Free-air Chamber Research Articles

Establishment of Hp(3) calibration system for eye dose monitoring in Taiwan.

In response to the ICRP's amending the occupational exposure limit for the eye lens, the Institute of Nuclear Energy Research (INER) established the Hp(3) calibration system for eye dose monitoring in Taiwan to accurately assess the dose received in the eye lens. INER employed the narrow-spectrum series radiation according to the ISO 4037 as the X-ray radiation qualities, and the measured half-value layer consistent with a 5% difference. The air kerma rate standard was determined by the self-made free air chamber, and through dose conversion coefficient referring to ISO 4037 to obtain the Hp(3) on an ISO cylinder phantom. Furthermore, the calibration system was provided as the characteristics tests for DOSIRIS headset dosemeters. Finally, the Hp(3) calibration system has been established in Taiwan, and it can be used to provide calibration services for eye lens dosemeters and be applied to the proficiency testing that will be held in 2023.

Determination of an air kerma-rate correction factor for the S7600 Xoft AxxentⓇ source model

PurposeThe purpose of this work was to provide guidance for the lack of an air-kerma rate standard for the S7600 Xoft Axxent® source by providing a correction factor to apply to the National Institute of Standards and Technology (NIST) traceable S7500 well chamber (WC) calibration coefficient before the development of an S7600 standard at NIST. METHODS AND MATERIALSThe Attix free air chamber (FAC) at the University of Wisconsin Medical Radiation Research Center was used to measure the air-kerma rate at 50 cm for six S7500 and six S7600 sources. These same sources were then measured using five standard imaging HDR1000+ WCs. The measurements made with the FAC were used to calculate source-specific WC calibration coefficients for the S7500 and S7600 source. These results were compared to the NIST traceable calibration coefficients for the S7500 source. The average results for each WC were then averaged together, and a ratio of the S7600 to S7500 WC calibration coefficients was determined. ResultsThe average S7600 air-kerma rate measurement with the FAC was 7% lower than the average air-kerma rate measurements of the S7500 source. On average, the S7500 determined WC calibration coefficients agreed within ±1% of the NIST traceable S7500 values. The S7600 WC calibration coefficients were up to 16% less than the NIST traceable S7500 values. The final correction factor determined to be applied to the NIST traceable S7500 value was 0.8415 with an associated uncertainty of ±8.1% at k = 2. ConclusionsThis work provides a suggested correction factor for the S7600 Xoft Axxent source such that the sources can be accurately implemented in the clinical setting.

Performance evaluation of a homemade extrapolation chamber in low energy radiation field

Extrapolation chambers are widely used to detect the weakly-penetrating types of radiation beams. Normally, the sensitive volume of them is limited, only a few cubic centimeters. Their collecting electrodes use sheet structure, which lead to more backscattering photons. In this work, a large volume extrapolation chamber with (235–1413) cm3 was designed for the absolute measurement of air kerma strength of 125I brachytherapy seeds. The developed chamber has the following innovation: large sensitive volume and the Aluminized Polyethylene terephthalate (PET) collecting electrode foil. In order to give a more comprehensive performance evaluation for the designed chamber, a series of experiments were conducted. Evaluation parameters include leakage current, saturation curve, polarity effect, ion collecting efficiency, linearity of response, collecting area and zero point. Those tests were measured in the standard X-ray radiation field. A comparative experiment with the primary standard free air chamber (FAC) was performed. Moreover, extrapolation curve was measured in 125I radiation field. Evaluation results show a good behavior of the extrapolation chamber. The Al-coated PET collecting electrode influence the response of the chamber to 0.07% with the simulation of MCNP5 Monte Carlo. All the results indicate the usefulness of the extrapolation chamber for the absolute measurement for 125I seeds.

