Linac Photon Research Articles

Energy response of CaSO4:Dy thermoluminescence detector to medical linear accelerator photon beams.

The study aims to evaluate the energy response of the thermoluminescent dosemeter (TLD) CaSO4:Dy from high energy photon beams produced from medical linear accelerator. The test was performed on the polymethyl methacrylate phantom surface and at the depth of dose maximum with a source-to-surface distance of 100cm and the radiation field size of 12×12cm. The results were compared with the TLD response exposed to 60Co standard source. The results show that the TLD response to the linac's photon differs >30% compared to the TLD response to 60Co exposure when measured at the phantom surface, while at the depth of maximum dose the response is similar (< ±10%), indicating that the TLD response is no longer dependent on the energy. This implies that the current dose reading evaluation based on 60Co calibration curve should be corrected to obtain more accurate dose report of the radiation workers.

Chemical and physical analysis of synthesized coumarin compounds: Investigating responses to gamma and neutron radiations

This research investigates the chemical and physical characteristics of three recently synthesized coumarin compounds produced via the von Pechmann condensation mechanism. These compounds were successfully and precisely synthesized, as confirmed by a comprehensive understanding of their chemical composition obtained through combined infrared and NMR analyses. The compounds underwent examination regarding their responses to gamma and neutron radiations, and their mass attenuation coefficients (MAC) were determined using both GATE/Geant4 simulation and NIST-XCOM data. Additionally, the effective removal cross-section values for fast neutrons were calculated employing various methods. All acquired data has been compared with that of water and other materials in the existing literature. Although coumarin compounds might not have intrinsic shielding properties, they have the potential to be part of composite materials designed for specific applications. Comparative analysis with established radiation dosimetry systems showed the coumarin compounds’ attenuation coefficients closely aligning with water, indicating their potential suitability for nuclear applications requiring water-equivalent properties. Further assessment with Geant4 simulation under linac photon beams demonstrated the water-equivalent properties of the coumarin compounds for 6 MV and 18 MV.

The Effect of Dose Enhancement in Tumor With Silver Nanoparticles on Surrounding Healthy Tissues: A Monte Carlo Study.

Objectives: Cancer-related death rates account for approximately one-third of all deaths, and this rate is increasing remarkably every year. In this study, we examined the dose enhancement factor (DEF) in the tumor and surrounding tissues by adding different concentrations of silver nanoparticles (AgNPs) to the brain tumor using the Monte Carlo (MC) technique. Methods: This study used MCNP6.2 simulation software. A Planning Target Volume (PTV) of 1 × 1 × 1 cm3 was placed in the center of a cubic cranial model with dimensions of 5 × 5 × 5 cm3. Five different simulations were initially generated using the simple method. These simulations included pure PTV and PTV consisting of 4 different silver concentrations (5, 10, 20, and 30 mg/g). Additionally, a model was created using the nanolattice method, considering the size, position, and distribution of the AgNPs. Irradiation was performed using a source with a 6 MV linac photon spectrum. Measurements were performed using the *f8 tally, and DEF values were calculated. Results: In the simulation study using the simple method, the DEF value of PTV increased linearly with concentration, whereas the DEF values were lower than the simulation results with the nanolattice model (1.9 vs 1.4 for 30 mg/g NP concentration). Performing the simple method, we observed no remarkable dose increase in lateral OARs surrounding PTV. While a remarkable dose decrease was observed in distal OARs, a dose increase in the proximal OAR was observed, which was consistent with that of PTV. However, according to the results obtained by performing the nanolattice method, the dose increase was observed in both the proximal OAR and the distal OAR and was similar to that of PTV. Conclusion: While enhancing the dose in the tumor by adding NPs into the tumor, it is essential to consider whether it also increases the OAR dose. In addition, simulation studies on NPs showed that the dose increase varied significantly with particle size, position, and distribution. Hence, these factors should be considered carefully.

Using KMeans Clustering to Evaluate and Alert for Deviations of Linac Photon Beam Parameters.

