Dosimetric Tool Research Articles

A technique for quantifying the sensitivity of dosimetric tool gamma with 2D detector array in pretreatment IMRT plans by segment deletion method.

Motivation of this study is to check the sensitivity of dosimetric tool gamma with 2D detector array combination when unexpected errors occur while transferring intensity-modulated radiation therapy treatment plans from planning system to treatment unit. This study consists of 17 head and neck cancer patient's treatment plans. Nine types of verification plans are created for all 17 clinically approved treatment plans by consecutively deleting different segments (up to eight) one by one from each field of the plan. Decrement factor (χ) is introduced in our study which illustrated the degree of decay of gamma passing rate when intentional errors are introduced. We analyzed the data by two different methods-one without selecting the region of interest (ROI) in dose distributions and the other by selecting the region of interest. By linear regression, the absolute value of slopes is 0.025, 0.024 and 0.015 without ROI and 0.030, 0.027 and 0.015 with ROI for 2%/2mm, 3%/3mm and 5%/5mm criteria, respectively. The higher absolute value of the fitted slope indicates the higher sensitivity of this method to identify erroneous plan in treatment unit. The threshold value for 2%/2mm equivalent to 95% passing criteria in 3%/3mm used in clinical practice is obtained as 83.44%. The 2D detector array with dosimetric tool gamma is less sensitive in detecting errors when unprecedented errors of segment deletion occur within the treatment plans.

OpenDose: Open-Access Resource for Nuclear Medicine Dosimetry.

Radiopharmaceutical dosimetry depends on the localization in space and time of radioactive sources and requires the estimation of the amount of energy emitted by the sources deposited within targets. In particular, when computing resources are not accessible, this task can be performed using precomputed tables of specific absorbed fractions (SAFs) or S values based on dosimetric models. The aim of the OpenDose collaboration is to generate and make freely available a range of dosimetric data and tools. Methods: OpenDose brings together resources and expertise from 18 international teams to produce and compare traceable dosimetric data using 6 of the most popular Monte Carlo codes in radiation transport (EGSnrc/EGS++, FLUKA, GATE, Geant4, MCNP/MCNPX, and PENELOPE). SAFs are uploaded, together with their associated statistical uncertainties, in a relational database. S values are then calculated from monoenergetic SAFs on the basis of the radioisotope decay data presented in International Commission on Radiological Protection Publication 107. Results: The OpenDose collaboration produced SAFs for all source region and target combinations of the 2 International Commission on Radiological Protection Publication 110 adult reference models. SAFs computed from the different Monte Carlo codes were in good agreement at all energies, with SDs below individual statistical uncertainties. Calculated S values were in good agreement with OLINDA/EXM 2.0 (commercial) and IDAC-Dose 2.1 (free) software. A dedicated website (www.opendose.org) has been developed to provide easy and open access to all data. Conclusion: The OpenDose website allows the display and downloading of SAFs and the corresponding S values for 1,252 radionuclides. The OpenDose collaboration, open to new research teams, will extend data production to other dosimetric models and implement new free features, such as online dosimetric tools and patient-specific absorbed dose calculation software, together with educational resources.

Minibeam radiation therapy at a conventional irradiator: Dose-calculation engine and first tumor-bearing animals irradiation

Purpose: Minibeam radiation therapy (MBRT) is a novel therapeutic strategy, whose exploration was hindered due to its restriction to large synchrotrons. Our recent implementation of MBRT in a wide-spread small animal irradiator offers the possibility of performing systematic radiobiological studies. The aim of this research was to develop a set of dosimetric tools to reliably guide biological experiments in the irradiator.Methods: A Monte Carlo (Geant4)-based dose calculation engine was developed. It was then benchmarked against a series of dosimetric measurements performed with gafchromic films. Two voxelized rat phantoms (ROBY, computer tomography) were used to evaluate the treatment plan of F98 tumor-bearing rats. The response of a group of 7 animals receiving a unilateral irradiation of 58 Gy was compared to a group of non-irradiated controls.Results: The good agreement between calculations and the experimental data allowed the validation of the dose-calculation engine. The latter was first used to compare the dose distributions in computer tomography images of a rat’s head and in a digital model of a rat’s head (ROBY), obtaining a good general agreement. Finally, with respect to the in vivo experiment, the increase of mean survival time of the treated group with respect to the controls was modest but statistically significant.Conclusions: The developed dosimetric tools were used to reliably guide the first MBRT treatments of intracranial glioma-bearing rats outside synchrotrons. The significant tumor response obtained with respect to the non-irradiated controls, despite the heterogenous dose coverage of the target, might indicate the participation of non-targeted effects.

