Intrinsic Energy Dependence Research Articles

Performance evaluation of an inorganic optical fibre dosimeter for use in external beam radiotherapy with pulsed beams

Objective. Optical fibre dosimeters (OFDs) offer great promise for real-timein vivodose measurement in radiation-based treatment modalities such as radiotherapy and brachytherapy. This is attributed to their many useful qualities such as high spatial resolution and sensitivity. However, there are several requirements that an optical fibre dosimeter must meet to be acceptable for dose measurement in a specified treatment modality. In this work, the dosimetric performance of a novel optical fibre dosimeter for use in external beam radiotherapy is presented.Approach. The dosimeter was characterised for photon beam energies between 6-15 MV using a Varian TrueBeam Linac at dose rates between 100-2400 MU/min and assessed based on its repeatability, dose dependence, dose rate dependence, energy dependence and dose-per-pulse dependence.Main Results. The results demonstrated excellent short-term repeatability of 0.3%, good linearity in response (R2>0.9997), and minor dose rate dependence between 0.53%-2.49% for all beam qualities investigated. As the scintillator of the OFD is non-water equivalent, Monte-Carlo-TOPAS simulations were used to calculate the absorbed dose energy dependence. A dose-per-pulse dependence was also investigated and compared with dosimetry measurements made with an ionisation chamber and simulated from the treatment planning system. An over-response of 20%was found at the lowest investigated dose-per-pulse, and an under-response of 34%was found at the highest investigated dose-per-pulse. This is believed to be due to an intrinsic energy dependence making this type of OFD unsuitable for external beam radiotherapy dosimetry.Significance. The OFD evaluated in this work was primarily designed for high-dose-rate brachytherapy whereas this study includes the first measurements made in external beam radiotherapy and highlights the challenges of transferability of the dosimeter to a different radiation source.

Determination of intrinsic energy dependence of point-like inorganic scintillation detector in brachytherapy.

Inorganic scintillation detectors (ISDs) are promising for in vivo dosimetry in brachytherapy (BT). ISDs have fast response, providing time resolved dose rate information, and high sensitivity, attributed to high atomic numbers. However, the conversion of the detector signal to absorbed dose-to-water is highly dependent on the energy spectrum of the incident radiation. This dependence is comprised of absorbed dose energy dependence, obtainable with Monte Carlo (MC) simulation, and the absorbed dose-to-signal conversion efficiency or intrinsic energy dependence requiring measurements. Studies have indicated negligible intrinsic energy dependence of ZnSe:O-based ISDs in Ir-192 BT. A full characterization has not been performed earlier. This study characterizes the intrinsic energy dependence of ZnSe:O-based ISDs for kV X-ray radiation qualities, with energies relevant for BT. Three point-like ISDs made from fiber-coupled cuboid ZnSe:O-based scintillators were calibrated at the Swedish National Metrology Laboratory for ionizing radiation. The calibration was done in terms of air kerma free-in-air, , in 13 X-ray radiation qualities, , from 25 to 300 kVp (CCRI 25-250kV and ISO 4037 N-series), and in terms of absorbed dose to water, , in a Co-60 beam, . The mean absorbed dose to the ISDs, relative to and , were obtained with the MC code TOPAS (Geant4) using X-ray spectra obtained with SpekPy software and laboratory filtration data and a generic Co-60 source. The intrinsic energy dependence was determined as a function of effective photon energy, , (relative to Co-60). The angular dependence of the ISD signal was measured in a 25 kVp (0.20mm Al HVL) and 135 kVp beam (0.48mm Cu HVL), by rotating the ISDs 180° around the fiber's longitudinal axis (perpendicular to the beam). A full 360° was not performed due to setup limitations. The impact of detector design was quantified with MC simulation. Above 30keV the intrinsic energy dependence varied with less than 5±4% from unity for all detectors (with the uncertainty expressed as the mean of all expanded measurement uncertainties for individual above 30keV, k=2). Below 30keV, it decreased with up to 17% and inter-detector variations of 13% were observed, likely due to differences in detector geometry not captured by the simulations using nominal geometry. In the 25 kVp radiation quality, the ISD signal varied with 24% over a ∼45° rotation. For 135 kVp, the corresponding variation was below 3%. Assuming a 0.05mm thicker layer of reflective paint around the sensitive volume changed the absorbed dose with 6.3% at the lowest , and with less than 2% at higher energies. The study suggests that the ISDs have an intrinsic energy dependence relative to Co-60 lower than 5±4% in radiation qualities with >30keV. Therefore, they could in principle be calibrated in a Co-60 beam quality and transferred to such radiation qualities with correction factors determined only by the absorbed dose energy dependence obtained from MC simulations. This encourages exploration of the ISDs' applications in intensity modulated BT with Yb-169 or other novel intermediate energy isotopes.