To analyse the daily measured Dosimetric Quality Assurance (QA) parameters of linear accelerator (linac) using Unsupervised Machine Learning (ML) Algorithm thereby evaluating the current machine status and to highlight the probable cause of the 'out-of-range' measured parameter. Five Parameters measured using PTW QuickCheckwebline device in a linac is subjected to KMeans clustering technique. The measured parameters comprise of Central Axis Dose (CAX), Beam Flatness, SymmetryLR, SymmetryGT and Beam Quality (BQF). Data from Varian with 55- and 107-day's measurements and from Elekta with 75 days measurements from 2 beam matched linacs were used in this clustering technique. This evaluation is used to review the current linac status and obtain 1) Upper and lower limits of each parameter (CAX, Flatness, Symmetry, Beam Quality), 2) Frequency of the days when the linac parameters are closer to the target value and when they deviate from the target value. 3) The date when these parameters deviate from the estimated limits. 4) The probable reason for the deviation and 5) Finally if the machine requires maintenance. This methodology ensures that the machine is always closest to the target value, thus providing quality radiation treatment for the cancer patients. Moreover, the performance of the linac is studied meticulously and the need for maintenance is alerted before the linac beam shows marked deviation from the base value. KMeans clustering is a very simple and easy to use ML tool. With quick computation time and with lesser data it can arrive at the actual limits of the linac parameters and help to determine if the linac needs maintenance well in advance.

PLASTIC SCINTILLATOR BASED 2D DETECTOR FOR PHOTON RADIOTHERAPY: PRELIMINARY RESULTS

A proof-of-concept study of a new detector based on a thin plastic scintillator monitored by a Charge-Coupled Device (CCD) camera designed for monitoring and characterisation of Linac photon beams is presented. The response of the detector is compared with radiochromic film using 6 and 18 MV radiotherapeutic beams. We have observed: (i) all instruments survived the secondary radiation fields during Linac operation, (ii) it was possible to process the measured data using statistical techniques and (iii) the processed data from the CCD camera qualitatively correspond to film dosimetry results. A statistical technique based on the selection of minimal values provides the clearest results. Quantitatively, CCD and film results can only be compared as 6 to 18 MV response rates. We have observed that the rates from the CCD data are systematically higher than the rates from film dosimetry. Differences are not too high, namely 1.9-2.4 times the combined standard deviation.

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.

Medical linac photon skyshine: Monte Carlo calculations and a methodology for estimates.

It has been shown that a widely quoted formula for estimating medical linac photon skyshine equivalent doses is erroneous. Monte Carlo calculations have been performed to develop an easy method for quickly and accurately estimating skyshine radiation levels and to gain improved physical insight into the skyshine phenomenon. Calculations of linac photon skyshine have been performed for 4, 6, 10, 15, and 18 MV beams for 10 × 10 cm2 and 40 × 40 cm2 fields and for a range of room dimensions and roof thicknesses. The effect of flattening filter free beams has been considered. Air kerma rates (AKRs) can be accurately fitted to a simple algebraic formula that is a function of the horizontal distance from the isocenter with a single energy dependent fitting parameter. The AKR, at a height of 1.3 m above level ground, reaches a local maximum at a distance d max = 1.5dw + 1.1h, where dw is the horizontal distance from the isocenter to the outside of the side wall, and h is the vertical distance from the isocenter to the top of the roof. For thin roofs, low energy beams lead to significantly more skyshine than high energy beams because low energy photons are more easily scattered through large angles. In the absence of a roof, the maximum skyshine dose rate is on the order of 8 × 10−7 times the dose rate at isocenter. The average energy of the skyshine photons is about 0.15 MeV, and it is remarkably independent of almost all parameters. A simple methodology is outlined for the evaluation of photon skyshine.