New application of polymer gels in medical radiation dosimetry: Plasmonic sensors

Polymer gels are amongst the most promising future dosimetric tools for medical applications, since irradiated gels are capable to represent a phantom and a dosimeter with a 3D-dose distribution measurement capability and biological tissue equivalency in one. High spatial resolution of these gels but limited dose sensitivity, especially in low dose region, as well complicated and time consuming read out methods are the main issues needing improvement.Operating principle of plasmonic sensors which are commonly used in biotechnology to assess the concentration and reaction rate of various organic compounds, is based on the excitation of strong free electron oscillations (surface plasmon resonance, SPR) in the boundary layer between metal and dielectric using polarized light and measurement of the refracted light characteristics. Plasmonic sensors stand out from the other sensing technologies due to their high resolution and relatively low uncertainties.Combination of plasmonic sensors’ characteristics and capabilities of dosimetric gels is key when developing dosimetric gel based SPR sensors for point dose measurements and corresponding relatively simple, however sensitive, in-house made dosimetric system working in angular mode, all of which is presented in this paper.Developed plasmonic sensor itself comprised out of Au diffraction grating, extracted from commercial optical media disc, spin-coated with a thin dosimetric gel (nPAG or VIPET) layer. Fabrication procedure of the plasmonic sensor and creation of the corresponding optical read out system are discussed in this paper. Irradiation experiments performed with dosimetric gel based SPR sensor (point dosimeter) have shown the feasibility of this new type dosimetry system to measure low absorbed doses (up to 5Gy) and revealed the potential of this system to perform measurements with the dosimetric sensitivity up to two times higher as compared to other known optical dose evaluation methods. Key aspects regarding sensitivity and resolution of plasmonic dosimeters are also discussed.

Benchmarking a GATE/Geant4 Monte Carlo model for proton beams in magnetic fields.

PurposeMagnetic resonance guidance in proton therapy (MRPT) is expected to improve its current performance. The combination of magnetic fields with clinical proton beam lines poses several challenges for dosimetry, treatment planning and dose delivery. Proton beams are deflected by magnetic fields causing considerable changes in beam trajectories and also a retraction of the Bragg peak positions. A proper prediction and compensation of these effects is essential to ensure accurate dose calculations. This work aims to develop and benchmark a Monte Carlo (MC) beam model for dose calculation of MRPT for static magnetic fields up to 1 T.MethodsProton beam interactions with magnetic fields were simulated using the GATE/Geant4 toolkit. The transport of charged particle in custom 3D magnetic field maps was implemented for the first time in GATE. Validation experiments were done using a horizontal proton pencil beam scanning system with energies between 62.4 and 252.7 MeV and a large gap dipole magnet (B = 0–1 T), positioned at the isocenter and creating magnetic fields transverse to the beam direction. Dose was measured with Gafchromic EBT3 films within a homogeneous PMMA phantom without and with bone and tissue equivalent material slab inserts. Linear energy transfer (LET) quenching of EBT3 films was corrected using a linear model on dose‐averaged LET method to ensure a realistic dosimetric comparison between simulations and experiments. Planar dose distributions were measured with the films in two different configurations: parallel and transverse to the beam direction using single energy fields and spread‐out Bragg peaks. The MC model was benchmarked against lateral deflections and spot sizes in air of single beams measured with a Lynx PT detector, as well as dose distributions using EBT3 films. Experimental and calculated dose distributions were compared to test the accuracy of the model.ResultsMeasured proton beam deflections in air at distances of 465, 665, and 1155 mm behind the isocenter after passing the magnetic field region agreed with MC‐predicted values within 4 mm. Differences between calculated and measured beam full width at half maximum (FWHM) were lower than 2 mm. For the homogeneous phantom, measured and simulated in‐depth dose profiles showed range and average dose differences below 0.2 mm and 1.2%, respectively. Simulated central beam positions and widths differed <1 mm to the measurements with films. For both heterogenous phantoms, differences within 1 mm between measured and simulated central beam positions and widths were obtained, confirming a good agreement of the MC model.ConclusionsA GATE/Geant4 beam model for protons interacting with magnetic fields up to 1 T was developed and benchmarked to experimental data. For the first time, the GATE/Geant4 model was successfully validated not only for single energy beams, but for SOBP, in homogeneous and heterogeneous phantoms. EBT3 film dosimetry demonstrated to be a powerful dosimetric tool, once the film response function is LET corrected, for measurements in‐line and transverse to the beam direction in magnetic fields. The proposed MC beam model is foreseen to support treatment planning and quality assurance (QA) activities toward MRPT.