Correction Factors for the LSDG Free-Air Ionization Chamber for Low and Medium-Energy X-Rays with Monte Carlo Calculations

Advances in radiation dosimetry continue to improve the accuracy of calibrating photon and electron beams of radiotherapy. This accuracy in the dose delivery to the patient in radiotherapy depends to a large extent on the absolute dosimetry of the irradiation unit. This necessitates the use of an ionization chamber such as Free air ionization chambers (FAC), calibrated in a recognised Standard Dosimetry Laboratory [1]. FACs are widely used by Primary Standard Dosimetry Labs (PSDL) as primary standard for absolute measurement of air kerma in beam of low and medium energy x-rays. It can accurately measure air kerma of x-ray beams of energy up to about 300kV. The purpose of this investigation, was to determine the correction factors of the Free Air Chamber (FAC) using the Monte Carlo calculations.

Effective Volume of the Korea Research Institute of Standards and Science Free Air Chamber L1 for Low-Energy X-Ray Measurement

Effective Volume of the Korea Research Institute of Standards and Science Free Air Chamber L1 for Low-Energy X-Ray Measurement

Comparison of air kerma rate between the S7500 and S7600 xoft axxent sources

The purpose of this work was to evaluate differences in air-kerma rate of the older, S7500 water-cooled Xoft Axxent source and newer, S7600 Galden-cooled source. The Attix Free Air Chamber (FAC) at the UWMRRC was used to measure the air-kerma rate at 50 cm for six S7600 Xoft Axxent sources. The average measured air-kerma of the S7600 sources was compared with the measured average air-kerma rate from five S7500 sources. The air-kerma rates of the S7500 sources were measured in a Standard Imaging HDR 1000+ well chamber. The FAC measurements were used to determine a well chamber calibration coefficient for the S7600 source. The S7500 calibration coefficients were incorrectly applied to the S7600 sources to indicate the magnitude of error that can occur if the incorrect calibration coefficient is used. A 10.3% difference was observed between the average air-kerma rates of the two sources although a 17% difference was observed between their calibration coefficients. The application of the S7500 calibration coefficient to the S7600 sources resulted in measured air-kerma rates that were 20% greater than the true value. This work indicates the need for a new air-kerma rate standard for the S7600 sources, and the results presented in this work are indicative of values that would be obtained at National Institute of Standards and Technology.

CALIBRATION METHOD AND ABSOLUTE MEASUREMENTS OF AIR KERMA OF DIAGNOSTIC X-RAY REFERENCE QUALITIES USING A FREE-AIR IONISATION CHAMBER.

Accurate dose measurement is very important for diagnosing X-ray equipment. These equipment should be traced directly or indirectly to primary standard with as little uncertainty as possible. A parallel-plate free-air chamber was designed to measure air kerma of diagnostic X-ray absolutely. Different thickness of aluminium filters were used to obtain different radiation qualities according with IEC 61267. Correction factors were obtained by experiments and simulation, especially the air attenuation factor measured with vacuum method. The relative standard uncertainty of air kerma rate measurement was 0.46% and calibration factor was 0.53%. A transfer ionisation chamber was calibrated with Radiation Qualities in Radiation beams (RQR) qualities in National Research Council of Canada and National Institute of Metrology (NIM), the differences of calibration factors are less than 0.68%.

Development of Standard X-Ray Beams for Calibration of Radiobiology Cabinet and Conformal Irradiators.

This work seeks to develop standard X-ray beams that are matched to radiobiology X-ray irradiators. The calibration of detectors used for dose determination of these irradiators is performed with a set of standard X rays that are more heavily filtered and/or lower energy, which leads to a higher uncertainty in the dose measurement. Models of the XRad320, SARRP, and the X-ray tube at the University of Wisconsin Medical Radiation Research Center (UWMRRC) were created using the BEAMnrc user code of the EGSnrc Monte Carlo code system. These models were validated against measurements, and the resultant modeled spectra were used to determine the amount of added filtration needed to match the X-ray beams at the UWMRRC to those of the XRad320 and SARRP. The depth profiles and half-value layer (HVL) simulations performed using BEAMnrc agreed to measurements within 3% and 3.6%, respectively. A primary measurement device, a free-air chamber, was developed to measure air kerma in the medium energy range of X rays. The resultant spectra of the matched beams had HVL's that matched the HVL's of the radiobiology irradiators well within the 3% criteria recommended by the International Atomic Energy Agency (IAEA) and the average energies agreed within 2.4%. In conclusion, three standard X-ray beams were developed at the UWMRRC with spectra that more closely match the spectra of the XRad320 and SARRP radiobiology irradiators, which will aid in a more accurate dose determination during calibration of these irradiators.

