Range Of Field Sizes Research Articles

Effect of Field Size Dependence on Dose Rate on Co-60 Teletherapy Unit at BINOR

Background: Cancer, defined as the uncontrolled proliferation of malignant cells, is a serious hazard. Radiotherapy, which employs high-energy radiation, seeks to break cancer cell DNA. Due to the significant consequences involved, accuracy is of utmost importance. Before commencing radiation therapy, thorough quality assurance procedures are carried out. The Percentage Depth Dose (PDD) is employed to ensure accurate dose measurement. The research utilized Cobalt-60 Teletherapy, which produces gamma radiation at energy levels of 1.17 MeV and 1.33 MeV, to assess the various field widths observed in PDD. In this experiment, the percentage depth dose (PDD) was evaluated by using a Farmer chamber, water phantom, PC electrometer, ionization chamber, computer software and barometer. The chamber was used throughout a range of field sizes, which varied from 5x5cm2 to 20x20cm2.The TRS-398 approach was employed to assess the radiation dose. Comparable findings were observed when conducting comparisons with published PDD data for cobalt-60 beams. The accumulation dose zone was found to be located approximately 0.5 cm below the surface, coinciding with the maximum (Zmax) dose site. The absorbed amount of radiation resulted in a decrease in the percentage depth dose (PDD) below the given depth. DOI: https://doi.org/10.52783/pst.643

Correction method for depth and field size dependence in Octavius 1000 SRS patient-specific QA

Stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) are advanced techniques capable of delivering higher doses of radiation precisely to small and complex targets. As these higher doses are delivered, the significance of patient-specific quality assurance becomes increasingly crucial to ensure both the effectiveness and safety of treatment. This study investigates the depth and field size dependence in Octavius 1000 SRS patient-specific quality assurance (PSQA), which is vital for ensuring accuracy in modern radiotherapy treatments. It comprehensively compares the performance of the Octavius 1000 SRS array against standard treatment planning system calculations, focusing on various field sizes and depths. An extensive measurement process was performed, wherein output factors were analyzed across a range of field sizes, from very small (1 × 1 cm2) to larger fields (10 × 10 cm2), at different depths (5 cm, 10 cm, and 15 cm). Our results highlighted significant variations in the measurement accuracy, dependent on the field size and depth, which underscored the challenge involved in ensuring precise dosimetry in PSQA for small fields. The largest discrepancy observed was 2.65% for a 1 × 1 cm2 field size at a depth of 15 cm. Based on these findings, a novel correction method tailored for small field sizes and varying depths was proposed. This method enhances the accuracy and reliability of PSQA in advanced radiotherapy to ensure that the delivered dose conforms closely to the planned dose, which is a critical aspect in treatments targeting small and complex tumor volumes. Through the proposed correction, this study aims to bridge the gap between calculated and measured dose distributions, thus contributing significantly to the refinement of PSQA processes in radiotherapy.

Rapid Switching of a C-Series Linear Accelerator Between Conventional and Ultrahigh-Dose-Rate Research Mode With Beamline Modifications and Output Stabilization

PurposeIn this study, a decommissioned C-series linear accelerator (linac) was configured to enable rapid and reliable conversion between the production of conventional electron beams and an Ultrahigh-dose-rate (UHDR) electron beamline to the treatment room isocenter for FLASH radiation therapy. Efforts to tune the beam resulted in a consistent, stable UHDR beamline. Methods and MaterialsThe linac was configured to allow for efficient switching between conventional and modified electron output modes within two minutes. Additions to the air system allow for retraction of the X-ray target from the beamline when the 10MV photon mode is selected. With the carousel set to an empty port, this grants access to the higher current pristine electron beam normally used to produce clinical photon fields. Monitoring signals related to the automatic frequency control (AFC) system allows for tuning of the waveguide while the machine is in a hold state so a stable beam is produced from the initial pulse. A pulse counting system implemented on a FPGA-based controller platform controls the delivery to a desired number of pulses. Beam profiles were measured with Gafchromic film. Pulse-by-pulse dosimetry was measured using a custom electrometer designed around the EdgeTM diode. ResultsThis method reliably produces a stable UHDR electron beam. Open field measurements of the 16cm (FWHM) gaussian beam saw average dose rates of 432Gy/s at treatment isocenter. Pulse overshoots were limited and ramp up was eliminated. Over the last year, there have been no recorded incidents that resulted in machine downtime due to the UHDR conversions. ConclusionStable 10MeV UHDR beams were generated to produce an average dose rate of 432Gy/s at the treatment room isocenter. With a reliable pulse-counting beam control system, consistent doses can be delivered for FLASH experiments with the ability to accommodate a wide range of field sizes, source-to-surface distances, and other experimental apparatus that may be relevant for future clinical translation.