An Evaluation and Feasibility Study for the Need of New Dosimetric Tools and Metrics for Lung Cancer Patients Receiving Radiotherapy

Evaluation of potential toxicity to healthy lung tissue in radiotherapy of lung cancer is assessed via the percentage of the healthy lung volume receiving doses between 5 to 30 Gy. Respiration during treatment changes lung density, affecting the radiological path length. This results in differences in the absorbed dose and variations in volume metrics taken from the static DVH, making the V5, for example, dynamic. This retrospective analysis of 10 lung cancer patients demonstrates the percent change (%Δ) of V5, V10 and V20 within the same patient when the clinical dose calculated on the average-intensity-projection CT (avgCT) is recomputed on 10 4DCT phases. We also compute and propose alternative mass-based metrics generated from the same CT data. We assess if they are demonstrably more stable and statically significant in their differences from the volumetric data when compared to the avgCT. Lung contours were redrawn on each patient’s avgCT using auto segmentation and then propagated to all the breathing phases (0%-90%) using deformable registration. The mean Hounsfield units (HU) and the volume of the lungs was extracted. The data sets were pushed to the treatment planning system were the clinically administered plan was recomputed on each phase and the V5, V10 and V20 were tabulated for all 110 CT sets. The 5, 10 and 20 Gy isodose lines (IDL) on each CT were converted to contours and their volumes acquired. Using the HU as a surrogate for density, the mass of each patient’s lung, along with the mass of contours of 5, 10 and 20 Gy IDLs were computed on all breathing phases and compared to the avgCT, yielding the mass of lung receiving 5, 10 and 20Gy (M5, M10, M20). For consistency, data from avgCT was the benchmark for %Δ of the all the mass and volume metrics. We observed a mean standard deviation (SD) of lung mass %Δ of 0.94± 0.4%, versus a mean SD of lung volume %Δ of 4.32 ± 0.95%. Comparisons of patients’ M values and V values to the avgCT values yielded mean SD of %Δ of V5 vs. M5 as 2.64 ± 1.03% vs. 1.99 ± 0.48% respectively, V10 vs. M10 as 2.61 ± 0.94% vs. 1.75 ± 0.49%, and V20 vs. M20 as 2.81 ± 1.38% vs. 2.16 ± 0.67%, respectively. An Analysis of Variance (ANOVA) between the %Δ of mass data and the reciprocal volumetric data, all yielded p-values of < 0.02.One-sample, two tail t-tests of the %Δ in mass metrics from a hypothesized 0% variance from the avgCT also yield p-values of <0.01, showing that there is a low probability that the change observed from 0% variation is due to chance. The low p-values of the ANOVA test show that although the mass metrics are derived from the volume data, the %Δ between corresponding data points is independent of each other. The lower SD and low p-values of the %Δ in mass metrics in all the breathing phases may point to the mass of lung irradiated to be more reliable way to assess dose to healthy lung. This would allow for larger retrospective study where the association could be determined between radiation induced pneumonitis and mass of lung irradiated by specific doses.

Radiological tissue equivalence of deformable silicone-based chemical radiation dosimeters (FlexyDos3D).

FlexyDos3D, a silicone‐based chemical radiation dosimeter, has great potential to serve as a three‐dimensional (3D) deformable dosimetric tool to verify complex dose distributions delivered by modern radiotherapy techniques. To facilitate its clinical application, its radiological tissue needs to be clarified. In this study we investigated its tissue‐equivalence in comparison with water and Solid Water (RMI457). We found that its effective and mean atomic numbers were 40% and 20% higher and the total interaction probabilities for kV x‐ray photons were larger than those of water respectively. To assess the influence of its over‐response to kV photons, its HU value was measured by kV computed tomography (CT) and was found higher than all the soft‐tissue substitutes. When applied for dose calculation without correction, this effect led to an 8% overestimation in electron density via HU‐value mapping and 0.65% underestimation in target dose. Furthermore, depth dose curves (PDDs) and off‐axis ratios (profiles) at various beam conditions as well as the dose distribution of a full‐arc VMAT plan in FlexyDos3D and reference materials were simulated by Monte Carlo, where the results showed great agreement. As indicated, FlexyDos3D exhibits excellent radiological water‐equivalence for clinical MV x‐ray dosimetry, while its nonwater‐equivalent effect for low energy x‐ray dosimetry requires necessary correction. The key findings of this study provide pertinent reference for further FlexyDos3D characterization research.

