Relative Depth Dose Research Articles

Carbon-fiber alternative to the commercial gating surrogate for the Varian Truebeam™

Background In this study we present the Tracking Accessory 3 (TA3) as an alternative to the commercial gating block (GB) surrogate for the Varian Truebeam™ gating system (TGS). The TGS requires three visible reflectors to track the surrogate, presenting an opportunity for a surrogate to be made with less material and thus smaller dosimetric footprint than the commercial four reflector model. Materials and methods Relative dose and depth dose profiles below the TA3 and the GB were measured with radiosensitive film. Accuracy and reproducibility of the detected motion amplitude for three TA3s and one GB were determined using a respiratory phantom with surrogate to determine the camera’s tracking volume. Clinical performance was evaluated prospectively in 10 breast cancer patients treated with deep inspiration breath hold monitored with TA3 and compared to previously published results. Non-parametric statistics were applied to test for significance. Results and conclusions Surface doses were increased up to 94% and 187% for the TA3 and GB, respectively, compared to no surrogate. The surface area influenced by at least 25% increase in dose was 12 cm2 and 105 cm2 for the TA3 and GB, respectively. The water equivalent thickness of the surrogates was found to be 1 mm for the TA3 and 3 mm for GB. The difference in measured amplitude were <0.2 mm for TA3 compared to the GB. The TA3s and GB were detected at all extremes of the clinically relevant tracking volume of the TGS. Clinical performance showed no significant differences. The TA3 caused less surface dose increase compared to the commercial GB. In the tested range all surrogates measured motion amplitude within 0.2 mm of reference value, which is not a clinically relevant difference. The TA3 showed no significant differences in clinical performance to similarly positioned surrogates.

Optically Stimulated Luminescence in LiF–MgF2 system and its response as medical radiation dosimeter

Optically Stimulated Luminescence (OSL) based dosimeters are garnering attention in the field of radiation dosimetry due to a number of benefits like portable equipment, easy and efficient excitation and read out process, re-readability of the dose etc.. Though varieties of fluoroperovskites have been studied as OSL materials, no research on LiMgF3 (Lithium Magnesium Fluoride) as an OSL material has yet been conducted. In the current work, LiF–MgF2 system with a low effective atomic number was synthesized using the high temperature solid state reaction approach and its OSL dosimetry properties were investigated. The prepared sample was thoroughly characterized by employing a variety of techniques, such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and UV–Vis (ultraviolet–visible) spectrophotometry. The structure of the sample was found to be polycrystalline with size in the micrometre range. OSL properties of LMF have been investigated for different doses of gamma and beta along with related OSL kinetic parameters. It exhibits good dose linearity under blue stimulation for gamma (100 mGy - 700 Gy) and beta (30 mGy - 5Gy) radiation. We have also investigated the reusability (3%), reproducibility (2–3%), and fading (45% in 40 days) of the phosphor. To check the viability of the sample in radiation dosimetry under medical treatment in radiotherapy, various dosimetric properties were studied in Linear Accelerator (LINAC) x-rays in MV energy range and Cobalt-60 gamma radiation. These studies include the response of the phosphor to various energies (called energy dependence), field size effect, the effect of the dose rate of the radiation beam, and relative depth dose response. The overall response of the phosphor (comparable with ion chamber in RT experiments) makes the material a potential candidate to be used as an OSL phosphor in medical radiation dosimetry.