Dose-rate constant and air-kerma strength evaluation of a new 125I brachytherapy source using Monte Carlo

Brachytherapy is a modality of radiotherapy which treats tumors using ionizing radiation with sources located close to the tumor. The sources can be produced from several radionuclides in various formats, such as Iodine‑125 seeds and Iridium‑192 wires. In order to produce a new Iodine‑125 seed in IPEN/CNEN and ensure its quality, it is essential to describe the dosimetry of the seed, so when applied in a treatment the lowest possible dose to neighboring healthy tissues can be reached. The report by the AAPM’s Task Group 43 U1 is a document that indicates the dosimetry procedures in brachytherapy based on physical and geometrical parameters. In this study, dose-rate constant and air-kerma strength parameters were simulated using the Monte Carlo method radiation transport code MCNP4C. The air-kerma strength is obtained from an ideal modeled seed, since its actual value should be measured for seeds individually in a specialized lab with a Wide-Angle Free-Air Chamber (WAFAC). Dose-rate constant and air-kerma strength are parameters that depends on intrinsic characteristics of the source, i.e. geometry, radionuclide, encapsulation, and together they define the dose-rate to the reference point, defined as the dose-rate to a point 1 cm away from the geometric center of the source, in its transverse plane. This study presents the values found for these parameters with associated statistical uncertainty, and is part of a larger project that aims the full dosimetry of this new seed model, including experimental measures.

Update of the CLRP TG-43 parameter database for low-energy brachytherapy sources.

To update the Carleton Laboratory for Radiotherapy Physics (CLRP) TG-43 dosimetry database for low-energy (≤50keV) photon-emitting low-dose rate (LDR) brachytherapy sources utilizing the open-source EGSnrc application egs_brachy rather than the BrachyDose application used previously for 27 LDR sources in the 2008 CLRP version (CLRPv1). CLRPv2 covers 40 sources ( Pd, I, and Cs). A comprehensive set of TG-43 parameters is calculated, including dose-rate constants, radial dose functions with functional fitting parameters, 1D and 2D anisotropy functions, along-away dose-rate tables, Primary-Scatter separation dose tables (for some sources), and mean photon energies at the surface of the sources. The database also documents the source models which will become part of the egs_brachy distribution. Datasets are calculated after a systematic recoding of the source geometries using the egs++ geometry package and its egs_brachy extensions. Air-kerma strength per history is calculated for models of NIST's Wide-Angle Free-Air chamber (WAFAC) and for a point detector located at 10cm on the source's transverse axis. Full scatter water phantoms with varying voxel resolutions in cylindrical coordinates are used for dose calculations. New statistical uncertainties of source volume corrections for phantom voxels which overlap with brachytherapy sources are implemented in egs_brachy, and all CLRPv2 data include these uncertainties. For validation, data are compared to CLRPv1 and other data in the literature. Data are available at https://physics.carleton.ca/clrp/egs_brachy/seed_database_v2, http://doi.org/10.22215/clrp/tg43v2. As well as being presented graphically in comparisons to previous calculations, data are available in Excel (.xlsx) spreadsheets for each source. The database has applications in research, dosimetry, and brachytherapy treatment planning. This comprehensive update provides the medical physics community with more accurate TG-43 dose evaluation parameters, as well as fully benchmarked and described source models which are distributed with egs_brachy.

