BACKGROUND:Intraoperative radiation therapy (IORT) using the INTRABEAM system has shown promise in glioblastoma treatment. However, accurate dosimetry remains challenging due to the low-energy photons used and the heterogeneity of tissues in the brain. Current clinical practice relies on the TARGIT method, but more robust approaches, including the TG-43 formalism and Monte Carlo (MC) simulations, warrant investigation for potential improvements in dose calculation accuracy. PURPOSE:To evaluate the TG-43 dosimetry formalism for IORT dose calculations in glioblastoma treatment using the INTRABEAM system, comparing it with the TARGIT method and MC simulations. METHODS:We analyzed the dose distributions in 20 patients from the INTRAGO trial. The TG-43 formalism was validated against MC simulations in water ( MC w ${\rm MC}_{\rm w}$ ) using global/local dose differences and gamma analysis (1%/1mm). Organ at risk (OAR) doses were calculated using TG-43, TARGIT, MC w ${\rm MC}_{\rm w}$ , and MC in heterogeneous media ( MC het ${\rm MC}_{\rm het}$ ). Combined IORT and external beam radiotherapy (EBRT) doses were evaluated. RESULTS:TG-43 showed good agreement with MC w ${\rm MC}_{\rm w}$ , with a 98.0% gamma pass rate. The mean global dose difference was 0.07%  ± $\pm$  0.29%, with TG-43 slightly overestimating dose compared to MC w ${\rm MC}_{\rm w}$ . For OAR dose comparisons using TG-43 as reference, TARGIT underestimated doses by 0.1%-1.7%, while MC het ${\rm MC}_{\rm het}$ showed larger differences near bony structures (up to 1.9%  ± $\pm$  1.4% for optic nerves). Combined IORT+EBRT analysis revealed more OAR constraint violations than identified by current clinical practice. Calculation times for TG-43 (0.6 s on average) were significantly shorter than for MC simulations (16-18 h on a computing cluster). CONCLUSIONS:The TG-43 formalism provides a reasonable compromise between accuracy and computational efficiency for IORT dose calculations in glioblastoma treatment. It offers improved accuracy over TARGIT while being significantly faster and thus more feasible for intraoperative use than full MC simulations. Implementation of validated volumetric dose calculation methods like TG-43 has the potential to improve the accuracy of IORT treatment planning and OAR dose assessment.

Ultrashort electron beams with femtosecond to picosecond bunch durations offer unique opportunities to explore active research areas ranging from ultrafast structural dynamics to ultra-high dose-rate radiobiological studies. It presents a straightforward method to generate relativistic electron beams in ambient air via the tight focusing of a few-cycle, mJ-class femtosecond infrared laser. It demonstrates experimentally that electrons can reach up to 1.4 MeV at a dose-rate of 0.15 Gy/s, providing enough dose rate for radiation therapy applications. 3D Particle-In-Cell simulations confirm that the acceleration mechanism is based on the relativistic ponderomotive force and show theoretical agreement with the measured electron energies and divergence. Relativistic peak intensities up to 1019 Wcm-2 are reached in ambient air due to a very low B-integral accumulation during focusing, which prevents intensity clamping. Furthermore, it discusses the scalability of this method with the continuing development of mJ-class high average power lasers, and providing a promising approach for FLASH radiation therapy. The generation of a high dose-rate (0.15 Gy/s), 1 MeV electron beam produced through pondermotive laser acceleration simply by tight focusing a mJ-class, femtosecond, 100 Hz, infrared laser in ambient air is reported. The measured beam characteristics are supported by 3D Particle-In-Cell simulations. The technique is scalable and provides a promising approach for FLASH radiation therapy.image