Characterization of EBT3 radiochromic films for dosimetry of proton beams in the presence of magnetic fields.

PurposeRadiochromic film dosimetry is extensively used for quality assurance in photon and proton beam therapy. So far, GafchromicTM EBT3 film appears as a strong candidate to be used in future magnetic resonance (MR) based therapy systems. The response of Gafchromic EBT3 films in the presence of magnetic fields has already been addressed for different MR‐linacs systems. However, a detailed evaluation of the influence of external magnetic fields on the film response and calibration curves for proton therapy has not yet been reported. This study aims to determine the dose responses of EBT3 films for clinical proton beams exposed to magnetic field strengths up to 1 T in order to investigate the feasibility of EBT3 film as an accurate dosimetric tool for a future MR particle therapy system (MRPT).MethodsThe dosimetric characteristics of EBT3 films were studied for a proton beam passing through magnetic field strengths of B = 0, 0.5, and 1 T. Absorbed dose calibration and measurements were performed using clinical proton beams in the nominal energy range of 62.4–252.6 MeV. Irradiations were done using an in‐house developed PMMA slab phantom placed in the center of a dipole research magnet. Monte Carlo (MC) simulations using the GATE/Geant4 toolkit were performed to predict the effect of magnetic fields on the energy deposited by proton beams in the phantom. Planned and measured doses from 3D box cube irradiations were compared to assess the accuracy of the dosimetric method using EBT3 films with/without the external magnetic field.ResultsNeither for the mean pixel value nor for the net optical density, any significant deviations were observed due to the presence of an external magnetic field (B ≤ 1T) for doses up to 10 Gy. Dose‐response curves for the red channel were fitted by a three‐parameter function for the field‐free case and for B = 1T, showing for both cases an R‐square coefficient of unity and almost identical fitting parameters. Independently of the magnetic field, EBT3 films showed an under‐response as high as 8% in the Bragg peak region, similarly to previously reported effects for particle therapy. No noticeable influence of the magnetic field strength was observed on the quenching effect of the EBT3 films.ConclusionsFor the first time detailed absorbed dose calibrations of EBT3 films for proton beams in magnetic field regions were performed. Results showed that EBT3 films represent an attractive solution for the dosimetry of a future MRPT system. As film response functions for protons are not affected by the magnetic field strenght, they can be used for further investigations to evaluate the dosimetric effects induced due to particle beams bending in magnetic fields regions.

A linear relationship for the LET-dependence of Gafchromic EBT3 film in spot-scanning proton therapy

Radiochromic film (RCF) is a valuable dosimetric tool, primarily due to its sub-millimeter spatial resolution. For accurate proton dosimetry, the dependence of film response on linear energy transfer (LET) must be characterized and calibrated. In this work, we characterized film under-response, or ‘quenching’, as a function of dose-weighted linear energy transfer (LETd) in several proton fields and established a simple, linear relationship with LETd.We performed measurements as a function of depth in a PMMA phantom irradiated by a spot-scanning proton beam. The fields had energies of 71.3 MeV, 71.3 MeV with filter, and 159.9 MeV. At each depth (measurements taken in depth step sizes of 0.5–1 mm in the Bragg peak), we measured dose with a PTW Markus ion chamber and EBT3 RCF. EBT3 under-response was characterized by the ratio of dose measured with film to that with ion chamber. LETd values for our experimental setup were calculated using in-house clinical Monte Carlo code. Measured film under-response increased with LETd, from approximately 10% under-response for LETd = 5 keV µm−1 to approximately 20% for LETd = 8 keV µm−1. The under-response for all measurements was plotted versus LETd. A linear fit to the data was performed, yielding a function for under-response, , with respect to LETd: . Finally, the linear under-response relationship was applied to a film measurement within a spread-out Bragg peak (SOBP). Without correcting for LETd-dependence in the SOBP measurement, the discrepancy between film and Monte Carlo profiles was greater than 15% at the distal edge. With correction, the corrected film profile was within 2% and 1 mm of the Monte Carlo profile.RCF response depends on LETd, potentially under-responding by >15% in clinically-relevant scenarios. Therefore, it is insufficient to perform only a dose calibration; LET calibration is also necessary for accurate proton film dosimetry. The LETd-dependence of EBT3 can be described by a single, linear function over a range of clinically-relevant proton therapy LETd values.