Establishment of diagnostic X-ray air kerma standard in Taiwan

Diagnostic X-ray air kerma standards were established at the Institute of Nuclear Energy Research (INER, Taiwan) in accordance with the recommendations of IEC 61267. The radiation qualities established by INER include IEC 61267 RQR (RQR 3-10), RQA (RQA 3-10) and RQT. Among them, RQR is used to simulate the energy spectrum generated from the X-ray machine, RQA is used to simulate the energy spectrum after passing through the human body, and RQT is used to simulate the energy spectrum in computed tomography (CT).A tungsten target X-ray machine, 99.9% pure aluminum and copper filters were used to build the IEC 61267 radiation qualities. A cylindrical free-air chamber (FAC) is used as the X-ray air kerma primary standard. The FAC correction factors, including air attenuation, window attenuation, ion recombination, photon-scattering, electron-loss and shadow-effect, were evaluated by experiments or Monte Carlo methods.The half value layers (HVL) of these radiation qualities were measured by FAC, and the HVL differences between INER and IEC 61267 are less than 3%. The uncertainty of measurement was evaluated according to ISO GUM, and the relative standard uncertainty of primary standard is 0.53% (k = 1). An ionization chamber (Exradin A5) was sent to PTB and calibrated with IEC 61267 RQR and RQA qualities. When comparing the calibration factors between PTB and INER, the differences are less than 1.25%.The air kerma standard of diagnostic X-ray has been established at INER, and it will provide dose calibration services in Taiwan to improve medical quality.

Inverse square corrections for FACs and WAFACs

Inverse square correction factors for wide-angle free-air chambers (WAFACs) and free-air chambers (FACs) for cylindrical, conical and square-prism detectors are required for determining the on-axis air kerma from measurements or Monte Carlo calculations made with these different shaped detectors. Values of air kerma measured with these detectors use an effective volume technique related to the inverse square correction factors. This paper presents these factors in a consistent framework and the relationships between them are made clear. Using Monte Carlo simulations, the various corrections and techniques are shown to be accurate within a statistical precision of about 0.04% or better with the exception of the published correction for square prism detectors which is shown to hold only for thin detectors which have an opening angle corresponding to the NIST and NRCC WAFAC primary standards. A more accurate correction for square prism detectors is presented which properly averages 1/d2 rather than d2 where d is the distance away from the source.

P037] Implementation of a new free-air ionization chamber as a primary air kerma standard for mammography radiation qualities

Purpose The Physikalisch-Technische Bundesanstalt (PTB) offers air kerma calibrations of secondary standards for standard dosimetry laboratories worldwide. Due to the significant increase in the demand for such calibrations in mammography X-ray beams (tube high voltages ranging from 20 kV to 50 kV), it was decided to construct and manufacture a novel free-air ionization chamber (FAC) with a design especially optimized for radiation qualities used in mammography. This work describes the final technical construction of the new FAC and its implementation as a primary air kerma standard at PTB. Methods The new FAC is of the parallel-plate type optimized for the photon energy range of 5 keV to 50 keV. Special emphasis has been given to the design of a chamber without potential wires in the beamline to minimize the scattering and attenuation of the X-rays. The design of the chamber was optimized with respect to electrical field homogeneity using the software package SIMION Version 8.1. This software was also used to calculate k d , the correction factor for the distortion of the electric field of the final chamber design. The other correction factors for photon attenuation, scattering, aperture transmission and scattering, electron loss and violating charged-particle equilibrium have been calculated using the user code ’egs_fac’ of the EGSnrc Monte Carlo transport simulation software package. Finally, air kerma rates measured with the PK50, the new free-air chamber, were compared with those measured with the other well-established primary air kerma standards of PTB. Results The PK50 was fully characterized by all the necessary correction factors. The correction for the field distortion is below 0.8%, thus the design goal has been achieved. For the ISO narrow series, the correction factors calculated with egs_fac are below 1.0%, proving that the chamber is well optimized for energies below 50 keV. Comparisons of air kerma measurements with the PK50 and established FACs yielded results which were consistent within the uncertainties of a few parts in 1000. Conclusions The new chamber designated as the PK50 is now ready for use as a primary air kerma standard for low-energy X-rays at PTB.