MR compatible detectors assessment for a 0.35 T MR-linac commissioning

PurposeTo assess a large panel of MR compatible detectors on the full range of measurements required for a 0.35 T MR-linac commissioning by using a specific statistical method represented as a continuum of comparison with the Monte Carlo (MC) TPS calculations. This study also describes the commissioning tests and the secondary MC dose calculation validation.Material and methodsPlans were created on the Viewray TPS to generate MC reference data. Absolute dose points, PDD, profiles and output factors were extracted and compared to measurements performed with ten different detectors: PTW 31010, 31021, 31022, Markus 34045 and Exradin A28 MR ionization chambers, SN Edge shielded diode, PTW 60019 microdiamond, PTW 60023 unshielded diode, EBT3 radiochromic films and LiF µcubes. Three commissioning steps consisted in comparison between calculated and measured dose: the beam model validation, the output calibration verification in four different phantoms and the commissioning tests recommended by the IAEA-TECDOC-1583.Main resultsThe symmetry for the high resolution detectors was higher than the TPS data of about 1%. The angular responses of the PTW 60023 and the SN Edge were − 6.6 and − 11.9% compared to the PTW 31010 at 60°. The X/Y-left and the Y-right penumbras measured by the high resolution detectors were in good agreement with the TPS values except for the PTW 60023 for large field sizes. For the 0.84 × 0.83 cm2 field size, the mean deviation to the TPS of the uncorrected OF was − 1.7 ± 1.6% against − 4.0 ± 0.6% for the corrected OF whereas we found − 4.8 ± 0.8% for passive dosimeters. The mean absolute dose deviations to the TPS in different phantoms were 0 ± 0.4%, − 1.2 ± 0.6% and 0.5 ± 1.1% for the PTW 31010, PTW 31021 and Exradin A28 MR respectively.ConclusionsThe magnetic field effects on the measurements are considerably reduced at low magnetic field. The PTW 31010 ionization chamber can be used with confidence in different phantoms for commissioning and QA tests requiring absolute dose verifications. For relative measurements, the PTW 60019 presented the best agreement for the full range of field size. For the profile assessment, shielded diodes had a behaviour similar to the PTW 60019 and 60023 while the ionization chambers were the most suitable detectors for the symmetry. The output correction factors published by the IAEA TRS 483 seem to be applicable at low magnetic field pending the publication of new MR specific values.

Clinical Linear Accelerator-Based Electron FLASH: Pathway for Practical Translation to FLASH Clinical Trials

Ultrahigh-dose-rate (UHDR) radiation therapy (RT) has produced the FLASH effect in preclinical models: reduced toxicity with comparable tumor control compared with conventional-dose-rate RT. Early clinical trials focused on UHDR RT feasibility using specialized devices. We explore the technical feasibility of practical electron UHDR RT on a standard clinical linear accelerator (LINAC). We tuned the program board of a decommissioned electron energy for UHDR electron delivery on a clinical LINAC without hardware modification. Pulse delivery was controlled using the respiratory gating interface. A short source-to-surface distance (SSD) electron setup with a standard scattering foil was configured and tested on an anthropomorphic phantom using circular blocks with 3- to 20-cm field sizes. Dosimetry was evaluated using radiochromic film and an ion chamber profiler. UHDR open-field mean dose rates at 100, 80, 70, and 59 cm SSD were 36.82, 59.52, 82.01, and 112.83 Gy/s, respectively. At 80 cm SSD, mean dose rate was ∼60 Gy/s for all collimated field sizes, with an R80 depth of 6.1 cm corresponding to an energy of 17.5 MeV. Heterogeneity was <5.0% with asymmetry of 2.2% to 6.2%. The short SSD setup was feasible under realistic treatment conditions simulating broad clinical indications on an anthropomorphic phantom. Short SSD and tuning for high electron beam current on a standard clinical LINAC can deliver flat, homogenous UHDR electrons over a broad, clinically relevant range of field sizes and depths with practical working distances in a configuration easily reversible to standard clinical use.