2 Dosimetry comparison for brain metastases treatment by stereotactic radiotherapy

Introduction Technical capabilities for brain metastases treatment in stereotactic radiotherapy are in-house Dynamic Conformal Arc Therapy (DCA, with non-coplanar arcs DCAr and without DCA0) and non-coplanar Volumetric-Modulated Arc Therapy (VMAT). This study is to decide on the most efficient method use to get the optimum PTV coverage and dose escalation, OAR sparing, especially healthy brain tissue, and minimum monitor units. Methods On Elekta’s Monaco treatment planning system, eight patients with brain metastases and an average target volume of 9.13 cc [3.58–26.5] were selected. Six of these patients were treated with DCAR / DCA0 and two with VMAT on VERSA HD radiation therapy system. For all these patients, the two other planning techniques were carried out retrospectively. Dosimetric analysis tools HI, CI, CO, OCO [1] as well as PTV coverage, PTV overdose, the 50% isodose line and healthy brain constraints ( V 12 Gy 5.9 cc, V 18 Gy 1 cc) enabled this comparison. The selected indicators were compared using non-parametric Wilcoxon’s rank tests. Results Homogeneity index is significantly higher for DCA0, as well as for DCA R compared to VMAT. The overdose for DCA0 is significantly lower than for the other two techniques. For the eight patients, the 50% isodose line is significantly smaller with DCA R . The healthy brain tissue gain (OCO) is significant using VMAT dosimetric planning rather than DCA0 (p 0.05). VMAT significantly saves healthy brain tissue for 12 Gy isodose line compared to DCA0 (p = 0.031). The same applies for DCA R compared to DCA0 (p = 0.047). Concerning average monitor units, there is 1738 UM for VMAT treatment planning against 1449 (−16%) for DCA R and 1359 (−21.8%) for DCA0. Conclusions Comparison between DCA with coplanar ( DCA r ) and non-coplanar arcs (DCA0) shows up that coplanar arcs were of significant interest for overdose at the center of PTV, for a 50% isodose line narrower and help sparing healthy brain tissue. DCA R and VMAT techniques are comparable on this last point. However, using non-coplanar arcs involves considerable additional processing time and controls. The study will be followed up with other patient data to confirm the use of VMAT treatment planning technique on target volumes from 3.58 to 26.5 cc.

Patient-specific quality control for intensity-modulated radiation therapy and volumetric-modulated arc therapy using electronic portal imaging device and two-dimensional ion chamber array

AbstractAimThe purpose of this study was to develop the patient-specific quality control (QC) process by most commonly used dosimeters in Bangladesh and recommend a suitable passing rate for QC, irrespective of the dosimetric tools used.Materials and methodsIntensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans of five head-and-neck (HN) and five prostate patients were selected for the patient-specific QC. These plans were generated using the Eclipse TPS v11·0 (Varian Medical Systems, Inc., Palo Alto, CA, USA) 6 MV X-ray from a Varian TrueBeam linear accelerator (Varian Medical Systems, Inc.) for each case. Each IMRT and VMAT plans were measured by two-dimensional (2D) ion chamber arrays (I’matriXX) and electronic portal imaging devices (EPID), respectively. The passing rates of the dosimetric tools were calculated using criteria of 3%/3 mm.ResultsThe average passing rates (±SD) of I’matriXX for prostate and HN were 97·9±0·76 and 98·88±0·24, respectively. For VMAT verification, the average passing rates of EPID for prostate for arc1 and arc2 were 96·15±0·49 and 97·8±0·70, respectively; similarly, for HN the rates were 97·85±0·63 and 97·2±0·56, respectively.ConclusionThe results showed that both the dosimeters can be used in patient-specific QC, although the EPID-based IMRT and VMAT QC is more advantageous in terms of time-saving and ease of use. Hence, for patient-specific QC, one can use the ion chamber arrays (I’matriXX) or EPID in hospital, but the systems need to be cross-checked.