A Comparison of Conventional Empirical Formula and MCNPX Code in the Estimations of Photon and Neutron Skyshine Rates for an 18MV Radiotherapy Bunker

Purpose: A physical phenomenon, scattering the radiation by the atmosphere above the room to the points at ground level around the linac treatment room is known as skyshine radiation. This study aimed to estimate photon and neutron skyshine from a linac in a high-energy radiation therapy facility. Materials and Methods: The empirical method of NCRP report 151 and MC simulations were employed to estimate skyshine radiation dose from the 18MV linac photon beam. A linac and its bunker were modeled and skyshine dose equivalent from photons and secondary neutrons were derived and compared in the control room, corridor, sidewalk and, parking. Results: The photon skyshine dose rates calculations by the MC method varied from 0.43 µSv/h at the sidewalk to 6.2 µSv/h at the control room. The ratios of NCRP to MCNP calculations varied from 3.58 for the corridor to 16.14 for the control room. For the neutron skyshine dose rate at distances shorter than 20m, it was found to be 10.4 nSv/h and the ratios of the NCRP to MCNP were 1.26 at the control room and 3.34 at the sidewalk. Conclusion: It was concluded that the empirical method overestimates photon and neutron skyshine dose rates in comparison to the MCNPX code. The refinement of the proposed empirical method of NCRP 151 and application of MC methods are strongly suggested for more reliable calculations of skyshine radiations.

Skyshine Dose Estimations for an 18 MeV Photon Beam Using MCNPX Code: A Comparison of Flattened and Flattening Filter-Free Beam

Purpose: The current study aimed to estimate photon skyshine dose rate from a Varian linac equippedwith a Flattening Filter (FF) and its FF-Free (FFF) mode. The skyshine photons from a Linac bunker can influence the radiation dose received by personnel and the public in radiation therapy centers. Materials and Methods: In the current study skyshine dose from the conventional flattened beam and the flattening-free beam were compared. The MCNPX Monte Carlo code was used to model an18 MeV photon beam of Varian linac. The skyshine radiation was calculated for FF and FFF linac photon beams at the control room, parking, sidewalk, and corridor around the linac room. Results: For the conventional beam, the skyshine dose rates of 0.53, 0.42, 0.45, and 0.50 mSv/h were estimated for the control room, corridor, sidewalk, and parking, respectively. While for the FFF beam, dose rates of 0.21, 0.20, 0.20, and 0.23 mSv/h were estimated for the same positions, respectively. The results indicated that the empirical method of NCRP 151 can not distinguish between FF and FFF beams in skyshine dose calculations. Our results found a 50% lower level dose rate from the FFF beam at distant and nearby locations. Conclusion: The findings of current can be helpful in the radiation dose calculations and the radiation protection designation of radiation therapy bunkers.

Investigation the trend of different magnetic fields types on linac photon beam mode by Monte Carlo method using Geant4 toolkit

The present study is aimed at investigating the behavior of the secondary particles due to the photon beam interaction at the presence of the longitudinal, radial, tangential (angles), and uniformly springy magnetic fields with different intensities. The photon mode of a 15 MV Siemens Primus linac is used as an example. On the validity of the accelerator simulation, the percentage of depth dose and dose profile were also measured in the water phantom in 5-Azar hospital. The simulation has been performed by the Geant4.10.3 version in order to assess investigate the effect of different magnetic fields with a variety of intensity on dose profiles. Based on the profile dose distribution obtained for magnetic fields with different types and intensities it can be realized that the field of 3 T has the optimal condensation of the dose profile distribution. The striking feature the flux of electrons dramatically decreases for the tangential field compared to other magnetic fields, as in the dose profile graphs of this filed were shown.

Simultaneous Optimization of Radiation-Imaging Coincidence for a Multi-Energy Linac.

Introduction:Medical physics guidelines stress the importance of radiation-imaging coincidence, especially for stereotactic treatments. However, multi-energy linear accelerators may only allow a single imaging isocenter. A procedure was developed to simultaneously optimize radiation-imaging isocenter coincidence for all linac photon energies on a Versa HD.Materials and Methods:First, the radiation beam center of each energy was adjusted to match the collimator rotation axis using a novel method that combined ion chamber measurements with a modified Winston-Lutz (WL) test using images only at gantry, couch, and collimator angles of 0°. With all energies properly steered, an 8-field WL test was performed to determine average linac isocenter position across all energies, gantry, and collimator angles. Lasers and the kV imaging isocenter were calibrated to the average linac isocenter of all photon energies. Finally, A 12-field WL test consisting of gantry, couch, and collimator rotations was used to adjust the couch rotation axis to the average linac isocenter, thereby minimizing overall radiation-imaging isocentricity of the system.Results:Using this method, the beam centers were calibrated within 0.10 mm of collimator rotation axis, and linac isocenter coincidence was within 0.20 mm for all energies. Couch isocenter coincidence was adjusted within 0.20 mm of average linac isocenter. Average radiation-imaging isocentricity for all energies was 0.89 mm (0.80–0.98 mm) for a single imaging isocenter.Conclusion:This work provides a method to adjust radiation-imaging coincidence within 1.0 mm for all energies on Elekta's Versa HD.