The intrinsic x-ray energy dependence of beryllium oxide (BeO) ceramic dosimeters

Beryllium oxide (BeO) ceramic is a radioluminescent and optically stimulated luminescent material which has been shown to be quite energy independent. However, BeO ceramic is a highly self-optically attenuating material. The aim of this study is to evaluate the effect self-optical attenuation has on the radioluminescence of the BeO ceramic, and how this effects the measured energy dependence. The energy dependence of a BeO ceramic fibre-coupled luminescent dosimeter was measured using a superficial x-ray unit with beams of 30–150 kVp. The energy dependence was measured with the dosimeter oriented parallel and perpendicular to the beam in order to enhance the effect of the self-optical attenuation from the BeO ceramic. From the measurements, energy dependence curves were generated showing the detector response in both orientations. The largest difference in response was >25% observed at the lowest energy.Monte-Carlo simulations of the dose distribution within the BeO ceramic were convolved with ray tracing modelling of the light collection from the BeO ceramic probe in order to simulate the total detector response. The light collection corrected the Monte Carlo simulation results, and the intrinsic energy dependence was obtained with the trends between the two orientations agreeing within 8%. Therefore, it can be concluded that the measured orientational energy dependence is due to the self-optical attenuation of the BeO ceramic.

Investigation of a synthetic diamond detector response in kilovoltage photon beams.

An important characteristic of radiation dosimetry detectors is their energy response which consists of absorbed-dose and intrinsic energy responses. The former can be characterized using Monte Carlo (MC) simulations, whereas the latter (i.e., detector signal per absorbed dose to detector) is extracted from experimental data. Such a characterization is especially relevant when detectors are used in nonrelative measurements at a beam quality that differs from the calibration beam quality. Having in mind the possible application of synthetic diamond detectors (microDiamond PTW 60019, Freiburg, Germany) for nonrelative dosimetry of low-energy brachytherapy (BT) beams, we determined their intrinsic and absorbed-dose energy responses in 25-250kV beams relative to a 60 Co beam, which is usually the reference beam quality for detector calibration in radiotherapy. Three microDiamond detectors and, for comparison, two silicon diodes (PTW 60017) were calibrated in terms of air-kerma free in air in six x-ray beam qualities (from 25 to 250kV) and in terms of absorbed dose to water in a 60 Co beam at the national metrology laboratory in Sweden. The PENELOPE/penEasy MC radiation transport code was used to calculate the absorbed-dose energy response of the detectors (modeled based on blueprints) relative to air and water depending on calibration conditions. The MC results were used to extract the relative intrinsic energy response of the detectors from the overall energy response. Measurements using an independent setup with a single ophthalmic BEBIG I25.S16 125 I BT seed (effective photon energy of 28keV) were used as a qualitative check of the extracted intrinsic energy response correction factors. Additionally, the impact of the thickness of the active volume as well as the presence of extra-cameral components on the absorbed-dose energy response of a microDiamond detector was studied using MC simulations. The relative intrinsic energy response of the microDiamond detectors was higher by a factor of 2 in 25 and 50kV beams compared to the 60 Co beam. The variation in the relative intrinsic energy response of silicon diodes was within 10% over the investigated photon energy range. The use of relative intrinsic energy response correction factors improved the agreement among the absorbed dose to water values determined using microDiamond detectors and silicon diodes, as well as with the TG-43 formalism-based calculations for the 125 I seed. MC study of microDiamond detector design features provided a possible explanation for inter-detector response variation at low-energy photon beams by differences in the effective thickness of the active volume. MicroDiamond detectors had a non-negligible variation in the relative intrinsic energy response (factor of 2) which was comparable to that in the absorbed-dose energy response relative to water at low-energy photon beams. Silicon diodes, in contrast, had an absorbed-dose energy dependence on photon energy that varied by a factor of 6, whereas the intrinsic energy dependence on beam quality was within 10%. It is important to decouple these two responses for a full characterization of detector energy response especially when the user and reference beam qualities differ significantly, and MC alone is not enough.

Composition and thickness dependence of TLD relative dose sensitivity: A Monte Carlo study