Elaboration and experimental validation of a Monte Carlo source model for linac 6 MV photon beams with and without Flattening Filter

The concept of virtual source model has been developed to provide an efficient and precise particle generator mainly involved in standard photon beam treatment planing and Monte Carlo dosimetry simulations. In this work, we propose a source model for 6 MV linac photon beam for both irradiation modes, with and without flattening filter. Two sources are considered to represent primary and a scattered radiations. Energy spectra taking into account the correlation between energy and diffusion angle have been used. Model parameters were tuned through a dose distribution iterative optimisation approach. The convergence condition relies on γindex evaluation. The simulations were performed in two steps, phase space files were generated using Geant4 toolkit, then energy deposit in water were computed using DPM code. Monte Carlo dose distributions of the model were compared to experimental data of two linac systems, an Elekta Synergy® with flattening filter and a Varian TrueBeam® with and without flattening filter. Good results were obtained in terms of γindex 2%/2 mm criterion, dose difference and distance to agreement (DTA) analysis for field size range from 3 × 3 cm2 to 30 × 30 cm2.

Geometrical source modeling of 6MV flattening-filter-free (FFF) beam from TrueBeam linear accelerator and its commissioning validation using Monte Carlo simulation approach for radiotherapy

PurposeTo study geometrical source modeling of 6 MV flattening-filter-free (FFF) beam from TrueBeam linear accelerator and its commissioning validation using Monte Carlo simulation approach. Methods & materialSource modeling of 6MVFFF beam from TrueBeam linear accelerator has been accomplished by using PRIMO as the Monte-Carlo (MC) simulation software based on PENELOPE code. Initially, source modeling of 6MVFFF beam was validated against experimentally measured Percentage-depth-dose (PDD) and profile for 10 × 10cm2 field. Followed by commissioning validation of 6MVFFF beam has been carried out by simulating PDDs, profiles and output factors for range of field sizes. Besides, verification of absolute dose, beam quality index, performance analysis of dose distribution in heterogeneous phantom and simulation of radiation treatment plan for validation of Treatment planning system (TPS) was performed. Simulated results compared against measured; dose distributions were analyzed using gamma analysis with acceptance criteria of 2% Dose-difference (DD) and 2 mm Distance-to-agreement (DTA). ResultsGamma analysis for all the simulated and measured PDDs and profiles shows minimum passing rate of 100% and 98.04% respectively. Simulated values of absolute point dose, beam quality index and output factors shows good agreements with measurements. Gamma analysis of depth dose distribution in heterogeneous phantom obtained using MC simulation and calculated with Analytical anisotropic algorithm (AAA) agreed within 95.89%. However, heterogeneous medium of very high and low-density region shows larger dose discrepancy between MC and AAA due to the poor modeling of AAA compared to MC. The percentage of 99.81% of gamma passing and comparison of predicted Dose-volume-histogram (DVH) shows good resemblance between PRIMO simulated and AAA calculated plan. ConclusionStudy concludes that PRIMO is an independent MC simulation tools for verification and commissioning validation of external beam radiotherapy in treatment of cancer. The MC validation of external beam therapy modules grants higher confidence in accuracy of radiation dose delivery.