P172] Establishment and transfer of a primary dosimetry standard for breast cancer IORT using the INTRABEAM(R) system

Purpose Intraoperative radiation therapy (IORT) using low-energy X-rays (⩽50 keV) delivers a concentrated dose by placing, during surgery, a miniaturized X-ray tube in contact with the treated tumor bed. In France, the most used IORT treatment system is the INTRABEAM one, manufactured by the Zeiss Company, which is delivered with a database used to assess the absolute dose distribution. This work aims at establishing traceability to a dosimetry reference independent from the manufacturer, as recommended in the report published in 2016 by the French National Authority for Health on the evaluation of such treatment. Methods Several types of IORT low-energy photon sources are commercially available and any metrology laboratory cannot afford to be equipped with all of them. So a multi-step methodology, applicable to any of these sources, was developed both to establish and transfer a dosimetry reference. This methodology removes the requirement to bring together, within the same laboratory, the source and the dedicated, but non-transportable, primary standard Free-Air-Chamber (FAC). Firstly, the photon energy spectrum of the commercial source is characterized at the hospital, then the spectrum is reproduced at the primary laboratory using a conventional X-ray generator. The reproduced beam is then characterized, in terms of air kerma rate, using a primary FAC and also used to calibrate a transfer cavity ionization chamber, which is in turn used to calibrate the INTRABEAM® system. Finally, a Monte Carlo model of the source is developed to calculate a conversion factor, from air kerma to absorbed dose to water under reference conditions. Results This methodology was applied to the INTRABEAM® system of a hospital associated with a 4 cm spherical applicator. The absorbed dose to water was established at a depth of 1 cm in water. The Zeiss Company recently changed its dosimetry procedure, going from the TARGIT method to the non-TARGIT one. Our value is higher by 33% and 16% respectively than those given by the TARGIT and non-TARGIT methods. Conclusions The present study shows, for IORT X-ray sources, the importance of making available to end-users independent dosimetry measurements traceable to national standards.

Dosimetric characterization of a new directional low-dose rate brachytherapy source.

CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.TM The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatability and reproducibility of a primary source strength standard for the CivaDot and characterizes the CivaDot dose distribution by performing in-phantom measurements and Monte Carlo (MC) simulations. Existing dosimetric formalisms were adapted to accommodate a directional source, and other distinguishing characteristics including the presence of gold shield x-ray fluorescence were addressed in this investigation. Primary air-kerma strength (SK ) measurements of the CivaDots were performed using two free-air chambers namely, the Variable-Aperture Free-Air Chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center (UWMRRC) and the National Institute of Standards and Technology (NIST) Wide-Angle Free-Air Chamber (WAFAC). An intercomparison of the two free-air chamber measurements was performed along with a comparison of the different assumed CivaDot energy spectra and associated correction factors. Dose distribution measurements of the source were performed in a custom polymethylmethacrylate (PMMA) phantom using GafchromicTM EBT3 film and thermoluminescent dosimeter (TLD) microcubes. Monte Carlo simulations of the source and the measurement setup were performed using MCNP6 radiation transport code. The CivaDot SK was determined using the two free-air chambers for eight sources with an agreement of better than 1.1% for all sources. The NIST measured CivaDot energy spectrum intensity peaks were within 1.8% of the MC-predicted spectrum intensity peaks. The difference in the net source-specific correction factor determined for the CivaDot free-air chamber measurements for the NIST WAFAC and UW VAFAC was 0.7%. The dose-rate constant analog was determined to be 0.555 cGy h-1 U-1 . The average difference observed in the estimated CivaDot dose-rate constant analog using measurements and MCNP6-predicted value (0.558 cGy h-1 U-1 ) was 0.6% ± 2.3% for eight CivaDot sources using EBT3 film, and -2.6% ± 1.7% using TLD microcube measurements. The CivaDot two-dimensional dose-to-water distribution measured in phantom was compared to the corresponding MC predictions at six depths. The observed difference using a pixel-by-pixel subtraction map of the measured and the predicted dose-to-water distribution was generally within 2-3%, with maximum differences up to 5% of the dose prescribed at the depth of 1 cm. Primary SK measurements of the CivaDot demonstrated good repeatability and reproducibility of the free-air chamber measurements. Measurements of the CivaDot dose distribution using the EBT3 film stack phantom and its subsequent comparison to Monte Carlo-predicted dose distributions were encouraging, given the overall uncertainties. This work will aid in the eventual realization of a clinically viable dosimetric framework for the CivaSheet based on the CivaDot dose distribution.