Absorbed dose sensitivity (SAD) of a dosimeter results from absorbed dose energy dependence (f) and intrinsic energy dependence (kbq) both influenced by compositional and dimensional details. Within attenuating media the SAD is also affected by the depth at which measurements are made as beam quality varies. Monte Carlo simulations allow for studying absorbed dose, importantly providing an effective means by which correction terms for detector response (frel) can be determined. Among the wide range of TLD materials that are available, most generally in particular geometries and size, the commercial products TLD-100 and TLD-100H are perhaps the types most commonly favoured by users. Comprising lithium fluoride (Li natural) co-doped with the activators Mg and Ti or Mg, Cu and P and compressed in a teflon binder to form thicknesses of from 1 mm to a few mm, these have been investigated in previous studies. For some common TLD materials available in small thickness, from 0.125 mm up to 1.000 mm, present study seeks to calculate and compare frel values obtained at different measurement depths in water relative to a reference depth. In simulating a reference radiation field, use has been made of the MCNPX code, calculation providing the correction factors at various depths in a water phantom. Results show that at sub-millimeter dimensions, the effect of detector thickness is more pronounced than the effective atomic number, Zeff, and density of the TLDs. Smaller thickness and higher Zeff TLDs need significant correction factors, with their deviation from unity increasing with the depth up to 15 cm; frel values in the range of 0.96–1.01 were obtained in this study. Perturbation resulting from the presence of the dosimeter was also evaluated in terms of dosimeter size and material composition. Although dosimeter size, density and Zeff are well-known parameters, all important in assessing the amount of perturbation, it has been shown herein that the effect of density is more pronounced than Zeff. For the particular TLDs it is also shown that 0.125 mm thick dosimeters produce a perturbation that might be considered negligible for radiation fields larger than a few centimeters.

Energy dependence of a radiophotoluminescent glass dosimeter for HDR 192 Ir brachytherapy source.

We determined correction factors for absorbed dose energy dependence and intrinsic energy dependence for measurements of absorbed dose to water around an 192 Ir source using a radiophotoluminescent glass dosimeter (RPLD) calibrated with a 4-MV photon beam. The ratio of the absorbed dose to the water and the average absorbed dose to RPLD for the 192 Ir beam relative to the same ratio in a 4MV photon beam defines the absorbed dose energy dependence and was determined at distances of 2-10cm (at intervals of 1cm) from the 192 Ir source in a water phantom using the egs_chamber user code. The RPLD was calibrated to measure absorbed dose to water, Dw , in a 4MV photon beam using an ionization chamber, which was also used to measure absorbed dose to water, Dw , in a water phantom using the 192 Ir source. The detector response radiophotoluminescence (RPL signal per average absorbed dose in the detector) in the 192 Ir beam relative to that in the 4MV photon beam (the relative intrinsic efficiency) was determined experimentally. Finally, the beam quality correction factor was obtained as the quotient between the absorbed dose energy dependence and the relative intrinsic efficiency and corrects for the difference between the beam quality Q0 used at calibration and the beam quality Q used in the measurements. The relative dose ratio of the average absorbed dose to water relative to RPLD ranged from 0.930 to 0.746, and the beam quality correction factor ranged from 0.999 to 0.794 for distances of 2-10cm from the 192 Ir source. The relative detector response to an 192 Ir source and a 4-MV photon beam was 0.930, and it did not vary significantly with distance. These results demonstrate that corrections for absorbed dose energy dependence and intrinsic energy dependence are required when using an RPLD to measure with sources different from the reference source providing the primary calibration.

OA153] EBT3 radiochromic film dosimetry in kV X-ray radiation therapy

Purpose To evaluate the suitability of EBT3 radiochromic films for the dosimetry of therapeutic kV X-ray beams and particularly examine their potential energy dependence. Methods Film samples from the same batch were irradiated at dose levels of 0.5, 1, 2 and 3 Gy using seven filters/qualities Q (80 ⩽ kVp ⩽ 220, 2.5mmAl ⩽ HVL ⩽ 2mmCu) of a unit calibrated using the in-air method [1] . Pre- and post-irradiation scanning facilitated the calculation of energy response, S AD , W (Q) = netOD(Q)/ D w (Q) for the red channel. S AD , W (Q) equals the inverse product of the intrinsic energy dependence, k bq (Q) = D film (Q)/netOD(Q), by the absorbed dose to water energy dependence, f(Q) = D w (Q)/ D film (Q). The latter was calculated using MCNP6 [2] employing secondary electron tracking in the experimental geometry with beam spectra from SpekCalc [3] and film composition from the literature [4] . k bq (Q) was calculated from experimental [ S AD , W (Q)] and simulation [f(Q)] results. Results EBT3 film does not exhibit energy dependence in the qualities studied. S AD , W (Q) results relative to the highest quality equal unity within 1%, which is well within experimental standard uncertainty (on the order of 4%). An increase of f - 1 (Q) with decreasing quality was found (up to 1.37 relative to the highest quality) suggesting an almost equal film under-response [ k bq - 1 (Q) 1], in accordance with the literature [4] . While S AD , W (Q) varies with dose due to the known non-linearity of film dose response, relative results are in close agreement regardless of dose level. Conclusions The energy dependence of EBT3 films is negligible in the energy range studied. Future studies will extend the studied energy range and focus on reducing experimental uncertainty, studying batch variability, and application of EBT3 for experimental dosimetry.

Experimental investigation of GafChromic® EBT3 intrinsic energy dependence with kilovoltage x rays, 137 Cs, and 60 Co.