Linac photon beam fine-tuning in PRIMO using the gamma-index analysis toolkit

BackgroundIn Monte Carlo simulations, the fine-tuning of linac beam parameters to produce a good match between simulated and measured dose profiles is a lengthy, time-consuming and resource-intensive process. The objective of this study is to utilize the results of the gamma-index analysis toolkit embedded inside the windows-based PRIMO software package to yield a truncated linac photon beam fine-tuning process.MethodsUsing PRIMO version 0.1.5.1307, a Varian Clinac 2100 is simulated at two nominal energy configurations of 6 MV and 10 MV for varying number of histories from 106 to more than 108. The dose is tallied on a homogeneous water phantom with dimensions 16.2 × 16.2 × 31.0 cm3 at a source-to-surface-distance of 100.0 cm. For each nominal energy setting, two initial electron beam energies are configured to reproduce the measured percent depth dose (PDD) distribution. Once the initial beam energy is fixed, several beam configurations are sequentially simulated to determine the parameters yielding good agreement with the measured lateral dose profiles. The simulated dose profiles are compared with the Varian Golden Beam Data Set (GBDS) using the gamma-index analysis method incorporating the dose-difference and distance-to-agreement criteria. The simulations are run on Pentium-type computers while the tuned 10 MV beam configuration is simulated at more than 108 histories using a virtual server in the Amazon.com Elastic Compute Cloud.ResultsThe initial electron beam energy configuration that will likely reproduce the measured PDD is determined by comparing directly the gamma-index analysis results of two different beam configurations. The configuration is indicated to yield good agreement with data if the gamma-index passing rates using the 1%/1 mm criteria generally increase as the number of histories is increased. Additionally at the highest number of histories, the matching configuration gives a much higher passing rate at the 1%/1 mm acceptance criteria over the other competing configuration. With the matching initial electron beam energy known, this input to the subsequent simulations allows the fine-tuning of the lateral beam profiles to proceed at a fixed yet lower number of histories. In a three-stage serial optimization procedure, the first remaining beam parameter is varied and the highest passing rate at the 1%/1 mm criteria is determined. This optimum value is input to the second stage and the procedure is repeated until all the remaining beam parameters are optimized. The final tuned beam configuration is then simulated at much higher number of histories and the good agreement with the measured dose distributions is verified.ConclusionsAs physical nature is not stingy, it reveals at low statistics what is hidden at high statistics. In the matter of fine-tuning a linac to conform with measurements, this characteristic is exploited directly by the PRIMO software package. PRIMO is an automated, self-contained and full Monte Carlo linac simulator and dose calculator. It embeds the gamma-index analysis toolkit which can be used to determine all the parameters of the initial electron beam configuration at relatively lower number of histories before the full simulation is run at very high statistics. In running the full simulation, the Amazon.com compute cloud proves to be a very cost-effective and reliable platform. These results are significant because of the time required to run full-blown simulations especially for resource-deficient communities where there could just be one computer as their sole workhorse.

A theoretical multileaf collimator model for fast Monte Carlo dose calculation of linac 6/10 MV photon beams

A complete linac photon beam Monte Carlo dose calculation, even on a fast computer, requires generally a substantial computation time. The existing approaches developed to improve the efficiency are mainly based on the use of variance reduction techniques, phase space sources or virtual source models. The collimation of the simulated photon beam, involving particle transport through the multileaf collimator, considerably increases the calculation time especially for intensity modulation radiation therapy, where multiple small and medium field shapes are taking place. In this paper an improvement of a previous model will aim to significantly enhance the calculation efficiency for fast intensity modulation computations mainly by the use of an analytical collimator. The linac photon beam and multileaf collimator model is controlled by several parameters, such as, the geometrical dimensions of the primary collimator, the flattening filter and the number of leaves of the linac’s collimator. The model uses only correlated energy spectra and particle fluence distributions derived from the International Atomic Energy Agency phase space files of Elekta Precise Varian TrueBeam.