Dose area product primary standards established by graphite calorimetry at the LNE-LNHB for small radiation fields in radiotherapy

PurposeTo present primary standards establishment in terms of Dose Area Product (DAP) for small field sizes. MethodsA large section graphite calorimeter and two plane-parallel ionization chambers were designed and built in-house. These chambers were calibrated in a 6MV FFF beam at the maximum dose rate of 1400 UM/min for fields defined by specifically designed circular collimators of 5, 7.5, 10, 13 and 15 mm diameter and jaws of 5, 7, 10, 13 and 15 mm side length on a Varian TrueBeam linac. ResultsThe two chambers show the same behaviour regardless of field shape and size. From 5 to 15 mm, calibration coefficients slightly increase with the field size with a magnitude of 1.8% and 1.1% respectively for the two chambers, and are independent of the field shape. This tendency was confirmed by Monte Carlo calculations. The average associated uncertainty of the calibration coefficients is around 0.6% at k=1. ConclusionsFor the first time, primary standards in terms of DAP were established by graphite calorimetry for an extended range of small field sizes. These promising results open the door for an alternative approach in small fields dosimetry.

A study of sea ice regime in the Obskaya guba Bay using modern satellite data in 2007–2017

The Obskay guba Bay is a region of rapidly developing oil and gas exploration. Knowing the current sea ice conditions including dangerous phenomena e. g. ridges and stamukhas is important for the safety of coastal and underwater construction as well as for ecological risk assessment. With this study, we aim to obtain new data on sea ice seasonal cycle in the southern and central part of the Obskay guba Bay for 10 years (from 2007 to 2017) and to demonstrate the capacity of satellite data in obtaining varying sea ice characteristics in the region. Analyzing daily visual MODIS and available Sentinel-1 SAR imagery, we derived dates of sea ice and fast ice formation, fast ice breakup and melt onset and the onset of ice-free period. For this purpose the satellite data were analyzed manually by sea ice expert. In addition, of sea ice ridges were derived and the sea ice drift data wea automatically processed in order to locate motionless sea ice features — stamukhas. The distribution of sea ice floes and field size in the region was derived from MODIS data. The analysis showed that there is a tendency towards a shorter ice covered period based on the data from 2007 to 2017. Overall, the formation of sea ice starts 9 days later and fast ice breakup occurs 16 days earlier compared to the long-term mean (1947–2010). The majority of ridges were located in the central part of the region and directed along the coast. The analysis confirmed absence of large stamuhas visible to be applied method (with a horizontal size of 100 m). The predominant sea ice field size range lies 500–1500 m. The study shows that a combination of images obtained in the optical range of the survey with radar data makes it possible to supplement the classical visual assessments with the results of automatic methods for detecting fast ice, detecting stamukha, as well as ice drift and deformation.

Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac.

PurposeThe aim of this study is to investigate off‐axis irradiation on the Australian MRI‐Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field.MethodsDosimetric measurements were performed using a microDiamond detector, Gafchromic® EBT3 film, and MOSkin TM. Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2. Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source‐to‐surface distance (SSD) of 1.8 m for fields collimated centrally and off‐axis. PDD measurements were also acquired at isocenter for each off‐axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination.ResultsOff‐axis irradiation separates the majority of electron contamination from an x‐ray beam and was found to significantly reduce in‐field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2 field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkin TM and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero.ConclusionsExperimental and simulation data were acquired for a range of field sizes to investigate off‐axis irradiation on an inline MRI‐Linac. The skin sparing effect was observed with off‐axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

Artificial intelligence based deconvolving on megavoltage photon beam profiles for radiotherapy applications

Objective. The aim of this work is an AI based approach to reduce the volume effect of ionization chambers used to measure high energy photon beams in radiotherapy. In particular for profile measurements, the air-filled volume leads to an inaccurate measurement of the penumbra. Approach. The AI-based approach presented in this study was trained with synthetic data intended to cover a wide range of realistic linear accelerator data. The synthetic data was created by randomly generating profiles and convolving them with the lateral response function of a Semiflex 3D ionization chamber. The neuronal network was implemented using the open source tensorflow.keras machine learning framework and a U-Net architecture. The approach was validated on three accelerator types (Varian TrueBeam, Elekta VersaHD, Siemens Artiste) at FF and FFF energies between 6 MV and 18 MV at three measurement depths. For each validation, a Semiflex 3D measurement was compared against a microDiamond measurement, and the AI processed Semiflex 3D measurement was compared against the microDiamond measurement. Main results. The AI approach was validated with dataset containing 306 profiles measured with Semiflex 3D ionization chamber and microDiamond. In 90% of the cases, the AI processed Semiflex 3D dataset agrees with the microDiamond dataset within 0.5 mm/2% gamma criterion. 77% of the AI processed Semiflex 3D measurements show a penumbra difference to the microDiamond of less than 0.5 mm, 99% of less than 1 mm. Significance. This AI approach is the first in the field of dosimetry which uses synthetic training data. Thus, the approach is able to cover a wide range of accelerators and the whole specified field size range of the ionization chamber. The application of the AI approach offers an quality improvement and time saving for measurements in the water phantom, in particular for large field sizes.