To determine experimentally the intrinsic energy response, kbq , of EBT3 GafChromic® radiochromic film with kilovoltage x rays, 137 Cs, and 60 Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved. The EBT3 film was irradiated with known doses using 60 Co, 137 Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002mGy, were chosen for this work. Monte Carlo methods were used to convert air-kerma rates to dose rates to the film active layer for each energy. A total of 738 film dosimeters, each measuring (1.2×1.2) cm2 , were cut from three film sheets out of the same lot of the latest version of EBT3 film, to allow for multiple dosimeters to be irradiated by each target dose and beam quality as well as unirradiated dosimeters to be used as controls. Net change in optical density in excess of the unirradiated controls was measured using the UWMRRC Laser Densitometry System (LDS). The dosimeter intrinsic energy response, kbq , for each dose level was determined relative to 60 Co, as the ratio of dosimeter response to each beam quality relative to the absorbed dose to the film active volume at the same dose level. A simplified, single-hit mathematical model was used to derive a single-free-parameter, β, which is a proportionality constant that is dependent on beam quality and describes the microdosimetric interactions within the active layer of film. The response of β for each beam quality relative to 60 Co was also determined. kbq was determined for a wide range of doses and energies. The results show a unique variation of kbq as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for kbq within the 500-7002mGy range outside of the expanded measurement uncertainty of 3.65% (k=2). For doses less than 500mGy, the signal-to-noise ratio was too low to determine kbq accurately. The single-free-parameter, β, fit calculations derived from the single-hit model show a correlation with kbq that suggests that β, at least in part, characterizes the microdosimetric interactions that determine kbq . For the beam qualities investigated, a single energy-dependent kbq correction can be used for doses between 500 and 7002mGy. Using the single-hit model with the single-free-parameter fit to solve for β shows promise in the determination of the intrinsic energy response of film, with β being the mathematical analog of the measured kbq .

Suitability of microDiamond detectors for the determination of absorbed dose to water around high-dose-rate 192 Ir brachytherapy sources.

Experimental dosimetry of high-dose-rate (HDR) 192 Ir brachytherapy (BT) sources is complicated due to high dose and dose-rate gradients, and softening of photon energy spectrum with depth. A single crystal synthetic diamond detector microDiamond (PTW 60019, Freiburg, Germany) has a small active volume, high sensitivity, direct readout, and nearly water-equivalent active volume. The purpose of this study was to evaluate the suitability of microDiamond detectors for the determination of absorbed dose to water around HDR 192 Ir BT sources. Three microDiamond detectors were used, allowing for the comparison of their properties. In-phantom measurements were performed using microSelectron and VariSource iX HDR 192 Ir BT treatment units. Their treatment planning systems (TPSs), Oncentra (v. 4.3) and BrachyVision (v. 13.6), respectively, were used to create irradiation plans for a cubic PMMA phantom with the microDiamond positioned at one of three source-to-detector distances (SDDs) (1.5, 2.5, and 5.5cm) at a time. The source was stepped in increments of 0.5cm over a total length of 6cm to yield absorbed dose of 2Gy at the nominal reference-point of the detector. Detectors were calibrated in 60 Co beam in terms of absorbed dose to water, and Monte Carlo (MC) calculated beam quality correction factors were applied to account for absorbed-dose energy dependence. Phantom correction factors were applied to account for differences in dimensions between the measurement phantom and a water phantom used for absorbed dose calculations made with a TPS. The same measurements were made with all three of the detectors. Additionally, dose-rate dependence and stability of the detectors were evaluated in 60 Co beam. The percentage differences between experimentally determined and TPS-calculated absorbed doses to water were from -1.3% to +2.9%. The values agreed to within experimental uncertainties, which were from 1.9% to 4.3% (k=2) depending on the detector, SDD and treatment delivery unit. No dose-rate or intrinsic energy dependence corrections were applied. All microDiamonds were comparable in terms of preirradiation dose, stability of the readings and energy response, and showed a good agreement. The results indicate that the microDiamond is potentially suitable for the determination of absorbed dose to water around HDR 192 Ir BT sources and may be used for independent verification of TPS's calculations, as well as for QA measurements of HDR 192 Ir BT treatment delivery units at clinical sites.

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, , was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4–6.4, 2.5–6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min−1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 03: Energy dependence of a clinical probe‐format calorimeter and its pertinence to absolute photon and electron beam dosimetry