Objective.Relative biological effectiveness (RBE) differs between radiation qualities. However, an RBE of 1.0 has been established for photons regardless of the wide range of photon energies used clinically, the lack of reproducibility in radiobiological studies, and outdated reference energies used in the experimental literature. Moreover, due to intrinsic radiosensitivity, different cancer types have different responses to radiation. This study aimed to characterize the RBE of clinically relevant high and low photon energiesin vitrofor three human cancer cell lines: HCT116 (colon), HeLa (cervix), and PC3 (prostate).Approach.Experiments were conducted following dosimetry protocols provided by the American Association of Physicists in Medicine. Cells were irradiated with 6 MV x-rays, an192Ir brachytherapy source, 225 kVp and 50 kVp x-rays. Cell survival post-irradiation was assessed using the clonogenic assay. Survival fractions were fitted using the linear quadratic model, and survival curves were generated for RBE calculations.Main results.Cell killing was more efficient with decreasing photon energy. Using 225 kVp x-rays as the reference, the HCT116 RBESF0.1for 6 MV x-rays,192Ir, and 50 kVp x-rays were 0.89 ± 0.03, 0.95 ± 0.03, and 1.24 ± 0.04; the HeLa RBESF0.1were 0.95 ± 0.04, 0.97 ± 0.05, and 1.09 ± 0.03, and the PC3 RBESF0.1were 0.84 ± 0.01, 0.84 ± 0.01, and 1.13 ± 0.02, respectively. HeLa and PC3 cells had varying radiosensitivity when irradiated with 225 and 50 kVp x-rays.Significance.This difference supports the notion that RBE may not be 1.0 for all photons through experimental investigations that employed precise dosimetry. It highlights that different cancer types may not have identical responses to the same irradiation quality. Additionally, the RBE of clinically relevant photons was updated to the reference energy of 225 kVp x-rays.

Purpose: The intraoperative radiotherapy in newly diagnosed glioblastoma multiforme (INTRAGO) clinical trial assesses sur-vival in patients with glioblastoma treated with intraoperative radiation therapy (IORT) using the INTRABEAM. Treatment plan-ning for INTRABEAM relies on vendor-provided in-water depth dose curves obtained according to the TARGeted Intraoperative radioTherapy (TARGIT) dosimetry protocol. However, recent studies have shown discrepancies between the estimated TARGIT and delivered doses. This work evaluates the effect of the choice of dosimetry formalism on organs at risk (OAR) doses.Methods and Materials: A treatment planning framework for INTRABEAM was developed to retrospectively calculate the IORT dose in 8 INTRAGO patients. These patients received an IORT prescription dose of 20 to 30 Gy in addition to external beam radiation therapy. The IORT dose was obtained using (1) the TARGIT method; (2) the manufacturer's V4.0 method; (3) the CQ method, which uses an ionization chamber Monte Carlo (MC) calculated factor; (4) MC dose-to-water; and (5) MC dose-to-tissue. The IORT dose was converted to 2 Gy fractions equivalent dose. Results: According to the TARGIT method, the OAR dose constraints were respected in all cases. However, the other formal-isms estimated a higher mean dose to OARs and revealed 1 case where the constraint for the brain stem was exceeded. The addition of the external beam radiation therapy and TARGIT IORT doses resulted in 10 cases of OARs exceeding the dose con-straints. The more accurate MC calculation of dose-to-tissue led to the highest dosimetric differences, with 3, 3, 2, and 2 cases (out of 8) exceeding the dose constraint to the brain stem, optic chiasm, optic nerves, and lenses, respectively. Moreover, the mean cumulative dose to brain stem exceeded its constraint of 66 Gy with the MC dose-to-tissue method, which was not evi-dent with the current INTRAGO clinical practice.Conclusions: The current clinical approach of calculating the IORT dose with the TARGIT method may considerably under-estimate doses to nearby OARs. In practice, OAR dose constraints may have been exceeded, as revealed by more accurate methods. (c) 2023 Elsevier Inc. All rights reserved.

We present a straightforward method to generate MeV-ranged high dose-rate electron beams in ambient air through the tight focusing of a mJ-class femtosecond laser. We demonstrate that relativistic intensities are reached through an intensity clamping suppression effect and that the technique is promising for performing FLASH radiation therapy.

Manipulating unconfined ultrashort pulsed laser beams is increasingly common in academic research laboratories. Normally, health risks associated with beam propagation and alignment can be mitigated using protective eyewear and following standard laser safety guidelines. A recent investigation [1] has uncovered an additional, unexpected health risk. While working with High Numerical Aperture (HNA) reflective optics in ambient air, it was demonstrated that the laser pulse intensity at the focus was sufficient to accelerate electrons to relativistic energies and generate high dose-rate ionizing radiation. Non-linear effects in these conditions usually distort the propagating wavefront and render difficult the proper focusing of the beam. We have shown this does not hold true when focusing the output of the ALLS 100Hz Mid-IR laser beamline (2.8 mJ, 12 fs pulse centered at 1800 nm) with a reflective optic having a Numerical Aperture (NA) close to 1. Using this configuration, a radiation dose of 100's of mGy per minute was measured at 50 cm from the focus in ambient air, which exceed public dose limits in a very short time. Non-negligible dose measurements have also been observed with longer pulse durations, shorter wavelengths and standard off-the-shelf tight focusing optics from traditional optical suppliers. In this specific case, when using HNA reflective optics with ultrashort pulsed laser beams, the Laser Safety Officer (LSO) should carefully assess the risk related to hazardous ionizing radiation emitted by the focused beam.