Three-dimensional (3D) optical imaging of electron beam and X-rays from medical linear accelerators (LINAC) using a plastic scintillator plate in water

Although optical imaging of electron beams and X-rays from medical linear accelerators (LINAC) is a possible method for dose distribution measurements, it has been limited to two-dimensional (2D) projection images. For the precise measurement of an optical image of electron beams and X-rays, three-dimensional (3D) imaging is desired. To measure 3D dose distributions, we conducted imaging of electron beams and X-rays using a plastic scintillator plate set in a water phantom. When this plate was immersed in the water phantom, irradiation with electron beams or X-rays was carried out from along the plate's sides. Optical images of the scintillator plate were acquired using a charge-coupled device (CCD) camera from the side during irradiation with electron beams and X-rays. Measurements were conducted at 6 MeV, 9 MeV and 12 MeV for electron beams and at 6 MV and 10 MV for X-rays. The imaging system was set on the bed of the LINAC and moved at 10-mm steps perpendicular to the beam direction to acquire a set of sliced optical images of the beams. A set of these sliced images were stacked and interpolated to form 3D optical images. For the 3D images, after the correction of the Cherenkov-light component in the images, the relative depth and lateral doses were evaluated. From the relative depth doses of electron beams, the half-value depths could be evaluated within an error of 1.3 mm. Lateral widths could be evaluated within an error of less than 2 mm parallel to the plastic scintillator and less than 6.5 mm perpendicular to it. From the relative depth doses of X-rays, the average difference between the measured value and that by a planning system was within an error of 2 %. Lateral widths could be evaluated within an error of less than 0.68 mm parallel to the plastic scintillator and less than 2.6 mm perpendicular to it. We confirmed that 3D imaging of electron beams and X-rays using plastic scintillator plate is feasible and is a promising method for measuring dose images at any position.

Characterization of CaF2:Dy (TLD-200) dosimeters and determination of dosimetric quantities in mammography

This study aimed to characterize CaF2:Dy (TLD-200) thermoluminescence dosimeter (TLD) in mammography, and to determine entrance surface dose (ESD), relative depth dose (RDD), and mean glandular dose (MGD) values. The angular and energy dependencies, dose-response, and TL intensity reproducibility characteristics of dosimeters were investigated. The dosimeters showed a linear dose-response behavior corresponding to air kerma values ranging between 2.11 and 21.98 mGy and a reproducibility of ±5%. The response of the dosimeters showed by ∼48% over response when the tube potential increased from 22 to 32 kVp, indicating that they have a substantial energy dependence. On the other hand, the TL intensity at 90° decreased by 70% compared with that at 0°, when the angular dependence of dosimeters was investigated. The ESD and MGD values obtained for both polymethyl methacrylate (PMMA) and BR-12 phantoms were compatible with existing literature, and the MGD values were also agreed with the European Federation of Organisations for Medical Physics (EFOMP). The RDD value decreased by nearly 90% at 6 cm depth. These dosimetric quantities did not change considerably for PMMA and BR-12 phantoms. The results showed that TLD-200 could be used as an alternative dosimeter in mammography to determine dosimetric quantities.

TOPAS Monte Carlo simulation for a scanning proton therapy system in SPHIC

PurposeThis study aims to use TOPAS Monte Carlo (MC) code to model the beamline of a scanned proton therapy system in Shanghai Proton and Heavy Ion Center (SPHIC). This tool can be useful for treatment plan quality assurance. Methods and materialsIn-air lateral profiles of single proton spots at five positions along the Z-axis were measured, which can then be used to calculate the angular variance, spatial angular covariance, and spatial variance at the beam exit position. The mean energy and energy spread of the primary incident beam for 15 energies were tuned by comparing with measured relative integrated depth dose at the central axis. Three spread-out Bragg peak plans in homogeneous water cubes and one plan with a half anthropomorphic head phantom were created to verify the accuracy of the model. In addition, the comparison was also performed to RayStation TPS with an MC dose calculation engine for the plans. ResultsThe average difference of spot sizes was 1.3% for all five energies. For all 15 energies, the average range (D90) difference was 0.04 mm, the mean distal-fall-off (DDF) difference was 0.04 mm, and the mean full width at half maximum (FWHM) difference was 0.10 mm. For the three homogeneous cube plans, a maximum difference of 0.2 mm for the D90 was observed, and the absolute differences in the lateral penumbras for each plan all agreed within 1 mm. For the plan on the anthropomorphic half head phantom, the mean dose deviation is less than 1.2% compared to measurements. ConclusionWe successfully developed an MC model for the proton scanning system used in SPHIC, which can be helpful for dose verification of the Treatment Planning System (TPS) and quality assurance in the future.