Purpose:To evaluate the intrinsic and absorbed‐dose energy dependence of a small‐scale graphite calorimeter probe (GPC) developed for use as a routine clinical dosimeter. The influence of charge deposition on the response of the GPC was also assessed by performing absolute dosimetry in clinical linac‐based electron beams.Methods:Intrinsic energy dependence was determined by performing constant‐temperature calorimetry dose measurements in a water‐equivalent solid phantom, under otherwise reference conditions, in five high‐energy photon (63.5 &lt; %dd(10)X &lt; 76.3), and five electron (2.3 cm &lt; R50 &lt; 8.3 cm) beams. Reference dosimetry was performed for all beams in question using an Exradin A19 ion chamber with a calibration traceable to national standards. The absorbed‐dose component of the overall energy dependence was calculated using the EGSnrc egs_chamber user code.Results:A total of 72 measurements were performed with the GPC, resulting in a standard error on the mean absorbed dose of better than 0.3 % for all ten beams. For both the photon and electron beams, no statistically‐significant energy dependence was observed experimentally. Peak‐to‐peak, variations in the relative response of the GPC across all beam qualities of a given radiation type were on the order of 1 %. No effects, either transient or permanent, were attributable to the charge deposited by the electron beams.Conclusions:The GPC's apparent energy‐independence, combined with its well‐established linearity and dose rate independence, make it a potentially useful dosimetry system capable measuring photon and electron doses in absolute terms at the clinical level.

SU-G-201-08: Energy Response of Thermoluminescent Microcube Dosimeters in Water for Kilovoltage X-Ray Beams

Purpose: To characterize the energy dependence for TLD-100 microcubes in water at kilovoltage energies. Methods: TLD-100 microcubes with dimensions of (1 × 1 × 1) mm3 were irradiated with kilovoltage x-rays in a custom-built thin-window liquid water phantom. The TLD-100 microcubes were held in Virtual Water™ probes and aligned at a 2 cm depth in water. Irradiations were performed using the M-series x-ray beams of energies ranging from 50-250 kVp and normalized to a 60Co beam located at the UWADCL. Simulations using the EGSnrc Monte Carlo Code System were performed to model the x-ray beams, the 60Co beam, the water phantom and the dosimeters in the phantom. The egs_chamber user code was used to tally the dose to the TLDs and the dose to water. The measurements and calculations were used to determine the intrinsic energy dependence, absorbed-dose energy dependence, and absorbed-dose sensitivity. These values were compared to TLD-100 chips with dimensions of (3.2 × 0.9 × 0.9) mm3. Results: The measured TLD-100 microcube response per dose to water among all investigated x-ray energies had a maximum percent difference of 61% relative to 60Co. The simulated ratio of dose to water to the dose to TLD had a maximum percent difference of 29% relative to 60Co. The ratio of dose to TLD to the TLD output had a maximum percent difference of 13% relative to 60Co. The maximum percent difference for the absorbed-dose sensitivity was 15% more than the used value of 1.41. Conclusion: These results confirm that differences in beam quality have a significant effect on TLD response when irradiated in water. These results also indicated a difference in TLD-100 response between microcube and chip geometries. The intrinsic energy dependence and the absorbed-dose energy dependence deviated up to 10% between TLD-100 microcubes and chips.

SU-G-201-10: Experimental Determination of Modified TG-43 Dosimetry Parameters for the Xoft Axxent® Electronic Brachytherapy Source

Purpose: The establishment of an air kerma rate standard at NIST for the Xoft Axxent® electronic brachytherapy source (Axxent® source) motivated the establishment of a modified TG-43 dosimetry formalism. This work measures the modified dosimetry parameters for the Axxent® source in the absence of a treatment applicator for implementation in Xoft's treatment planning system. Methods: The dose-rate conversion coefficient (DRCC), radial dose function (RDF) values, and polar anisotropy (PA) were measured using TLD-100 microcubes with NIST-calibrated sources. The DRCC and RDF measurements were performed in liquid water using an annulus of Virtual Water™ designed to align the TLDs at the height of the anode at fixed radii from the source. The PA was measured at several distances from the source in a PMMA phantom. MCNP-determined absorbed dose energy dependence correction factors were used to convert from dose to TLD to dose to liquid water for the DRCC, RDF, and PA measurements. The intrinsic energy dependence correction factor from the work of Pike was used. The AKR was determined using a NIST-calibrated HDR1000 Plus well-type ionization chamber. Results: The DRCC was determined to be 8.6 (cGy/hr)/(µGy/min). The radial dose values were determined to be 1.00 (1cm), 0.60 (2cm), 0.42 (3cm), and 0.32 (4cm), with agreement ranging from (5.7% to 10.9%) from the work of Hiatt et al. 2015 and agreement from (2.8% to 6.8%) with internal MCNP simulations. Conclusion: This work presents a complete dataset of modified TG-43 dosimetry parameters for the Axxent® source in the absence of an applicator. Prior to this study a DRCC had not been measured for the Axxent® source. This data will be used for calculating dose distributions for patients receiving treatment with the Axxent® source in Xoft's breast balloon and vaginal applicators, and for intraoperative radiotherapy. Sources and partial funding for this work were provided by Xoft Inc. (a subsidiary of iCAD). This work was also supported by the Radiological Sciences T32 Training Grant through the University of Wisconsin-Madison Medical Physics department (5T32CA009206-37).

