive nitrogen-vacancy color centers retain their bulk properties to the benefit of numerous exciting potential applications in photonics, biomedical labeling, and imaging.