Optimization of bromine-based radical initiators using leucomalachite green and solvents in PRESAGE® dosimeter

This study has two main objectives. The first objective is to create an ideal dosimeter with improved sensitivity and tissue-like equivalent characteristics. Formulas F1 and F2 were fabricated for this specific purpose. The second objective is to investigate the effect of two bromine-based radical initiators and the effect of adding two types of solvents to PRESAGE® formula on the radiological properties and dose sensitivity of PRESAGE® dosimeter. Formulas F3, F4, F5, F6, and F7 were fabricated for this investigation. In total, seven different PRESAGE® dosimeters containing up to 2 wt% of tetrabromomethane (CBr4) and 1,1,1,2-tetrabromoethane (C2H2Br4) radical initiators were fabricated. They were irradiated with either 250 or 100-kVp X-rays ranging from 0 to 40 Gy. Their optical density changes pre and post irradiation were measured using a spectrophotometer. Besides, radiological properties and sensitivity of PRESAGE® dosimeters were investigated for different percentages of radical initiators and varying amounts of cyclohexanone with a fixed amount of dimethyl sulfoxide (DMSO; 2 wt%). Using Monte Carlo methods, the relative depth doses in water of F2, F3, and F5 were calculated for 50, 100, and 300- kVp X-ray beams and 1.25 MeV from cobalt-60. For all PRESAGE® formulas, an excellent linear correlation between dose response and applied radiation dose was observed. CBr4 showed much higher sensitivity than C2H2Br4 even when using the same weight percentage. The formula F2 exhibited improved sensitivity to radiation than the presented PRESAGE® dosimeter formula named as MOD3, along with radiological properties close to water. The PRESAGE® formula F5 showed a sensitivity increase of 46.1% with addition of DMSO; however, it showed an increase in effective atomic number by ∼1%. The PRESAGE® formula F2 is equivalent to water than formulas F3 and F5 in terms of depth-dose response and can be used in both kilo- and megavoltage beams without correction factors. The PRESAGE® dosimeters with CBr4 as a radical initiator can be used if high sensitivity is required. However, an effective atomic number is not water-equivalent when using a considerable amount of radical initiator (>0.50 wt%).

Technical note: Providing proton fields down to the few‐MeV level at clinical pencil beam scanning facilities for radiobiological experiments

The adequate performance of radiobiological experiments using clinical proton beams typically requires substantial preparations to provide the appropriate setup for specific experiments. Providing radiobiologically interesting low-energy protons is a particular challenge, due to various physical effects that become more pronounced with larger absorber thickness and smaller proton energy. This work demonstrates the generation of decelerated low-energy protons from a clinical protonbeam. Monte Carlo simulations of proton energy spectra were performed for energy absorbers with varying thicknesses to reduce the energy of the clinical proton beam down to the few-MeV level corresponding to m-ranges. In this way, a setup with an optimum thickness of the absorber with a maximum efficiency of the proton fluence for the provisioning of low-energy protons is supposed to be found. For the specific applications of 2.5-3.3MeV protons and -particle range equivalent protons, the relative depth dose was measured and simulated together with the dose-averaged linear energy transfer (LETd) distribution. The resulting energy spectra from Monte Carlo simulations indicate an optimal absorber thickness for providing low-energy protons with maximum efficiency of proton fluence at an user-requested energy range for experiments. For instance, providing energies lower than 5MeV, an energy spectrum with a relative total efficiency of to the initial spectrum was obtained with the optimal setup. The measurements of the depth dose, compared to the Monte Carlo simulations, showed that the dosimetry of low-energy protons works and protons with high LETd down to the range of -particles can be produced. This work provides a method for generating all clinically and radiobiologically relevant energies - especially down to the few-MeV level - at one clinical facility with pencil beam scanning. Thereby, it enables radiobiological experiments under environmentally uniform conditions.

