Various in vivo experimental works carried out on different animals and organs have shown that it is possible to reduce the damage caused to healthy tissue still preserving the therapeutic efficacy on the tumor tissue, by drastically reducing the total time of dose delivery (<200 ms). This effect, called the FLASH effect, immediately attracted considerable attention within the radiotherapy community, due to the possibility of widening the therapeutic window and treating effectively tumors which appear radioresistant to conventional techniques. Despite the experimental evidence, the radiobiological mechanisms underlying the FLASH effect and the beam parameters contributing to its optimization are not yet known in details. In order to fully understand the FLASH effect, it might be worthy to investigate some alternatives which can further improve the tools adopted so far, in terms of both linac technology and dosimetric systems. This work investigates the problems and solutions concerning the realization of an electron accelerator dedicated to FLASH therapy and optimized for in vivo experiments. Moreover, the work discusses the saturation problems of the most common radiotherapy dosimeters when used in the very high dose-per-pulse FLASH conditions and provides some preliminary experimental data on their behavior.

The Novac7 and Liac are linear accelerators (linacs) dedicated to intraoperative radiation therapy (IORT), which produce high energy, very high dose‐per‐pulse electron beams. The characteristics of the accelerators heads of the Novac7 and Liac are different compared to conventional electron accelerators. The aim of this work was to investigate the specific characteristics of the Novac7 and Liac electron beams using the Monte Carlo method. The Monte Carlo code BEAMnrc has been employed to model the head and simulate the electron beams. The Monte Carlo simulation was preliminarily validated by comparing the simulated dose distributions with those measured by means of EBT radiochromic film. Then, the energy spectra, mean energy profiles, fluence profiles, photon contamination, and angular distributions were obtained from the Monte Carlo simulation. The Spencer‐Attix water‐to‐air mass restricted collision stopping power ratios (normalsW,air) were also calculated. Moreover, the modifications of the percentage depth dose in water (backscatter effect) due to the presence of an attenuator plate composed of a sandwich of a 2 mm aluminum foil and a 4 mm lead foil, commonly used for breast treatments, were evaluated. The calculated normalsW,air values are in agreement with those tabulated in the IAEA TRS‐398 dosimetric code of practice within 0.2% and 0.4% at zref (reference depth in water) for the Novac7 and Liac, respectively. These differences are negligible for practical dosimetry. The attenuator plate is sufficient to completely absorb the electron beam for each energy of the Novac7 and Liac; moreover, the shape of the dose distribution in water strongly changes with the introduction of the attenuator plate. This variation depends on the energy of the beam, and it can give rise to an increase in the maximum dose in the range of 3%–9%.PACS number: 87.56.‐v

The aim of this study was to investigate the dosimetric characteristics of the electron beams generated by the light intraoperative accelerator, Liac® (SORDINA, Italy), using Monte Carlo (MC) calculations. Moreover we investigated the possibility of characterizing the Liac® dosimetry with a minimal set of dosimetric data. In fact accelerator commissioning requires measurements of both percentage depth doses (PDDs) and off-axis profiles for all the possible combinations of energy, applicator diameter and bevelled angle. The Liac® geometry and water phantom were simulated in a typical measurement setup, using the MC code EGSnrc/BEAMnrc. A simulated annealing optimization algorithm was used in order to find the optimal non-monoenergetic spectrum of the initial electron beam that minimizes the differences between calculated and measured PDDs. We have concluded that, for each investigated nominal energy beam, only the PDDs of applicators with diameters of 30, 70 and 100 mm and the PDD without an applicator were needed to find the optimal spectra. Finally, the output factors of the entire set of applicator diameters/bevelled angles were calculated. The differences between calculated and experimental output factors were better than 2%, with the exception of the smallest applicator which gave differences between 3% and 4% for all energies. The code turned out to be useful for checking the experimental data from various Liac® beams and will be the basis for developing a tool based on MC simulation to support the medical physicist in the commissioning phase.

This report describes the present status of the detector design for SuperB. It is one of four separate progress reports that, taken collectively, describe progress made on the SuperB Project since the publication of the SuperB Conceptual

Our data confirm that neutron exposure levels around the new dedicated IORT accelerator are very low. Mobile shielding panels can be used to reduce x-ray levels to below regulatory levels without necessarily providing permanent shielding in the operating room.

Background: Conventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra-high dose per pulse as used for FLASH radiotherapy. Purpose: We present the design, development, and characterization of an ultrathin parallel plate IC that can be used in ultra-high dose rate (UHDR) deliveries with minimal recombination. Methods: The charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 μ s pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra-high pulse dose rate electron beams. Results: It has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra-high dose per pulse range, at standard voltage of clinical grade electrometers,well into several Gy per pulse.The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so-called ultrathin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 μ s pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 μ s pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch-Technische Bundesanstalt, PTB). Conclusions: This work demonstrates the ability to extend the dose rate operating range of ICs to ultra-high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams.

Purpose FLASH radiotherapy (RT) is an emerging technique in which beams with ultra‐high dose rates (UH‐DR) and dose per pulse (UH‐DPP) are used. Commercially available active real‐time dosimeters have been shown to be unsuitable in such conditions, due to severe response nonlinearities. In the present study, a novel diamond‐based Schottky diode detector was specifically designed and realized to match the stringent requirements of FLASH‐RT. Methods A systematic investigation of the main features affecting the diamond response in UH‐DPP conditions was carried out. Several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW‐Freiburg. Such devices were tested under electron UH‐DPP beams. The linearity of the prototypes was investigated up to DPPs of about 26 Gy/pulse and dose rates of approximately 1 kGy/s. In addition, percentage depth dose (PDD) measurements were performed in different irradiation conditions. Radiochromic films were used for reference dosimetry. Results The response linearity of the diamond prototypes was shown to be strongly affected by the size of their active volume as well as by their series resistance. By properly tuning the design layout, the detector response was found to be linear up to at least 20 Gy/pulse, well into the UH‐DPP range conditions. PDD measurements were performed by three different linac applicators, characterized by DPP values at the point of maximum dose of 3.5, 17.2, and 20.6 Gy/pulse, respectively. The very good superimposition of three curves confirmed the diamond response linearity. It is worth mentioning that UH‐DPP irradiation conditions may lead to instantaneous detector currents as high as several mA, thus possibly exceeding the electrometer specifications. This issue was properly addressed in the case of the PTW UNIDOS electrometers. Conclusions The results of the present study clearly demonstrate the feasibility of a diamond detector for FLASH‐RT applications.

The present study confirms that parallel-plate ionization chambers can properly and accurately substitute ferrous sulphate detectors in reference dosimetry of LIAC and NOVAC mobile linear accelerators. Therefore, we hope that the most commonly used protocols for reference dosimetry in external-beam radiotherapy will be updated in order to provide guidance in the calibration of electron beams from linear accelerators dedicated to IORT, so that users may benefit from specific, authoritative and up-to-date recommendations.