Upper limits to the mean annual optical efficiency of solar mono-tower systems

This study presents an analysis of the theoretical upper limits to the mean annual optical efficiency of the Light Collection and Concentration (LCC) subsystem of a solar tower system, which is the subsystem composed of the heliostat field and the active surfaces of the receiver that capture the concentrated solar radiation sent by the heliostat field to transform it into thermal energy. The “ideal” LCC subsystem is modelled gradually, by incrementally introducing into the model factors that limit the mean annual optical efficiency of any LCC subsystem and whose influence in the optical efficiency cannot be reduced to zero, starting with the cosine factor, followed by the atmospheric transmittance, and then by the product of the receiver intercept factor and a factor related to the shading of the heliostat field by the receiver. At each degree of complexity of the ideal LCC subsystem model, the optimal configuration of the “ideal” LCC subsystem that achieves the highest mean annual optical efficiency in terms of heliostat field shape and receiver size is identified for a selected range of latitudes, tower heights and heliostat field sizes or total mirror areas. Thus, the upper-limits of the mean annual optical efficiency of the LCC subsystem of solar towers are obtained as a function of the specified parameters. The results obtained show that at a latitude of 30° North, for tower heights between 200 and 300 meters and heliostat fields with mirror areas between 1.3km2 and 2.5km2, the upper limit of the mean annual optical efficiencies vary between 79.2% and 74.7%, with the highest upper limit corresponding to the combinations of the tallest towers and smallest heliostat fields. This strongly indicates that the mean annual optical efficiency of actual solar mono-tower systems of commercially realistic heliostat field sizes and tower heights will hardly reach 70% at any location on Earth.

Measuring small field profiles and output factors with a stemless plastic scintillator array.

To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors. An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array. A fifth scintillator was used that had been segmented by laser etching and the septa filled with black paint. Each detector array was first calibrated using a reference field of 95cm SSD, 5cm depth, and 10×10 cm2 field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5×0.5 cm2 , 1×1 cm2 , and 2×2 cm2 . Using the central pixel of the array, output factors were measured for field sizes of 0.5×0.5 cm2 to 25×25 cm2 . Small field profiles were compared to film measurements and output factors compared to ion chamber measurements. The segmented scintillator measured profiles that were in good agreement with film for all three field sizes. Output factors agreed to within 1.2% of ion chamber over the field size range of 1×1 cm2 to 25×25 cm2 . At 0.5×0.5 cm2 the segmented scintillator underestimated the output factor compared to film and a microDiamond detector. Bulk scintillators failed to produce a good agreement with film for measured profiles and deviations from ion chamber for output factors were apparent at field sizes below 5×5 cm2 . In comparison to a bulk scintillator of dimensions 5×5×0.5 cm3 the etched scintillator saw a reduction of 5.1, 7.1, and 10.5 times the signal for field sizes of 0.5×0.5 cm2 , 1×1 cm2 , and 2×2 cm2 , respectively. The reduction of signal comes from reduced cross-talk that was present in all of the bulk scintillator geometries to various degrees. A 1D SPSD array was demonstrated with various scintillator designs. The etched scintillator array demonstrated excellent small field profile measurements when compared to film and output factors (down to 1×1cm2 field size) when compared to micro ion chamber and diamond detector measurements.

The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation.