Evaluation of bone dose arising from skin cancer brachytherapy: A comparison between 192Ir and 60Co sources through Monte Carlo simulations

PurposeThis study aimed to calculate and compare absorbed dose to bone following exposure to 192Ir and 60Co sources in high dose rate (HDR) skin brachytherapy. Moreover, effects of the bone thickness and soft tissue thickness before the bone on absorbed dose to the bone are evaluated . Materials and methods192Ir and 60Co sources inserted in Leipzig applicators with internal diameters of 1, 2 and 3 cm with/without their optimal flattening filters were simulated by MCNPX Monte Carlo code. Then, heterogeneous phantoms (including skin, soft tissue before and after the bone, cortical bone and bone marrow) were defined. Finally, relative depth dose values for the bone and other tissues in the heterogeneous phantoms were obtained and compared. ResultsThe average relative depth dose values of the skin, soft tissue before and after bone and bone marrow were almost similar for both 192Ir and 60Co sources, with a maximum difference less than 2%. However, a 0.1–6.8% difference was observed between average relative depth dose values of these two sources for the cortical bone. The results showed that with increasing the bone thickness and bone distance from the skin surface, the average relative depth dose values of the bone marrow and cortical bone decreased for both 192Ir and 60Co sources inserted in the applicators without/with their optimal flattening filters. For most of evaluated the applicators without/with their flattening filters, the average relative depth dose values of the bone marrow arisen from the 60Co source were higher than those obtained from the 192Ir source, while an opposite trend was observed for the cortical bone . ConclusionThe obtained findings showed that the average relative depth dose values of 192Ir and 60Co sources at the corresponding depths of the designed heterogeneous phantoms were almost similar (expect for the cortical bone). Hence, it is concluded that 60Co source can be used instead of 192Ir source in HDR skin brachytherapy, particularly in developing countries.

A Comparison between Electron Gamma Shower, National Research Council/Easy Particle Propagation (EGSnrc/Epp) and Monte Carlo N-Particle Transport Code (MCNP) in Simulation of the INTRABEAM ® System with Spherical Applicators.

Background: Online Monte Carlo (MC) treatment planning is very crucial to increase the precision of intraoperative radiotherapy (IORT). However, the performance of MC methods depends on the geometries and energies used for the problem under study.Objective: This study aimed to compare the performance of MC N-Particle Transport Code version 4c (MCNP4c) and Electron Gamma Shower, National Research Council/easy particle propagation (EGSnrc/Epp) MC codes using similar geometry of an INTRABEAM® system.Material and Methods: This simulation study was done by increasing the number of particles and compared the performance of MCNP4c and EGSnrc/Epp simulations using an INTRABEAM® system with 1.5 and 5 cm diameter spherical applicators. A comparison of these two codes was done using simulation time, statistical uncertainty, and relative depth-dose values obtained after doing the simulation by each MC code.Results: The statistical uncertainties for the MCNP4c and EGSnrc/Epp MC codes were below 2% and 0.5%, respectively. 1e9 particles were simulated in 117.89 hours using MCNP4c but a much greater number of particles (5e10 particles) were simulated in a shorter time of 90.26 hours using EGSnrc/Epp MC code. No significant deviations were found in the calculated relative depth-dose values for both in the presence and absence of an air gap between MCNP4c and EGSnrc/Epp MC codes. Nevertheless, the EGSnrc/Epp MC code was found to be speedier and more efficient to achieve accurate statistical precision than MCNP4c. Conclusion: Therefore, in all comparisons criteria used, EGSnrc/Epp MC code is much better than MCNP4c MC code for simulating an INTRABEAM® system.

Gradient corrections for reference dosimetry using Farmer-type ionization chambers in single-layer scanned proton fields.