Gafchromic EBT3 film dosimetry in electron beams - energy dependence and improved film read-out.

For megavoltage photon radiation, the fundamental dosimetry characteristics of Gafchromic EBT3 film were determined in 60Co gamma ray beam with addition of experimental and Monte Carlo (MC)‐simulated energy dependence of the film for 6 MV photon beam and 6 MeV, 9 MeV, 12 MeV, and 16 MeV electron beams in water phantom. For the film read‐out, two phase correction of scanner sensitivity was applied: a matrix correction for scanning area and dose‐dependent correction by iterative procedure. With these corrections, the uniformity of response can be improved to be within ±50 pixel values (PVs). To improve the read‐out accuracy, a procedure with flipped film orientations was established. With the method, scanner uniformity can be improved further and dust particles, scratches and/or dirt on scanner glass can be detected and eliminated. Responses from red and green channels were averaged for read‐out, which decreased the effect of noise present in values from separate channels. Since the signal level with the blue channel is considerably lower than with other channels, the signal variation due to different perturbation effects increases the noise level so that the blue channel is not recommended to be used for dose determination. However, the blue channel can be used for the detection of emulsion thickness variations for film quality evaluations with unexposed films. With electron beams ranging from 6 MeV to 16 MeV and at reference measurement conditions in water, the energy dependence of the EBT3 film is uniform within 0.5%, with uncertainties close to 1.6% (k=2). Including 6 MV photon beam and the electron beams mentioned, the energy dependence is within 1.1%. No notable differences were found between the experimental and MC‐simulated responses, indicating negligible change in intrinsic energy dependence of the EBT3 film for 6 MV photon beam and 6 MeV–16 MeV electron beams. Based on the dosimetric characteristics of the EBT3 film, the read‐out procedure established, the nearly uniform energy dependence found and the estimated uncertainties, the EBT3 film was concluded to be a suitable 2D dosimeter for measuring electron or mixed photon/electron dose distributions in water phantom. Uncertainties of 3.7% (k=2) for absolute and 2.3% (k=2) for relative dose were estimated.PACS numbers: 87.53.Bn, 87.55.K‐, 87.55.Qr

Technical note: Influence of the phantom material on the absorbed-dose energy dependence of the EBT3 radiochromic film for photons in the energy range 3 keV-18 MeV.

Water is the reference medium for radiation therapy dosimetry, but for film dosimetry it is more practical to use a solid phantom. As the composition of solid phantoms differs from that of water, the energy dependence of film exposed within solid phantoms may also differ. The energy dependence of a radiochromic film for a given beam quality Q (energy for monoenergetic beams) has two components: the intrinsic energy dependence and the absorbed-dose energy dependence f(Q), the latter of which can be calculated through a Monte Carlo simulation of radiation transport. The authors used Monte Carlo simulations to study the influence of the phantom material on the f(Q) of the EBT3 radiochromic film (Ashland Specialty Ingredients, Wayne, NJ) for photon beams with energies between 3 keV and 18 MeV. All simulations were carried out with the general-purpose Monte Carlo code penelope 2011. The geometrical model consisted of a cylindrical phantom, with the film positioned at different depths depending on the initial photon energy. The authors simulated monoenergetic parallel photon beams and x-ray beams from a superficial therapy system. To validate their choice of simulation parameters, they also calculated f(Q) for older film models, EBT and EBT2, comparing with published results. In addition to water, they calculated f(Q) of the EBT3 film for solid phantom materials commonly used for film dosimetry: RW1 and RW3 (PTW-Freiburg, Freiburg, Germany), Solid Water (Gammex-RMI, Madison, WI), and PMMA. Finally, they combined their calculated f(Q) with published overall energy response data to obtain the intrinsic energy dependence of the EBT3 film in water. The calculated f(Q) for EBT and EBT2 films was statistically compatible with previously published data. Between 10 keV and 18 MeV, the variation found in f(Q) of the EBT3 film for water was within 2.3%, with a standard statistical uncertainty less than 1%. If the quantity dose-to-water in the phantom is considered, which is the common practice in radiation dosimetry, the maximum difference of energy dependence for the solid phantoms with respect to water is about 6%, at an energy of 50 keV. The EBT3 film shows a reasonably constant absorbed-dose energy dependence when irradiated in water. If the dose-to-water in the phantom is considered, the maximum difference of EBT3 film energy dependence with the solid phantoms studied with respect to water is about 6% (at an energy of 50 keV). The reported overall energy dependence of the EBT3 film in water at energies below 100 keV is mainly due to the intrinsic energy dependence.

Effect of improved TLD dosimetry on the determination of dose rate constants for (125)I and (103)Pd brachytherapy seeds.