We have experimentally and computationally characterized the PTW microSilicon 60023-type diode's performance in 6 and 15 MV photon fields ≥5×5 mm2 projected to isocenter. We tested the detector on- and off-axis at 5 and 15cm depths in water, and investigated whether its response could be improved by including within it a thin airgap. Experimentally, detector readings were taken in fields generated by a Varian TrueBeam linac and compared with doses-to-water measured using Gafchromic film and ionization chambers. An unmodified 60023-type diode was tested along with detectors modified to include 0.6, 0.8, and 1.0 mm thick airgaps. Computationally, doses absorbed by water and detectors' sensitive volumes were calculated using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Detector response was characterized using , a factor that corrects for differences in the ratio of dose-to-water to detector reading between small fields and the reference condition, in this study 5cm deep on-axis in a 4×4 cm2 field. The greatest errors in measurements of small field doses made using uncorrected readings from the unmodified 60023-type detector were over-responses of 2.6%±0.5% and 5.3%±2.0% determined computationally and experimentally, relative to the reading-per-dose in the reference field. Corresponding largest errors for the earlier 60017-type detector were 11.9%±0.6% and 11.7%±1.4% over-responses. Adding even the thinnest, 0.6mm, airgap to the 60023-type detector over-corrected it, leading to under-responses of up to 4.8%±0.6% and 5.0%±1.8% determined computationally and experimentally. Further, Monte Carlo calculations indicate that a detector with a 0.3mm airgap would read correctly to within 1.3% on-axis. The ratio of doses at 15 and 5cm depths in water in a 6 MV 4×4 cm2 field was measured more accurately using the unmodified 60023-type detector than using the 60017-type detector, and was within 0.3% of the ratio measured using an ion chamber. The 60023-type diode's sensitivity also varied negligibly as dose-rate was reduced from 13 to 4Gy min-1 by decreasing the linac pulse repetition frequency, whereas the sensitivity of the 60017-type detector fell by 1.5%. The 60023-type detector performed well in small fields across a wide range of beam energies, field sizes, depths, and off-axis positions. Its response can potentially be further improved by adding a thin, 0.3mm, airgap.

Modeling Physiological Sources of Heading Bias from Optic Flow.

Human heading perception from optic flow is accurate for directions close to the straight-ahead and systematic biases emerge in the periphery (Cuturi and Macneilage, 2013; Sun et al., 2020). In pursuit of the underlying neural mechanisms, primate brain dorsal medial superior temporal (MSTd) area has been a focus because of its causal link with heading perception (Gu et al., 2012). Computational models generally explain heading sensitivity in individual MSTd neurons as a feedforward integration of motion signals from medial temporal (MT) area that resemble full-field optic flow patterns consistent with the preferred heading direction (Britten, 2008; Mineault et al., 2012). In the present simulation study, we quantified within the structure of this feedforward model how physiological properties of MT and MSTd shape heading signals. We found that known physiological tuning characteristics generally supported the accuracy of heading estimation, but not always. A weak-to-moderate overrepresentation of peripheral headings in MSTd garnered the highest accuracy and precision out of the models that we tested. The model also performed well when noise corrupted high proportions of the optic flow vectors. Such a peripheral MSTd model performed well when units possessed a range of receptive field (RF) sizes and were strongly direction tuned. Physiological biases in MT direction tuning toward the radial direction also supported heading estimation, but the tendency for MT preferred speed and RF size to scale with eccentricity did not. Our findings help elucidate the extent to which different physiological tuning properties influence the accuracy and precision of neural heading signals.

Proposing a novel graphical method to determine effective bremsstrahlung focal spot size and shape of the therapeutic beam from a linear accelerator in radiation therapy

The purpose of this study is to introduce and demonstrate a novel graphical method to determine effective bremsstrahlung focal spot size and shape of the therapeutic beam from a linear accelerator. The energy distribution within the beam spot allows in determining effective bremsstrahlung size and shape of beam spot. The transverse and radial dose profiles were measured for a range of field sizes in an isocentric setup. The Length of direct focal region (LDFR) on either side of profiles were determined through the de-convolution of dose profile. The graph plotted for LDFR versus field size helps in determining properties of beam spot. The dimensions of FWHM of effective bremsstrahlung focal spot were found elliptical in nature close to the order of 1 mm. This method allows determination of FWHM of the effective bremsstrahlung focal spot that fairly falls within the range of dimension of FWHM of beamspot published in literature.