The local depth dose gradient and the displacement correction factor for Farmer-type ionization chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields. Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement. Depth dose gradients are as low as 0.1-0.4% per mm at measurement depths from 1 to 5cm in the highest clinical proton energies (with residual ranges higher than 15cm) and increase to 1% per mm at a residual range of 4cm and become larger than 3% per mm for residual ranges lower than 2cm. The literature review shows that the effective point of measurement of Farmer-type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5cm for which the residual range is 4cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2cm and for residual ranges of 15cm or higher. The literature review confirmed that for proton beams the effective point of measurement of Farmer-type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4cm while the third option only for residual ranges of at least 15cm.

Commissioning, dosimetric characterization and machine performance assessment of the LIAC HWL mobile accelerator for Intraoperative Radiotherapy

BackgroundThe LIAC HWL (Sordina IORT Technologies, Vicenza, Italy) is a recently designed mobile linear accelerator for intraoperative electron radiotherapy (IOeRT), producing high dose rate electron beams at four different energy levels. It features a software tool for the visualization of 2D dose distributions, which is based on Monte Carlo simulations. The aims of this work were to (i) assess the dosimetric characteristics of the accelerator, (ii) experimentally verify calculated data exported from the software and (iii) report on commissioning as well as performance of the system during the first year of operation. MethodsThe electron energies of the LIAC HWL used in this study are 6, 8, 10 and 12 MeV. Diameters of the cylindrically shaped applicators range from 3 to 10cm. We studied two applicator sets with different length ratios of proximal and terminal applicator sections. Reference dosimetry, linearity as well as short- and long-term stability were measured with a PTW Advanced Markus chamber, relative depth dose and profiles were measured using an unshielded diode. Percentage-depth-dose (PDD) and transversal dose profile (TDP) data were exported from the simulation software LIACSim and compared with our measurements. ResultsThe device reaches dose rates up to 40Gy/min (for 12 MeV). Surface doses for the 10cm applicators are higher than 90%, X-ray background is below 0.6% for all energies. Simulations and measurements of PDD agreed well, with a maximum difference in the depth of the 50% isodose of 0.7mm for the flat-ended applicators and 1mm for the beveled applicators. The simulations slightly underestimate the dose in the lateral parts of the field (difference < 1.8% for flat-ended applicators). The two different applicator sets were dosimetrically equivalent. Long-term stability measurements for the first year of operation ranged from -2.1% to 1.6% (mean: -0.1%). ConclusionsThe system is dosimetrically well suited for IOeRT and performed stably and reliably. The software tool for visualization of dose distributions can be used to support treatment planning, following thorough validation.

Relative depth-doses from square and rectangular Y-90 beta applicators used in skin cancer therapy

Plaques containing beta-emitting isotopes have several medical applications. In particular, due to the short range of beta particles and the rapid fall off of dose rates they are suitable to be used in small and superficial skin cancer therapy, killing the tumor cells and preserving the underlying healthy structures. The analytical/numerical beta-point dose function formalism is used to a simple estimate of the relative dose rates around square and rectangular beta applicator containing the Yttrium-90 isotope. Maximum differences in relative dose rates along central axis of 13% at 6.6 mm depth and 20% at 5 mm depth were found, respectively, for the square and rectangular plaques as compared with the literature measurements data. Results have also indicated that for both plaques dose rates fall off to 50% of the reference value at about 2 mm depth; drop to just 10% at 4.5 mm depth, and become zero from 8 mm on. This fact confirms that these sources are suitable to be used in superficial skin cancer therapy. The method presented here may be used to estimate the relative dose rates around square and rectangular Y-90 beta applicators in a simple and fast way.