PIN diodes for radiation therapy use: Their construction, characterization, and implementation

PurposeDevelopment and implementation of a PIN diode measurement system that can measure dose on the surface of a patient, which can be compared to dose calculated by a treatment planning system. Measurements are to be possible for static or rotational photon beams and electrons. Simple calibration procedures are to be devised that require a minimum set of correction factors. MethodsReadily available PIN type photodiodes are fabricated into devices that can be used for detection of ionizing radiation. Single or dual PIN diodes are soldered onto flexible shielded cables for static and rotational irradiation use. Diode signals are measured with an electrometer with zero input bias voltage. Diode temperature is determined by operating it as a thermistor. Linac photon and electron dose is measured. A commercial treatment planning system is used for calculating dose expected in various test geometries. ResultsThe scanning and surface diodes have an intrinsic buildup of 0.3 mm water equivalence and the IMRT device a buildup of 1.5 mm. Correction factors are determined for changes in diode sensitivity with the following factors: dose-per-pulse, dose rate, angle of radiation incidence, temperature, and field size. The surface or IMRT diode detector measurements agree within 1.0% with Eclipse calculations. ConclusionsClinically useable dose detection devices can be fabricated from PIN type photodiodes. These diodes are used to measure dose at the surface of a patient or under bolus. Correction factors are unnecessary if a surface or IMRT type device is chosen for use with static orthogonal or rotational IMRT delivered beams, respectively.

P143] Quality assurance of tomoedge dynamic jaws: A retrospective analysis with the statistical control charts approach

Purpose TomoEDGETM refers to the sliding window motion of the jaws along the superior-inferior edge of the treatment target. An automatic procedure, (“Dynamic Jaws Sweep” DJS), tests the movement functions with specific exercises and measures the alignment of the linac photon source in relation to jaw collimation evaluating the longitudinal dose profiles. The results of the DJS were retrospectively analyzed using the statistical control charts. Methods From June 2013 to September 2016 the DJS was performed, on a weekly basis, on a TomoHDATM system for a total amount of 130 runs. All measurements were analyzed with TomoTherapy Quality Assurance (TQA) software that provides: the fluence output factor (FOF) for the three clinical jaws aperture (J20, J14, J7), the time skew and the linac photon source position along the longitudinal direction (IECy SP). Qualitative graphs related to specific exercises are also produced. For each quantitative parameter the Individual (ICC) and the Moving Range chart control (MRCC) were implemented. For both charts the Central value, the Upper and Lower Control Limit are based on the average and on the statistical variance of the first 10 runs. The control chart results were compared to the TQA limits, suggested by the vendor. Results The FOF values outside the limits overlap with erroneous qualitative graphs, pointing out a real critical issue of jaw movements. Both the Individual and the Moving range control chart showed analogous results; indeed, with the ICC the FOF were outside limits in 12.3%, 13.1% and 20% of their measurements for J20, J14 and J7 jaw settings respectively and the time skew parameter only in 4.6%, while, with the MRCC, the corresponding values were 9.2%, 10.1%, 14.3% and 9.0% for the time skew parameter. The IECy SP, while remaining within vendor specific values, rapidly diverged from its initial value with an increasing trend, eventually approaching the zero value. Conclusions Control chart approach seems to be an useful tool for studying in depth the results of the DJS procedure. Much more stringent TQA limits, based on the own machine behavior, are useful to easily detect critical issue related to the dynamic jaws movements.

On the impact of ICRU report 90 recommendations on kQ factors for high-energy photon beams.