OA237] The output factor measurements of cyberknife system using various detectors

Purpose In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors. Conclusions 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume.

TL and OSL dosimetric characterization of different luminescent materials for clinical electron beams application in TSEB treatments

Thermoluminescent dosimeters (TLDs) play an important role in radiotherapy for the dosimetry of ionizing radiation. This type of dosimeter presents advantages that makes them a useful tool for measurements in anthropomorphic phantoms and in vivo dosimetry. Several dosimetric materials have been used in the radiotherapy sectors such as LiF, µLiF, CaSO4:Dy. The OSL dosimetry has also been widely applied using Aluminum Oxide (Al2O3:C). These dosimeters have advantages over TLDs due to their high sensitivity, extensive linearity in response to the dose, faster reading, possibility of multiple readings and the need to perform the heat treatment of the samples. The aim of this work was to compare and characterize, using TL and OSL techniques, different luminescent dosimeters (LiF, µLiF, CaSO4:Dy and Al2O3:C) to be applied in clinical electron beam used to TSEB treatments. Measurements were performed in order to study the applicability of these detectors as easy-to-take alternatives to calibration and measurements of TSEB treatments. Parameters such as dose-response curves; average sensitivity to radiation, intrinsic efficiency and energy and angular dependences were evaluated. The results show good agreement within CaSO4:Dy and TLD-100 measurements and, applying energy and angle dependence factors over the other two materials, all the four detectors can be applied as alternative easy-to-take dosimetric tools for commissioning and quality assurance of 6 MeV clinical electron beams used in TSEB treatments.

Development of Optical Fiber Based Measurement System for the Verification of Entrance Dose Map in Pencil Beam Scanning Proton Beam.

This study describes the development of a beam monitoring system for the verification of entrance dose map in pencil beam scanning (PBS) proton therapy based on fiber optic radiation sensors (FORS) and the validation of this system through a feasibility study. The beam monitoring system consisted of 128 optical fibers optically coupled to photo-multiplier tubes. The performance of the beam monitoring system based on FORS was verified by comparing 2D dose maps of square-shaped fields of various sizes, which were obtained using conventional dosimeters such as MatriXX and EBT3 film, with those measured using FORS. The resulting full-width at half maximum and penumbra were compared for PBS proton beams, with a ≤2% difference between each value, indicating that measurements using the conventional dosimetric tool corresponded to measurements based on FORS. For irregularly-shaped fields, a comparison based on the gamma index between 2D dose maps obtained using MatriXX and EBT3 film and the 2D dose map measured by the FORS showed passing rates of 96.9 ± 1.3% and 96.2 ± 1.9%, respectively, confirming that FORS-based measurements for PBS proton therapy agreed well with those measured using the conventional dosimetric tools. These results demonstrate that the developed beam monitoring system based on FORS is good candidate for monitoring the entrance dose map in PBS proton therapy.

Investigation of Temperature Dependence of Polymer Gels for Use with Scanning Magnetic Resonance Imaging

Polymer gels are three-dimensional dosimetric tools. The purpose of the present study was to investigate the temperature dependence of polymer gels during scanning Magnetic Resonance Imaging. Prepared gels were irradiated with a 6MV X-ray beam at intensities ranging from 0 to 20 Gy in order to investigate their dose-R2 and dose-R1 responses. Irradiated gels were evaluated from 1.5-T magnetic resonance R2 and R1 images for each 5°C change in temperature from 5°C to 41°C, and then the four-field box technique irradiation plan was used to deliver a total dose of 4 Gy using the same beam weight in each direction to the prepared gels. The profile of the dose map generated from the four-field irradiated gel data at 20°C was then compared with the planned data. The dose-R2 response curve was linear up to 20 Gy at 20°C, with a slope of 1.17 Gy-1&dot;s-1. The slopes of the fitted curves of the dose-R2 decreased as gel temperature increased. The slopes of the dose-R1 curves were more parallel than the slopes of the dose-R2 curves between 5 and 41°C. The difference in the full width of half maximum of the gel profile data obtained using the four-field box technique at 20°C and the planned data were below 5% on average. The dose map from the irradiated gels obtained using the dose-R2 curve was the same as that from the planned data under the same temperature conditions. Measurement of difference between various temperatures is significant with dose accuracy. It is suitable to evaluate the gel dosimeter under the thermal equilibrium condition, MRI room temperature from the point of view of the stability of the irradiated gels.