Technical Note: Characteristics of a microSilicon X shielded diode detector for photon beam dosimetry.

This study investigated the characteristics of a new shielded diode detector, microSilicon X (model 60022: MSX), for small-field and large-field dosimetry. The percent depth dose (PDD), beam profiles, detector output factor (OFdet ), temperature dependence, dose rate dependence, dose-per-pulse (DPP) dependence, and dose-response linearity of MSX were evaluated in Cyberknife and TrueBeam photon beams and compared with various detectors including microDiamond (PTW model 60019: MD), Sun Nuclear EDGE detector, Photon diode (PTW model 60016: PD), and semiflex ionization chamber (PTW model 31010: IC). For field sizes ranging from 50×50mm2 to 400×400mm2 , MSX-measured OFdet values were within 1% of the IC-measured values. For the CyberKnife small fields, the maximum difference between the MSX-measured OFdet and the MD-measured field output factor (Ω) was 4.0%, while the maximum differences were 8.8% and 10.9% for PD and EDGE, respectively. MSX showed a stable response within 0.7% for water temperatures of 5°C to 34°C, while PD and EDGE showed a linear correlation between the water temperature and the response. MSX showed small variations within 0.2% for the dose rate, and PD and EDGE showed logarithmic increases in the response with the dose rate. MSX and MD had smaller DPP dependences than PD and EDGE. The characteristics of MSX for measurements of small- and large-field photon beams are favorable. Compared to PD, MSX exhibited significant improvement in the over-response for small fields. The OFdet values measured by MSX were approximately in-between those measured by MD and PD. MSX showed stable responses against water temperature, dose rate, and DPP variations and provided suitable data for a wide range of field sizes. However, careful attention is required for measurements of OFdet for field sizes of <10×10mm2 and PDD for field sizes of ≥200×200mm2 .

Performance of a multileaf collimator system for a 1.5T MR-linac.

The purpose of this study was to evaluate the performance of a multileaf collimator (MLC) system for the high magnetic field of the Elekta Unity magnetic resonance linear accelerator (MR-linac). Performance was evaluated as recommended in AAPM Report Nos. 72 and 142, and MLC motor radiofrequency (RF) shielding was also evaluated. Fields of 99.8×99.8mm2 and 199.6×199.6mm2 were examined to calculate the leaf width. Profiles of 28.4×28.4mm2 , 57.0×57.0mm2 , 99.8×99.8mm2 , and 199.6×199.6mm2 fields at center and off-axis positions were measured using a microDiamond detector and BEAMSCAN® MR water phantom to determine the field penumbra. EBT3 film was exposed to a 190.0×220.0mm2 field composed of two strip segments to assess the tongue-and-groove (T&G) effect. Outputs of 100.0×100.0mm2 open and blocked fields at center and off-axis positions were measured using a PTW TW31021 ion chamber and a film with the full MLC-blocked field to determine transmission. The minimum gap fields were also exposed on EBT3 film at center and off-axis positions to assess leakage between leaf tips in the "closed" position. The picket fence test was performed using a megavoltage imager to analyze leaf positional reproducibility. RF shielding was measured via real-time MR imaging of a homogeneous phantom and quantified using the signal-to-noise ratio (SNR). The leaf widths were 7.00±0.00 and 7.10±0.00mm for 99.8×99.8 and 199.6×199.6mm2 fields, respectively. The leaf side penumbra ranged from 5.11±0.08 to 11.28±0.29mm and the leaf tips penumbra ranged from 4.47±0.03 to 6.25±0.15mm for the field size range of 28.4×28.4mm2 to 199.6×199.6mm2 . The T&G effect resulted in an underdose of up to 23.41±0.88%. The max leaf transmission was 0.33±0.00% when MLCs were all closed and diaphragms were all opened, and this value was reduced to 0.18±0.00% when the diaphragm blocked the field with a minimum MLC gap. The leaf tip leakage rates were 60.08±0.05% at field center and 2.94±0.00% approximately 100mm off-axis. At 6months, the short-term leaf position reproducibility was 0.11mm (2σ), and the long-term leaf position reproducibility was 0.19mm (2σ). Leaf motion led to small fluctuations of the SNR of up to 1.64%. The new MLC system has a comparable T&G effect, low radiation leakage, high positioning reproducibility, and good RF shielding for the Elekta Unity MR-linac. However, more attention is required for the positional change of the leaf side penumbra.