To assess the impact of the ICRU report 90 recommendations on the beam-quality conversion factor, kQ , used for clinical reference dosimetry of megavoltage linac photon beams. The absorbed dose to water and the absorbed dose to the air in ionization chambers representative of those typically used for linac photon reference dosimetry are calculated at the reference depth in a water phantom using Monte Carlo simulations. Depth-dose calculations in water are also performed to investigate changes in beam quality specifiers. The calculations are performed in a cobalt-60 beam and MV photon beams with nominal energy between 6MV and 25MV using the EGSnrc simulation toolkit. Inputs to the calculations use stopping-power data for graphite and water from the original ICRU-37 report and the new proposed values from the recently published ICRU-90 report. Calculated kQ factors are compared using the two different recommendations for key dosimetry data and measured kQ factors. Less than about 0.1% effects from ICRU-90 recommendations on the beam quality specifiers, the photon component of the percentage depth-dose at 10cm, %dd(10)x , and the tissue-phantom ratio at 20cm and 10cm, TPR1020, are observed. Although using different recommendations for key dosimetric data impact water-to-air stopping-power ratios and ion chamber perturbation corrections by up to 0.54% and 0.40%, respectively, we observe little difference (≤0.14%) in calculated kQ factors. This is contradictory to the predictions in ICRU-90 that suggest differences up to 0.5% in high-energy photon beams. A slightly better agreement with experimental values is obtained when using ICRU-90 recommendations. Users of the addendum to the TG-51 protocol for reference dosimetry of high-energy photon beams, which recommends Monte Carlo calculated kQ factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.

A photon source model based on particle transport in a parameterized accelerator structure for Monte Carlo dose calculations.

An accurate source model of a medical linear accelerator is essential for Monte Carlo (MC) dose calculations. This study aims to propose an analytical photon source model based on particle transport in parameterized accelerator structures, focusing on a more realistic determination of linac photon spectra compared to existing approaches. We designed the primary and secondary photon sources based on the photons attenuated and scattered by a parameterized flattening filter. The primary photons were derived by attenuating bremsstrahlung photons based on the path length in the filter. Conversely, the secondary photons were derived from the decrement of the primary photons in the attenuation process. This design facilitates these sources to share the free parameters of the filter shape and be related to each other through the photon interaction in the filter. We introduced two other parameters of the primary photon source to describe the particle fluence in penumbral regions. All the parameters are optimized based on calculated dose curves in water using the pencil-beam-based algorithm. To verify the modeling accuracy, we compared the proposed model with the phase space data (PSD) of the Varian TrueBeam 6 and 15MV accelerators in terms of the beam characteristics and the dose distributions. The EGS5 Monte Carlo code was used to calculate the dose distributions associated with the optimized model and reference PSD in a homogeneous water phantom and a heterogeneous lung phantom. We calculated the percentage of points passing 1D and 2D gamma analysis with 1%/1mm criteria for the dose curves and lateral dose distributions, respectively. The optimized model accurately reproduced the spectral curves of the reference PSD both on- and off-axis. The depth dose and lateral dose profiles of the optimized model also showed good agreement with those of the reference PSD. The passing rates of the 1D gamma analysis with 1%/1mm criteria between the model and PSD were 100% for 4×4, 10×10, and 20×20cm2 fields at multiple depths. For the 2D dose distributions calculated in the heterogeneous lung phantom, the 2D gamma pass rate was 100% for 6 and 15MV beams. The model optimization time was less than 4min. The proposed source model optimization process accurately produces photon fluence spectra from a linac using valid physical properties, without detailed knowledge of the geometry of the linac head, and with minimal optimization time.