36. Selective internal radiation therapy with yttrium-90: Optimization of pre-therapeutic dosimetry

Introduction During the simulation step of a selective internal radiation therapy (SIRT) with yttrium-90, SPECT/CT images using 99mTc-MAA (macroaggregated human albumin) are acquired in order to assess the lung shunt and the MAA biodistribution in the healthy liver and the tumour. A predictive dosimetry is then usually performed in order to estimate the mean absorbed dose delivered to the lungs, the healthy liver and the tumour during the treatment. The aim of this study was to find a simple and quantitative segmentation method that can easily be transferred to clinical routine. We chose to use 99mTc-MAA SPECT images alone and we compared this method with two other historically methods used in this procedure: anatomical segmentation obtained from CT alone and from co-registered SPECT/CT images. Methods The dosimetric reconstruction tool currently in development uses the Geant4 and the GATE Monte Carlo codes to provide dose maps in the area of an irradiation accident. An important feature of the simulation Two calibration curves were obtained from phantom studies using a NEMA/IEC phantom. The first curve was used to determine which threshold (SPECTthres) should be applied to match the actual organs volumes according to measured the signal to noise ratio (SNR) in the images. The second curve enabled the user to get quantitative data from the chosen threshold. This method was validated using the Stratos software with an anthropomorphic torso phantom: the mean absorbed doses to the tumour and the healthy liver were estimated by Voxel Dose Kernel (VDK) dosimetry and compared to the theoretical mean absorbed doses calculated with the MIRD formalism. The method was then compared with two other segmentation techniques and retrospectively applied to 5 patients. Results In phantom studies, the image derived characteristics of the healthy liver were preserved (volume deviation Conclusion Besides its ease of implementation in clinical routine, the SPECTthres technique allows operators to estimate accurately the volume and activity of 3D delineated object in the framework of SIRT with yttrium-90.

35. Conception of a numerical simulation tool for the deployable expert team CReDO (Operational Dose Reconstruction Cell)

Introduction In the context of a nuclear or radiological accident involving high doses of ionizing radiation, the treatment of the victims faces two different situations: – For contaminated patients, the priority goes to the treatment, which can often be “blindly” administered to reduce the committed dose as the main molecules don’t engage any harmful side effects. This therapeutic must take place early, before and during a precise assessment of the involved radionuclides. – For irradiated patients, priority goes to the diagnosis because it is essential to know how the dose is distributed among the organs in order to sort the victims according to the severity of the exposure. The victims can consequently be lead to the most appropriate health structures. At present there is yet only very few field techniques that are capable of rapidly characterizing an external radiation exposure in case of an accident involving a large amount of victims. Nevertheless scientific, industrial and military applications as well as terrorist menace generate a significant probability of such an event. Methods The dosimetric reconstruction tool currently in development uses the Geant4 and the GATE Monte Carlo codes to provide dose maps in the area of an irradiation accident. An important feature of the simulation device is to be able to operate in highly degraded situations. As it is integrated in a militarized and hardened case, it can be freed from any link to a remote computer cluster thanks to a powerful multicore calculator that allows performing dose simulations with total autonomy. Results Using a simple and intuitive graphical user interface, trained users will be able to quickly design the whole scene of the accident using mostly the mouse and navigate in this 3D virtual world with a first person camera. This simulation tool is also compatible to work with libraries of sources, voxel phantoms and shielding materials that can be quickly integrated into the modeled scene of the radiation accident. Numerical filters are currently in development to target the most exposed areas to help medical teams to guide victims through appropriate medical care management solutions. Conclusions This numerical simulation tool based on modern technologies for dose calculation is aimed to strengthen the current diagnostic arsenal by meeting the need of on-field dosimetric triage for radiation-exposed victims. Through a validation phase on standardized scenarios the relevance of this dosimetric solution is currently being tested for different type of possible external exposure situations.

