Variable-aperture Free-air Chamber Research Articles

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.

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.

Air-kerma strength determination of a new directional (103)Pd source.

A new directional (103)Pd planar source array called a CivaSheet™ has been developed by CivaTech Oncology, Inc., for potential use in low-dose-rate (LDR) brachytherapy treatments. The array consists of multiple individual polymer capsules called CivaDots, containing (103)Pd and a gold shield that attenuates the radiation on one side, thus defining a hot and cold side. This novel source requires new methods to establish a source strength metric. The presence of gold material in such close proximity to the active (103)Pd region causes the source spectrum to be significantly different than the energy spectra of seeds normally used in LDR brachytherapy treatments. In this investigation, the authors perform air-kerma strength (S(K)) measurements, develop new correction factors for these measurements based on an experimentally verified energy spectrum, and test the robustness of transferring S(K) to a well-type ionization chamber. S(K) measurements were performed with the variable-aperture free-air chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center. Subsequent measurements were then performed in a well-type ionization chamber. To realize the quantity S(K) from a directional source with gold material present, new methods and correction factors were considered. Updated correction factors were calculated using the MCNP 6 Monte Carlo code in order to determine S(K) with the presence of gold fluorescent energy lines. In addition to S(K) measurements, a low-energy high-purity germanium (HPGe) detector was used to experimentally verify the calculated spectrum, a sodium iodide (NaI) scintillating counter was used to verify the azimuthal and polar anisotropy, and a well-type ionization chamber was used to test the feasibility of disseminating S(K) values for a directional source within a cylindrically symmetric measurement volume. The UW VAFAC was successfully used to measure the S(K) of four CivaDots with reproducibilities within 0.3%. Monte Carlo methods were used to calculate the UW VAFAC correction factors and the calculated spectrum emitted from a CivaDot was experimentally verified with HPGe detector measurements. The well-type ionization chamber showed minimal variation in response (<1.5%) as a function of source positioning angle, indicating that an American Association of Physicists in Medicine (AAPM) Accredited Dosimetry Calibration Laboratory calibrated well chamber would be a suitable device to transfer an S(K)-based calibration to a clinical user. S(K) per well-chamber ionization current ratios were consistent among the four dots measured. Additionally, the measurements and predictions of anisotropy show uniform emission within the solid angle of the VAFAC, which demonstrates the robustness of the S(K) measurement approach. This characterization of a new (103)Pd directional brachytherapy source helps to establish calibration methods that could ultimately be used in the well-established AAPM Task Group 43 formalism. Monte Carlo methods accurately predict the changes in the energy spectrum caused by the fluorescent x-rays produced in the gold shield.

TU-AB-201-09: Calibration of An Element of a New Directional Pd-103 Planar Source Array

Purpose: The CivaSheet™ is a new directional Pd-103 planar source array, with a variable number of discrete source elements referred to as “dots”. Each dot consists of a polymer capsule containing 1⁰3Pd and a gold shield that attenuates radiation on one side of the device to define hot and cold dose regions. Fluorescence from the gold shield is observed in the dot spectrum. Since CivaSheet™ is a planar directional source, conventional methods used for calibration of azimuthally symmetric sources are not applicable. The purpose of this work is to establish an air-kerma-strength standard and a transfer to a well chamber for clinical calibration. Methods: Primary air-kerma strength measurement of the dots was performed using a variable-aperture free-air chamber (VAFAC). Charge measurements were recorded using a well chamber with a custom insert. Anisotropy measurements were performed using a Sodium-Iodide detector. Spectral measurements were performed using a low-energy germanium detector and compared to a source without gold. The dot geometry was modeled using the MCNP6 radiation transport code. Results: Air-kerma strength measurements of a batch of four dots performed with the VAFAC were within ±1.5% of the average measured value and the measurement precision was within ±0.5%. Anisotropy measurements indicated uniform emission within the measurement uncertainty for the solid angle defining the VAFAC aperture used. Charge measurements of each dot using the well chamber in four cardinal angle source orientations were within ±1.5% of the average measured values. The spectral study of a dot resulted in identification of fluorescence from the gold shield and primary spectral energies that were compared to MCNP6 simulations. Conclusion: Calibration procedures for the new directional Pd-103 source dot were established for future clinical use, based on the results of experimental and Monte Carlo investigations. This work was partially supported by NCI contract (HHSN261201200052C) through CivaTech Oncology Inc.