Objective: The relative TG-43 dosimetry parameters of the INTRABEAM (Carl Zeiss Meditec AG, Jena, Germany) bare probe were recently reported by Ayala Alvarez et al (2020 Phys. Med. Biol. 65 245041). The current study focuses on the dosimetry characterization of the INTRABEAM source with the eight available spherical applicators according to the TG-43 formalism using Monte Carlo (MC) simulations. Approach: This report includes the calculated dose-rate conversion coefficients that determine the absolute dose rate to water at a reference point of 10 mm from the applicator surface, based on calibration air-kerma rate measurements at 50 cm from the source on its transverse plane. Since the air-kerma rate measurements are not yet provided from a standards laboratory for the INTRABEAM, the values in the present study were calculated with MC. This approach is aligned with other works in the search for standardization of the dosimetry of electronic brachytherapy sources. As a validation of the MC model, depth dose calculations along the source axis were compared with calibration data from the source manufacturer. Main results: The calculated dose-rate conversion coefficients were 434.0 for the bare probe, and 683.5, 548.3, 449.9, 376.5, 251.0, 225.6, 202.8, and 182.6 for the source with applicators of increasing diameter from 15 to 50 mm, respectively. The radial dose and the 2D anisotropy functions of the TG-43 formalism were also obtained and tabulated in this document. Significance: This work presents the data required by a treatment planning system for the characterization of the INTRABEAM system in the context of intraoperative radiotherapy applications.

The INTRABEAM system (Carl Zeiss Meditec AG, Jena, Germany) is an electronic brachytherapy (eBT) device designed for intraoperative radiotherapy applications. To date, the INTRABEAM x-ray source has not been characterized according to the AAPM TG-43 specifications for brachytherapy sources. This restricts its modelling in commercial treatment planning systems (TPSs), with the consequence that the doses to organs at risk are unknown. The aim of this work is to characterize the INTRABEAM source according to the TG-43 brachytherapy dosimetry protocol. The dose distribution in water around the source was determined with Monte Carlo (MC) calculations. For the validation of the MC model, depth dose calculations along the source longitudinal axis were compared with measurements using a soft x-ray ionization chamber (PTW 34013) and two synthetic diamond detectors (microDiamond PTW TN60019). In our results, the measurements in water agreed with the MC model calculations within uncertainties. The use of the microDiamond detector yielded better agreement with MC calculations, within estimated uncertainties, compared to the ionization chamber at points of steeper dose gradients. The radial dose function showed a steep fall-off close to the INTRABEAM source ([Formula: see text]10 mm) with a gradient higher than that of commonly used brachytherapy radionuclides (192Ir, 125I and 103Pd), with values of 2.510, 1.645 and 1.232 at 4, 6 and 8 mm, respectively. The radial dose function partially flattens at larger distances with a fall-off comparable to that of the Xoft Axxent® (iCAD, Inc., Nashua, NH) eBT system. The simulated 2D polar anisotropy close to the bare probe walls showed deviations from unity of up to 55% at 10 mm and 155°. This work presents the MC calculated TG-43 parameters for the INTRABEAM, which constitute the necessary data for the characterization of the source as required by a TPS used in clinical dose calculations.

Despite being considered the gold standard for brachytherapy dosimetry, Monte Carlo (MC) has yet to be implemented into a software for brachytherapy treatment planning. The purpose of this work is to present RapidBrachyMCTPS, a novel treatment planning system (TPS) for brachytherapy applications equipped with a graphical user interface (GUI), optimization tools and a Geant4-based MC dose calculation engine, RapidBrachyMC. Brachytherapy sources and applicators were implemented in RapidBrachyMC and made available to the user via a source and applicator library in the GUI. To benchmark RapidBrachyMC, TG-43 parameters were calculated for the microSelectron v2 (192Ir) and SelectSeed (125I) source models and were compared against previously validated MC brachytherapy codes. The performance of RapidBrachyMC was evaluated for a prostate high dose rate case. To assess the accuracy of RapidBrachyMC in a heterogeneous setup, dose distributions with a cylindrical shielded/unshielded applicator were validated against film measurements in a Solid WaterTM phantom. TG-43 parameters calculated using RapidBrachyMC generally agreed within 1%–2% of the results obtained in previously published work. For the prostate case, clinical dosimetric indices showed general agreement with Oncentra TPS within 1%. Simulation times were on the order of minutes on a single core to achieve uncertainties below 2% in voxels within the prostate. The calculation time was decreased further using the multithreading features of Geant4. In the comparison between MC-calculated and film-measured dose distributions, at least 95% of points passed the 3%/3 mm gamma index criteria in all but one case. RapidBrachyMCTPS can be used as a post-implant dosimetry toolkit, as well as for MC-based brachytherapy treatment planning. This software is especially well suited for the development of new source and applicator models.