Biases and Cosmic Variance in Molecular Gas Abundance Measurements at High Redshift

Abstract Recent deep millimeter-wave surveys have attempted to measure the carbon monoxide (CO) luminosity function and mean molecular gas density through blind detections of CO emission lines. While the cosmic star formation rate density is now constrained in fields of hundreds of square arcminutes or more, molecular gas studies have been limited to ≤50 arcmin2. These small fields result in significant biases that have not been accounted for in published results. To quantify these biases, we assign CO luminosities to halos in cosmological simulations to produce mock observations for a range of field sizes. We find that fields of ≲10 arcmin2 alter the recovered shape of the luminosity function, causing underestimates of the number of bright objects. Our models suggest that current surveys are sensitive enough to detect sources responsible for approximately half of the cosmic molecular gas density at high redshift. However, uncertainties in the gas density measurement are large, and cosmic variance may double the uncertainty claimed in these surveys. As a result, the field size needed to detect redshift evolution in the molecular gas at high confidence may be more than one order of magnitude larger than what current surveys have achieved. Shot power intensity mapping measurements are particularly sensitive to Poisson variance and require yet larger areas to constrain the gas density or its evolution. We provide a simple prescription for approximating uncertainty in total CO emission as a function of survey area and redshift for both direct detection and intensity mapping surveys.

Measurement of the small field output factors for 10 MV photon beam using IAEA TRS-483 dosimetry protocol and implementation in Eclipse TPS commissioning

Dosimetry of small fields (SF) is vital for the success of highly conformal techniques. IAEA along with AAPM recently published a code of practice TRS-483 for SF dosimetry. The scope of this paper is to investigate the performance of three different detectors with 10 MV with-flatting-filter (WFF) beam using TRS-483 for SF dosimetry and subsequent commissioning of the Eclipse treatment planning system (TPS version-13.6) for SF data. SF dosimetry data (beam-quality TPR 20,10(10), cross-calibration, beam-profile, and field-output-factor (F.O.F)) measurements were performed for PTW31006-pinpoint, IBA-CC01 and IBA-EFD-3G diode detectors in nominal field size (F.S) range 0.5 × 0.5cm2 to 10 × 10 cm2 with water and solid water medium using Varian Truebeam linac. However, Eclipse-TPS commissioning data was acquired using IBA-EFD-3G diode, and absolute dose calibration was performed with FC-65G detector. The dosimetric performance of the Eclipse-TPS was validated using TLD-LiF chips, IBA-PFD, and IBA-EFD-3G diodes. Dosimetric performance of the PTW31006-pinpoint, IBA-CC01, and IBA-EFD-3G detectors was successfully tested for SF dosimetry. The F.O.Fs were generated and found in close agreement for all F.S except 0.5 × 0.5cm2. It is also found that TPR20,10(10) value can be derived within 0.5% accuracy from a non-reference field using Palmans equation. Cross-calibration can be performed in F.S 6 × 6 cm2 with a maximum variation of 0.5% with respect to 10 × 10cm2. During profile measurement, the full-width half-maxima (FWHM) of F.S 0.5 × 0.5cm2 was found maximum deviated from the geometric F.S. In addition, Eclipse-TPS was commissioned along with some limitations: F.O.F below F.S 1 × 1cm2 was ignored by TPS, PDD and profiles were dropped from configuration below F.S 2 × 2 cm2, and F.O.F which does not satisfy the condition 0.7 < A/B < 1.4 (A and B are FWHM in cross-line and in-line direction) have higher uncertainty than specified in TRS-483. Validation tests for Eclipse-TPS generated plans were also performed. The measured dose was in close agreement (3%) with TPS calculated dose up to F.S 1.5 × 1.5cm2.