Theoretical estimate of relative depth-doses for the asymmetric and spherical 106Ru/106Rh COC-type plaque used in radiotherapy

In this work we present results of relative dose rate along central and lateral axis of the 106Ru/106Rh COC model plaque used for the treatment of tumors close to the optical nerve. The calculations were based on the beta-point dose function and are compared with an available data obtained by means of Monte Carlo simulations. Even with the limitations of the method, it was capable to reproduce the general trend of the dose rates around this kind of plaque and a maximum difference of -18% with MC was obtained. Despite the limitations of the method, the results may be used as a guide to future experimental and theoretical studies on this applicator, since there is a lack of available data in literature.

Detector response in the buildup region of small MV fields.

The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen. We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface-near area of dose buildup for fields sized between 10×10 and 0.6×0.6cm2 . Depth dose curves were measured with 14 different detectors including plane-parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects. Depth-dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13cm3 scanning chamber CC13 and a plane-parallel chamber PPC05 agree very well in fields sized 4×4 and 10×10cm2 . For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used. A scanning chamber or a PPC40 chamber is suitable for fields larger than 4×4cm2 . Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half the inner radius of cylindrical ionization chambers for the effective point of measurement should be used. For small ionization chambers, averaging between both polarities is necessary for data obtained near the surface.

PO-0894 Characterization of a multilayer ionization chamber for relative depth-dose curves in particle beams

PO-0894 Characterization of a multilayer ionization chamber for relative depth-dose curves in particle beams

An Experimental Slope Method for a More Accurate Measurement of Relative Radiation Doses using Radiographic and Radiochromic Films and Its Application to Megavoltage Small-Field Dosimetry.

Purpose:An experimental method using the linear portion of the relative film dose–response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.Materials and Methods:The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.Results:The maximum percentage deviation of PDDFS with PDDIC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDDFS with PDDIC for electron beams was within 3% for energy range studied of 8–15 MeV. The gamma passing rates of PDDFS with PDDIC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDDFS with PDDIC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROFFS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDDFS and PDDMC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROFFS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.Conclusions:The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.

The potential impact of ultrathin filter design on dosimetry and relative biological effectiveness in modern image-guided small animal irradiators.

Modern image-guided small animal irradiators like the Xstrahl Small Animal Radiation Research Platform (SARRP) are designed with ultrathin 0.15 mm Cu filters, which compared with more heavily filtrated traditional cabinet-style biological irradiators, produce X-ray spectra weighted toward lower energies, impacting the dosimetric properties and the relative biological effectiveness (RBE). This study quantifies the effect of ultrathin filter design on relative depth dose profiles, absolute dose output, and RBE using Monte Carlo techniques. The percent depth-dose and absolute dose output are calculated using kVDoseCalc and EGSnrc, respectively, while a tally based on the induction of double-strand breaks as a function of electron spectra invoked in PENELOPE is used to estimate the RBE. The RBE increases by >2.4% in the ultrathin filter design compared to a traditional irradiator. Furthermore, minute variations in filter thickness have notable effects on the dosimetric properties of the X-ray beam, increasing the percent depth dose (at 2 cm in water) by + 0.4%/0.01 mm Cu and decreasing absolute dose (at 2 cm depth in water) by -1.8%/0.01 mm Cu for the SARRP. These results show that modern image-guided irradiators are quite sensitive to small manufacturing variations in filter thickness, and show a small change in RBE compared to traditional X-ray irradiators. We quantify the consequences of ultrathin filter design in modern image-guided biological irradiators on relative and absolute dose, and RBE. Our results show these to be small, but not insignificant, suggesting laboratories transitioning between irradiators should carefully design their radiobiological experiments.