Establishing IAEA TRS-457 diagnostic X-ray beam qualities at the Australian primary standard dosimetry laboratory.

This paper reports the process and findings in establishing the diagnostic beam qualities for the calibration of diagnostic dose meters in the Australian primary standard dosimetry laboratory at the Australian Radiation Protection and Nuclear Safety Agency, ARPANSA. A novel iterative method to achieve the RQR beam quality requirements described by IEC 61267 is presented. The correction factors for the primary standard free-air chamber and their uncertainties are reported. The calibration procedure with detailed uncertainty budget of a typical diagnostic ionisation chamber (model PTW 34060) is reported. Beam profile measurements and details of the use of an external monitor chamber (model PTW 34014) are also reported. The uncertainty in the ARPANSA calibration coefficients is estimated to be 1.2% at k = 2 for both RQR and RQA beam qualities. The laboratory is now able to provide calibration services for radiation detectors used in general X-ray radiography.

Causes of variation among rice models in yield response to CO2 examined with Free-Air CO2 Enrichment and growth chamber experiments

The CO2 fertilization effect is a major source of uncertainty in crop models for future yield forecasts, but coordinated efforts to determine the mechanisms of this uncertainty have been lacking. Here, we studied causes of uncertainty among 16 crop models in predicting rice yield in response to elevated [CO2] (E-[CO2]) by comparison to free-air CO2 enrichment (FACE) and chamber experiments. The model ensemble reproduced the experimental results well. However, yield prediction in response to E-[CO2] varied significantly among the rice models. The variation was not random: models that overestimated at one experiment simulated greater yield enhancements at the others. The variation was not associated with model structure or magnitude of photosynthetic response to E-[CO2] but was significantly associated with the predictions of leaf area. This suggests that modelled secondary effects of E-[CO2] on morphological development, primarily leaf area, are the sources of model uncertainty. Rice morphological development is conservative to carbon acquisition. Uncertainty will be reduced by incorporating this conservative nature of the morphological response to E-[CO2] into the models. Nitrogen levels, particularly under limited situations, make the prediction more uncertain. Improving models to account for [CO2] × N interactions is necessary to better evaluate management practices under climate change.

Photon-scattering, electron-loss and shadow-effect correction factors calculation for cylindrical free-air chamber

A cylindrical free-air ionization chamber is used as the medium X-ray air kerma primary standard at the Institute of Nuclear Energy Research (INER, Taiwan). Photon-scattering, electron-loss and shadow-effect correction factors are taken into account for the measurement of air kerma by the cylindrical free-air ionization chamber. The photon-scattering correction factor is used to deduct the ionizations caused by scattered photons. The electron-loss correction factor compensates for the loss of electrons striking the electrode shell without fully depositing their energies to charges in the air. The shadow-effect correction factor compensates for the loss of electrons striking the collecting rod inside the chamber. The photon scattering and electron loss correction factors previously used at INER were based on a least-squares fit with experimental data published in the NBS Handbook 64. The shadow-effect correction factor was not considered.In this study, photon-scattering, electron-loss and shadow-effect correction factors for each mono-energetic photon were calculated by a Monte Carlo code, EGS5. Then, the mono-energetic correction factors were substituted into the ISO 4037 radiation qualities spectrum, and the energy weighted correction factors were calculated. Comparing the calculated correction factors with the previous correction factors, the maximum differences were 0.51% and 1.22% for IN-250 and IN-300 radiation qualities, respectively.In a report detailing an international comparison of air kerma standards for the ISO 4037 narrow spectrum series (EUROMET.RI(I)-S3), which was conducted from 2004 to 2005, the ratio of differences and expanded uncertainties (Di/Ui) for INER's IN-250 and IN-300 radiation qualities were 0.9 and 1.8, respectively. If the correction factors obtained in this study are substituted, the differences can be reduced, and the (Di/Ui) ratios become 0.36 and 0.6 for IN-250 and IN-300, respectively.