To more accurately account for the relative intrinsic energy dependence and relative absorbed-dose energy dependence of TLDs when used to measure dose rate constants (DRCs) for (125)I and (103)Pd brachytherapy seeds, to thereby establish revised "measured values" for all seeds and compare the revised values with Monte Carlo and consensus values. The relative absorbed-dose energy dependence, f(rel), for TLDs and the phantom correction, Pphant, are calculated for (125)I and (103)Pd seeds using the EGSnrc BrachyDose and DOSXYZnrc codes. The original energy dependence and phantom corrections applied to DRC measurements are replaced by calculated (f(rel))(-1) and Pphant values for 24 different seed models. By comparing the modified measured DRCs to the MC values, an appropriate relative intrinsic energy dependence, kbq (rel), is determined. The new Pphant values and relative absorbed-dose sensitivities, SAD (rel), calculated as the product of (f(rel))(-1) and (kbq (rel))(-1), are used to individually revise the measured DRCs for comparison with Monte Carlo calculated values and TG-43U1 or TG-43U1S1 consensus values. In general, f(rel) is sensitive to the energy spectra and models of the brachytherapy seeds. Values may vary up to 8.4% among (125)I and (103)Pd seed models and common TLD shapes. Pphant values depend primarily on the isotope used. Deduced (kbq (rel))(-1) values are 1.074 ± 0.015 and 1.084 ± 0.026 for (125)I and (103)Pd seeds, respectively. For (1 mm)(3) chips, this implies an overall absorbed-dose sensitivity relative to (60)Co or 6 MV calibrations of 1.51 ± 1% and 1.47 ± 2% for (125)I and (103)Pd seeds, respectively, as opposed to the widely used value of 1.41. Values of Pphant calculated here have much lower statistical uncertainties than literature values, but systematic uncertainties from density and composition uncertainties are significant. Using these revised values with the literature's DRC measurements, the average discrepancies between revised measured values and Monte Carlo values are 1.2% and 0.2% for (125)I and (103)Pd seeds, respectively, compared to average discrepancies for the original measured values of 4.8%. On average, the revised measured values are 4.3% and 5.9% lower than the original measured values for (103)Pd and (125)I seeds, respectively. The average of revised DRCs and Monte Carlo values is 3.8% and 2.8% lower for (125)I and (103)Pd seeds, respectively, than the consensus values in TG-43U1 or TG-43U1S1. This work shows that f(rel) is TLD shape and seed model dependent suggesting a need to update the generalized energy response dependence, i.e., relative absorbed-dose sensitivity, measured 25 years ago and applied often to DRC measurements of (125)I and (103)Pd brachytherapy seeds. The intrinsic energy dependence for LiF TLDs deduced here is consistent with previous dosimetry studies and emphasizes the need to revise the DRC consensus values reported by TG-43U1 or TG-43U1S1.

Energy dependence of TLD-900 dosimeters exposed to low energy X-rays

This work investigates the energy dependence of CaSO4:Dy (TLD-900) thermoluminescent dosimeters to low energy X-rays. Dosimeters were exposed to X-ray radiation qualities between 30 and 250 kVp for three air kerma values: 100, 250 and 500 mGy. The detector thermoluminescent response as a function of air kerma and the detector's relative kerma sensitivity with respect to 60Co as a function of effective energy were obtained. Monte Carlo and analytical calculations were also performed. A maximum relative kerma sensitivity of 12 is found for a 22 keV effective energy corresponding to the 50 kVp X-ray beam. Monte Carlo and analytical calculations accurately describe trends in the energy dependence curve as a function of photon energy though they predict lower values for the relative kerma sensitivity. Maximum difference is observed at the lowest energy measured (16 keV) where experimental data is 2.1 and 2.3 times greater than Monte Carlo and analytical calculations, respectively. The difference between measurements and Monte Carlo-calculated predictions is attributed to the intrinsic energy dependence of TLD-900. Values of intrinsic energy dependence estimated from the measured relative TL kerma sensitivity together with the MC and analytical calculated values of kerma energy dependence were found to be independent of beam quality in the region from 33 to 142 keV effective energies.

SU‐F‐BRE‐03: Consideration of a Track‐Interaction Model for Radiochromic Film Response