Factors affecting exposure level for medical staff during orthopedic procedures under fluoroscopic control

Extended control of staff exposure in interventional radiology has been legally required over the last few years. This is determined by a number of factors, including the type of procedure, technical conditions and methodology. In orthopedic procedures fluoroscopy is used to control surgical reconstructions. An influence of particular factors on the registered values of doses received by the members of medical team performing osteosynthesis for limb fractures is presented in this paper. Doses received by individual interventional team members performing specific functions, operator, assisting physicians and scrub nurse, during a series of the procedures were measured. Each person was equipped with 4 dosimetric tools, containing thermoluminescent dosimeters, to measure the equivalent doses for the eyes, hand skin and the neck (outside the shield) and to evaluate effective doses. The investigations were performed in operational theatres of 3 hospitals in Łódź. The equivalent doses per one procedure for the eyes and hand skin of the operator were 0.029-0.073 mSv and 0.366-1.604 mSv, respectively. Significantly higher doses were noted during the procedures of intramedullary osteosynthesis, especially for the operator. An average age and body mass index (BMI) of patients treated in the monitored hospitals did not differ statistically. Based on the dosimetric measurements the following conclusions can be drawn: in orthopedic procedures of interventional radiology (IR) the exposure of the staff is mostly determined by the type of procedure and more precisely by its complexity and by the optimized use of X-ray unit, including pulsed fluoroscopy. It is also revealed that the operator is the most exposed person in the interventional team. Med Pr 2017;68(1):75-83.

Pixel response-based EPID dosimetry for patient specific QA.

Increasing use of high dose rate, flattening filter free (FFF), and/or small‐sized field beams presents a significant challenge to the medical physics community. In this work, we develop a strategy of using a high spatial resolution and high frame rate amorphous silicon flat panel electronic portal imaging device (EPID) for dosimetric measurements of these challenging cases, as well as for conventional external beam therapy. To convert a series of raw EPID‐measured radiation field images into water‐based dose distribution, a pixel‐to‐pixel dose–response function of the EPID specific to the linac is essential. The response function was obtained by using a Monte Carlo simulation of the photon transport in the EPID with a comprehensive calibration. After the raw image was converted into the primary incident photon fluence, the fluence was further convolved into a water‐based dose distribution of the dynamic field by using a pregenerated pencil‐beam kernel. The EPID‐based dosimetric measurement technique was validated using beams with and without flattening filter of all energies available in Varian TrueBeam STx™. Both regularly and irregularly shaped fields measured using a PTW 729 ion chamber array in plastic water phantom. The technique was also applied to measure the distribution for a total of 23 treatment plans of different energies to evaluate the accuracy of the proposed approach. The EPID measurements of square fields of 4 × 4 cm2 to 20 × 20 cm2, circular fields of 2–15 cm diameters, rectangular fields of various sizes, and irregular MLC fields were in accordance with measurements using a Farmer chamber and/or ion chamber array. The 2D absolute dose maps generated from EPID raw images agreed with ion chamber measurements to within 1.5% for all fields. For the 23 patient cases examined in this work, the average γ‐index passing rate were found to be 99.2 ± 0.6%, 97.4 ± 2.4%, and 72.6 ± 8.4%, respectively, for criterions of 3 mm/3%, 2 mm/2%, and 1 mm/1%. The high spatial resolution and high frame rate EPID provides an accurate and efficient dosimetric tool for QA of modern radiation therapy. Accurate absolute 2D dose maps can be generated from the system for an independent dosimetric verification of treatment delivery.

Investigation of Dose Distribution in Mixed Neutron-Gamma Field of Boron Neutron Capture Therapy using N -Isopropylacrylamide Gel

Gel dosimeters have unique advantages in comparison with other dosimeters. Until now, these gels have been used in different radiotherapy techniques as a reliable dosimetric tool. Because dose distribution measurement is an important factor for appropriate treatment planning in different radiotherapy techniques, in this study, we evaluated the ability of the N-isopropylacrylamide (NIPAM) polymer gel to record the dose distribution resulting from the mixed neutron-gamma field of boron neutron capture therapy (BNCT). In this regard, a head phantom containing NIPAM gel was irradiated using the Tehran Research Reactor BNCT beam line, and then by a magnetic resonance scanner. Eventually, the R-2 maps were obtained in different slices of the phantom by analyzing T2-weighted images. The results show that NIPAM gel has a suitable potential for recording three-dimensional dose distribution in mixed neutron-gamma field dosimetry. Copyright (C) 2016, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.