Mucosal dosimetry on unflattened photon beams: a Monte Carlo phantom study

Objective: This study investigated the dependences of mucosal dose and photon energy distribution on the flattened and unflattened photon beams using Monte Carlo simulation. Methods: A heterogeneous mucosa phantom with a variation of mucosal thickness (0.5–3 mm) was irradiated by the 6 and 10 MV flattened and unflattened photon beams generated by a Varian TrueBeam linac. The photon energy distributions at the mucosa and depth doses at the bones and mucosa were calculated using the EGSnrc-based Monte Carlo code. Results: It is found that the 6 MV unflattened photon beam contained more particle fluence in the low-energy range (0–100 keV) than the 10 MV. However, mucosal doses for the unflattened photon beams were found lower than the flattened. This is different from the skin dose having dose enhancement on the unflattened beam compared to the flattened, though both mucosal and skin structure involve an air-soft tissue interface. The particle fluence of the 6 and 10 MV unflattened photon beams increased with an increase of the mucosal thickness. This agreed well with the relative depth doses that increased in the ranges of 3.4%–3.6% and 2.5%–3.0% at the upper mucosa, and 5.1%–5.7% and 2.9%–3.2% at the lower mucosa, with their thickness increasing from 0.5–3 mm for the 6 and 10 MV unflattened photon beams. Conclusion: It is concluded that the unflattened photon beam results in a lower mucosal dose than the flattened, and the 6 MV unflattened beam has a larger dependence of mucosal dose on its thickness when compared to the 10 MV.

Toward a novel dosimetry system using acrylic disk radiation sensor for proton pencil beam scanning.

Fabricate an acrylic disk radiation sensor (ADRS) and characterize the photoluminescence signal generated from the optical device as basis for the development and evaluation of a new dosimetry system for pencil beam proton therapy. Based on the characteristics of the proposed optical dosimetry sensor, we established the relation between the photoluminescence output and the applied dose using an ionization chamber. Then, we obtained the relative integral depth dose profiles using the photoluminescence signal generated by pencil beam irradiation at energies of 99.9 and 162.1 MeV, and compared the results with the curve measured using a Bragg peak ionization chamber. The relation between the photoluminescence output and applied dose was linear. In addition, the ADRS was dose independent for beam currents up to 6 Gy/min, and the calibration factor for energy was close to 1. Hence, the energy dependence on the optical device can be disregarded. The integral depth dose profiles obtained for the ADRS suitable agreed with the curve measured in the Bragg peak ionization chamber without requiring correction. These results suggest that the ADRS is suitable for dosimetry measurements in pencil beam scanning, and it will be employed as a low-cost and versatile dosimetry sensor in upcoming developments.

Characterisation of two new radiochromic gel dosimeters TruView™ and ClearView™ in combination with the vista™ optical CT scanner: A feasibility study

PurposeThis study aims at characterising the properties of TruView™ and ClearView™ two new gel dosimeters (Modus Medical Devices Inc.) and at studying the feasibility of relative dosimetry using these dosimeters and the Vista™ Optical CT scanner to accurately evaluate dose. MethodsIn this work, we investigated key dosimetric aspects (dose response, energy and dose rate dependence) and stability of these radiochromic gels initiated in preliminary works (Huet et al., 2017; Colnot et al., 2017) using spectrophotometric measurements. Moreover, by mean of optical CT scanning (Vista™), their performances to measure relative depth dose (PDD) and cross profiles were analysed. ResultsTruView™ and ClearView™ present a linear dose response up to 20 Gy and up to 80 Gy respectively, independent of both photon beam energy (4–18 MV) and dose rate (up to 9.9 Gy/min) (Huet et al., 2017; Colnot et al., 2017). ClearView™ response proves to be stable for a week post-irradiation and uniform within the batch whereas TruView™ presents an unstable but uniform response. Optical CT scanning generates errors due to stray light that need to be corrected in order to use these gels; ClearView™ scanning particularly requires important precautions. After corrections, those gels used in combination with the Vista™ scanner show promising spatial and dosimetric precision (dose difference <5%). Finally, TruView™ is reusable and presents excellent reproducible response (maximum 3% difference) and the ClearView™ dosimeter presents good spatial stability (0.5% difference after 6 days). ConclusionThis study provides important knowledge about two gel dosimeters presenting interesting dosimetric properties. A study is ongoing to benchmark those promising candidates for clinical dose verification.