Purpose:Conventional methods for characterizing the energy response of radiochromic film (RCF) typically involve assessing changes in response when exposed to various beam qualities and use Monte Carlo to determine absorbed dose. These methods represent RCF as a dose integrator of a homogeneous energy deposition volume. Apparent film saturation, nonlinearity, and intrinsic energy dependence are unpredicted with conventional methods. Recent work has shown significant RCF intrinsic energy dependence, which limits its use in absolute dosimetry. This work introduces a track‐interaction model (TIM) for RCF and assesses its ability to predict total energy response.Methods:A TIM based on Katz single‐hit theory was developed to accumulate energy flux along particle tracks within active crystals, represented as (1×1×20)um3 prisms about the Gafchromic™ EBT3 active volume using MCNP5 and Matlab. Energy flux contributed to film response only if near the threshold energy for polymerization in polydiacetylenes (2.5eV/monomer). Energy deposition in excess of maximum efficiency represented crystal saturation and did not contribute to film response. The TIM was applied to RCF exposed in air to various monoenergetic photon beams and Co‐60. Geometric distribution of energy flux was found for each beam quality in a (1×1)mm2 RCF area. RCF response relative to Co‐60 absorbed dose‐to‐water (S_TIM) was determined and compared to published values (S_PUB).Results:TIM successfully predicted that lower energy radiation is less effective at inducing polymerization, though the magnitude of the phenomenon was overpredicted. S_TIM was −29% and +20% for 20 and 40 keV, respectively. This agreed qualitatively with S_PUB of −27% and +16%. TIM‐generated sensitometric curves contained the non‐linearity and saturation apparent in RCF.Conclusion:This work indicates the possibility for TIMs to predict changes in RCF response to various energies. Future work will refine TIM by considering size distributions of active elements, other RCF formulations, and response in complex treatment fields.Acknowledgements: The authors wish to thank Dr. Christopher Soares at NIST (Gaithersburg, MD) for performing film measurements and Dr. David Lewis of Ashland, Inc. (Wayne, NJ) for making the EBT3 film available to us.

The use of TLDs for brachytherapy dosimetry

Thermoluminescent dosimeters (TLDs) are routinely used to measure the dose around brachytherapy sources due to their small size and high precision. This work presents a concise overview of the use of LiF:Mg,Ti TLDs for brachytherapy dosimetry including the experimental procedures required to achieve high-precision measurements as well as new results regarding the intrinsic energy dependence with some of the commonly used brachytherapy sources. Equations to correct TLD light output to air kerma are outlined and a description of the method to determine the intrinsic energy dependence is presented. For the intrinsic energy dependence investigation, a review of previously published results is presented as well as new experimental results using 125I, 103Pd, 192Ir, and miniature x-ray brachytherapy sources at the University of Wisconsin Medical Radiation Research Center (UWMRRC). The results of these experiments are consistent with previous work and give valuable insight to investigators using TLDs for brachytherapy measurements.

Improving the energy response of external beam therapy (EBT) GafChromicTM dosimetry films at low energies (≤100 keV)

Purpose of this work is to investigate the effects of varying the active layer composition of external beam therapy (EBT) GafChromic(TM) films on the energy dependence of the film, as well as try to develop a new prototype with more uniform energy response at low photon energies (⩽ 100 keV). First, the overall energy response (S(AD, W)(Q)) of different commercial EBT type film models that represent the three different generations produced to date, i.e., EBT, EBT2, and EBT3, was investigated. Pieces of each film model were irradiated to a fixed dose of 2 Gy to water for a wide range of beam qualities and the corresponding S(AD, W)(Q) was measured using a flatbed document scanner. Furthermore, the DOSRZnrc Monte Carlo code was used to determine the absorbed dose to water energy dependence of the film, f(Q). Moreover, the intrinsic energy dependence, kbq(Q), for each film model was evaluated using the corresponding S(AD, W)(Q) and f(Q). In the second part of this study, the authors investigated the effects of changing the chemical composition of the active layer on SAD, W(Q). Finally, based on these results, the film manufacturer fabricated several film prototypes and the authors evaluated their S(AD, W)(Q). The commercial EBT film model shows an under response at all energies below 100 keV reaching 39% ± 4% at about 20 keV. The commercial EBT2 and EBT3 film models show an under response of about 27% ± 4% at 20 keV and an over response of about 16% ± 4% at 40 keV.S(AD, W)(Q) of the three commercial film models at low energies show strong correlation with the corresponding f(-) (1)(Q) curves. The commercial EBT3 model with 4% Cl in the active layer shows under response of 22% ± 4% at 20 keV and 6% ± 4% at about 40 keV. However, increasing the mass percent of chlorine makes the film more hygroscopic which may affect the stability of the film's readout. The EBT3 film prototype with 7.5% Si shows a significant improvement in the energy response at very low energies compared to the commercial EBT3 films with 4% Cl. It shows under response of 15% ± 5% at about 20 keV to 2% ± 5% at about 40 keV. However, according to the manufacturer, the addition of 7.5% Si as SiO2 adversely affected the viscosity of the active fluid and therefore affected the potential use in commercial machine coating. The latest commercial EBT3 film model with 7% Al as Al2O3 shows an overall improvement in SAD, W(Q) compared to previous commercial EBT3 films. It shows under response at all energies <100 keV, varying from 20% ± 4% at 20 keV to 6% ± 4% at 40 keV. The energy response of films in the energy range <100 keV can be improved by adjusting the active layer chemical composition. Removing bromine eliminated the over response at about 40 keV. The under response at energies ≤ 30 keV is improved by adding 7% Al to the active layer in the latest commercial EBT3 film models.