SU‐F‐I‐13: Correction Factor Computations for the NIST Ritz Free Air Chamber for Medium‐Energy X Rays

Purpose:The National Institute of Standards and Technology (NIST) uses 3 free‐air chambers to establish primary standards for radiation dosimetry at x‐ray energies. For medium‐energy × rays, the Ritz free‐air chamber is the main measurement device. In order to convert the charge or current collected by the chamber to the radiation quantities air kerma or air kerma rate, a number of correction factors specific to the chamber must be applied.Methods:We used the Monte Carlo codes EGSnrc and PENELOPE.Results:Among these correction factors are the diaphragm correction (which accounts for interactions of photons from the x‐ray source in the beam‐defining diaphragm of the chamber), the scatter correction (which accounts for the effects of photons scattered out of the primary beam), the electron‐loss correction (which accounts for electrons that only partially expend their energy in the collection region), the fluorescence correction (which accounts for ionization due to reabsorption ffluorescence photons and the bremsstrahlung correction (which accounts for the reabsorption of bremsstrahlung photons). We have computed monoenergetic corrections for the NIST Ritz chamber for the 1 cm, 3 cm and 7 cm collection plates.Conclusion:We find good agreement with other's results for the 7 cm plate. The data used to obtain these correction factors will be used to establish air kerma and it's uncertainty in the standard NIST x‐ray beams.

SU-G-201-06: Directional Low-Dose Rate Brachytherapy: Determination of the TG-43 Dose-Rate Constant Analog for a New Pd-103 Source

Purpose: The aim of this work is to determine the TG-43 dose-rate constant analog for a new directional low-dose rate brachytherapy source based on experimental methods and comparison to Monte Carlo simulations. The CivaSheet™ is a new commercially available planar source array comprised of a variable number of discrete directional source elements called “CivaDots”. Given the directional nature and non-conventional design of the source, modifications to the AAPM TG-43 protocol for dosimetry are required. As a result, various parameters of the TG-43 dosimetric formalism have to be adapted to accommodate this source. This work focuses on the dose-rate constant analog determination for a CivaDot. Methods: Dose to water measurements of the CivaDot were performed in a polymethyl methacrylate phantom (20×20×12 cm3) using thermoluminescent dosimeters (TLDs) and Gafchromic EBT3 film. The source was placed in the center of the phantom, and nine TLD micro-cubes were irradiated along its central axis at a distance of 1 cm. For the film measurements, the TLDs were substituted by a (3×3) cm2 EBT3 film. Primary air-kerma strength measurements of the source were performed using a variable-aperture free-air chamber. Finally, the source was modeled using the Monte Carlo N-Particle Transport Code 6. Results: Dose-rate constant analog observed for a total of eight CivaDots using TLDs and five CivaDots using EBT3 film was within ±7.0% and ±2.9% of the Monte Carlo predicted value respectively. The average difference observed was −4.8% and −0.1% with a standard deviation of 1.7% and 2.1% for the TLD and the film measurements respectively, which are both within the comparison uncertainty. Conclusion: A preliminary investigation to determine the doserate constant analog for a CivaDot was conducted successfully with good agreement between experimental and Monte Carlo based methods. This work will aid in the eventual realization of a clinically-viable dosimetric framework for the CivaSheet. This work was partially supported by NCI contract (HHSN261201200052C) through CivaTech Oncology Inc.