We have developed a total body photon irradiation technique using multiple overlapping open field arcs (TB‐ARC). This simple technique uses predetermined arc‐weights, with MUs calculated as a function of prescription depth only. Patients lie on a stretcher, in the prone/supine treatment position with AP/PA arcs. This treatment position has many advantages including ease of delivery (especially for tall, pediatric or compromised patients), dose uniformity, simplicity for organ shielding, and imaging capabilities.Using a Varian TrueBeam linac, 14 arcs using 40×40 cm2 6 MV open photon beams, sweeping across 10 degrees each, complete a 140 degree arc. The nominal SSD at zero degrees is 200 cm. Arcs at the sweep limits (+/− 70 degrees) are differentially weighted and deliver a dose within 10% of the prescription on central axis, at a depth of 10 cm over a superior‐inferior length of 275 cm.CT planning using Varian Eclipse enables dose evaluation. A custom made beam spoiler, consisting of a 2.5 m sheet of polycarbonate (6 mm thick) increases the surface dose from 45% to 90%. This beam spoiler also serves as a support in the event that differential attenuation is required for organs such as lung, heart, liver, kidneys. The geometry of the sweeping beam technique limits organ dose (using varying thicknesses of melting alloy) to about 20% and 40% of prescription at dmax and midplane respectively. Digital imaging with a portable DR cassette enables proper attenuator location prior to treatment.

PURPOSE With the emergence of flattening filter free (FFF) photon beams, several authors have noted many advantages to their use. One such advantage is the decrease in neutron production by photonuclear reactions in the linac head. In the present work we investigate the reduction in neutrons from a Varian TrueBeam linac using the Nested Neutron Spectrometer (NNS, Detec). The neutron spectrum, total fluence and source strength were measured and compared for 10 MV with and without flattening filter and the effect of moderation by the room and maze was studied for the 15 MV beam. METHODS The NNS, similar to traditional Bonner sphere detectors but operated in current mode, was used to measure the neutron fluence and spectrum. The NNS was validated for use in high dose rate environments using Monte Carlo simulations and calibrated at NIST and NRC Canada. Measurements were performed at several positions within the treatment room and maze with the linac jaws closed to maximize neutron production. RESULTS The measurements showed a total fluence reduction between 35-40% in the room and maze when the flattening filter was removed. The neutron source strength Qn was calculated from in-room fluence measurements and was found to be 0.042 × 102 n/Gy, 0.026 × 102 n/Gy and 0.59 × 1012 n/Gy for the 10 MV, the 10 MV FFF and 15 MV beams, respectively. We measured ambient equivalent doses of 11 mSv/hr, 7 mSv/hr and 218 mSv/hr for the 10 MV, 10 MV FFF and 15 MV by the head. CONCLUSION Our measurements revealed a decrease in total fluence, neutron source strength and equivalent dose of approximately 35-40% across the treatment room for the FFF compared to FF modes. This demonstrates, as expected, that the flattening filter is a major component of the neutron production for the TrueBeam. The authors greatly acknowledge support form the Canadian Nuclear Commission and the Natural Sciences and Engineering Research Council of Canada through the CREATE program. Co-authors Dubeau and Witharana are employees of Detec (Gatineau, Quebec), manufacturer of the Nested Neutron Spectrometer.

Remote afterloading devices used for high‐dose‐rate (HDR) brachytherapy may be supplied with different sources, and these sources typically have differing initial source strengths. In addition, the proposed frequency for source changes may also vary, depending upon the source type. Dosimetric parameters unique to each source are often used to compare source types. However, when considering the relative dosimetric efficiency between two HDR sources, the combined effect of source type, initial source strength, and source change scheme must be considered. A method of quantifying this combined effect by calculating mean dose rate from specific dosimetric source data is discussed. This method suggests an objective manner of comparing source scheme equivalency to facilitate performing a cost ratio analysis between different HDR sources and source change schemes.PACS numbers: 87.53.Jw, 87.56.bg