Ge-doped silica optical fibres as RL/OSL dosimeters for radiotherapy dosimetry

Various tailor-made doped-silica optical fibres are investigated as dosimeters in support of radiotherapy, based on radioluminescence (RL) and optically stimulated luminescence (OSL) technology. Investigations focus on the development of these glassy dosimetric media, offering a number of advantages going well beyond their water impervious nature and excellent spatial resolution (∼few microns). An RL/OSL photomultiplier-tube (PMT)-based reader was assembled, providing for study of the influence on the RL/OSL signal of different Ge-dopant concentrations (3.59, 4.74 and 7.03wt%) in silica fibres exposed to medical LINAC photon beams. Among the three arbitrary choices of dopant concentration, those fibres containing the least concentration of Ge (3.59wt%), denoted as Ge-1, gave rise to the greatest RL yield, at 1.67 and 2.34 times that of Ge-2 (fibres Ge-doped at 4.74wt%) and Ge-3 (fibres Ge-doped at 7.03wt%) respectively, reducing in yield with increasing Ge-dopant concentration. At 7.3×103 counts Gy−1min at 22°C, the Ge-1fibres provided the superior sensitivity, also being found to be reusable without noticeable variation in RL signal (<1%; 1 SD) for X-ray exposures delivered at a dose-rate of 600cGy/min. The RL signal was found to be free from spectral superposition or noise, also exhibiting energy independence in the use of X-rays generated at 6- and 10MV. In regard to percentage depth-dose (PDD), in measurements made using optical fibre dosimeters and 6MV X-ray photon beams, the maximum value of PDD, dmax, was obtained at a depth of 1.5cm, in accord with ionization chamber measurements. Using green stimulation light, for all three concentrations of Ge-dopant, linearity between the OSL signal and dose was observed across the 0.5- to 8Gy dose range investigated. Results from this ongoing study are intended to assist in efforts towards improving the performance of RL/OSL fibre sensors for radiotherapy dosimetry.

Investigation of X-ray focal spot alignment using a jig of novel design.

The X-ray focal spot and the centre of the flattening filter should be placed as close as possible to the central axis (CAX) on a linear accelerator (linac) to produce a radially symmetric beam. The aim of this study is to devise a method to easily optimise the focal spot position of Varian linac photon beams generated with a flattening filter. A simple and robust jig was designed and built to be inserted into the largest electron applicator. Accessory and jaw position interlocks were overridden to enable photon beam operation. The jig was made from aluminium and consists of a plate permanently fixed inside a low melting point alloy (LMPA) insert frame and a block machined to suspend an ion chamber below the plate, such that the axis of the chamber is at the linac isocentre. The jig was used to optimise the position of the X-ray beam focal spot with respect to the device central axis (CAX). This was achieved by minimising the percentage change in ionisation chamber signal between collimator rotations from 90° to 270° as position steering was changed. As part of the investigation, in-plane (radial) and cross-plane (transverse) profiles obtained from water phantom scans were used to quantify how large the percentage ionisation change can be before profiles are distorted in the region of the CAX.

Energy and field size dependence of a silicon diode designed for small-field dosimetry.

To investigate the energy dependence/spectral sensitivity of silicon diodes designed for small-field dosimetry and obtain response factors (RFs) for arbitrary photon spectra using Monte Carlo (MC) simulations. The EGSnrc user-code DOSRZnrc was used to calculate the dose deposition in water and in the active volume of a stereotactic diode field detector (SFD). Then, the RFs of the SFD were calculated for several circular field sizes and energies at 5 cm depth in water. Several low-energy photon spectra (mean energy 55 to 200 keV), as well as Co-60 radiation (mean energy 1.25 MeV) and a 6 MV Elekta Synergy beam (mean energy 2.9 MeV), in 10 × 10 cm2 field size were used to validate the MC calculations, using a simple beam model. The RFs of the SFD detector for a 6 MV Elekta Synergy linac photon beam in different field sizes were calculated. These were also measured with EBT3 Gafchromic film and the SFD detector. For the reference field size, differences between measured and calculated RFs were less than 5% at mean energies below 1 MeV and less than 1% at energies above 1 MeV. The calculated RFs for a 6 MV Elekta Synergy linac photon beam as a function of different field sizes showed a good agreement between the measurements and previously reported results. This agreement was within 2% for all considered field sizes. While at high photon energies, the change of response of the SFD is marginal, whereas it is extreme at low energies. Therefore, it is desirable to benchmark response calculations also in the low-energy domain. Our results, with a simple beam model and geometry, indicate that a validation of the simulations by experimental results is achievable. The present work provides a comprehensive table that can be used to calculate SFD detector response factors depending on both, field size and photon energy.