SU‐E‐T‐155: Calibration of Variable Longitudinal Strength 103Pd Brachytherapy Sources

Purpose:Brachytherapy sources with variable longitudinal strength (VLS) allow for a customized intensity along the length of the source. These have applications in focal brachytherapy treatments of prostate cancer where dose boosting can be achieved through modulation of intra‐source strengths. This work focused on development of a calibration methodology for VLS sources based on measurements and Monte Carlo (MC) simulations of five 1 cm 1⁰ 3Pd sources each containing four regions of variable 1⁰3Pd strength.Methods:The air‐kerma strengths of the sources were measured with a variable‐aperture free‐air chamber (VAFAC). Source strengths were also measured using a well chamber. The in‐air azimuthal and polar anisotropy of the sources were measured by rotating them in front of a NaI scintillation detector and were calculated with MC simulations. Azimuthal anisotropy results were normalized to their mean intensity values. Polar anisotropy results were normalized to their average transverse axis intensity values. The relative longitudinal strengths of the sources were measured via on‐contact irradiations with radiochromic film, and were calculated with MC simulations.Results:The variable 1⁰3Pd loading of the sources was validated by VAFAC and well chamber measurements. Ratios of VAFAC air‐kerma strengths and well chamber responses were within ±1.3% for all sources. Azimuthal anisotropy results indicated that ≥95% of the normalized values for all sources were within ±1.7% of the mean values. Polar anisotropy results indicated variations within ±0.3% for a ±7.6° angular region with respect to the source transverse axis. Locations and intensities of the 1⁰3Pd regions were validated by radiochromic film measurements and MC simulations.Conclusion:The calibration methodology developed in this work confirms that the VLS sources investigated have a high level of polar uniformity, and that the strength and longitudinal intensity can be verified experimentally and through MC simulations.103Pd sources were provided by CivaTech Oncology, Inc.

SU‐GG‐T‐292: The Response of LiF:Mg,Ti Thermoluminescent Dosimeters to Low‐Energy Photons

Purpose: To characterize the component of the LiF:Mg,Ti TLD response to the low‐energy photons of and LDR brachytherapy sources and x‐ray beam qualities of M40 and M80, that cannot be predicted by cavity theory or Monte Carlo methods. To provide a methodology for determining accurate energy correction factors for experiments performed in a variety of scatter conditions and to provide an example of how to apply the results of this work will also be presented. Method and Materials: TLD‐100 chips were exposed to and LDR brachytherapy sources using the known geometry of the University of Wisconsin Variable Aperture Free Air Chamber. Dose calculations were based on primary determinations of air‐kerma strength and Monte Carlo simulations of the full irradiation and source geometry. For comparison purposes and to examine the effects of dosimeter size and scatter conditions, a series of x‐ray experiments were performed to compare the response of chips (3×3×0.89mm) to microcubes (1×1×1mm) and free in air irradiations to irradiations in a PMMA holder. Results: The results of the x‐ray experiments agreed well with the work of Nunn et al. (Med. Phys. 2006) and confirmed that the “solid‐state” component of the energy response was largely independent of irradiation geometry. The results of the experiments exhibited good reproducibility for the single source, but the measurements seem to exhibit source‐to‐source variability. More experiments are needed for both isotopes to determine the nature of these observations. Conclusion: For all radiation qualities used in these experiments it was found that Monte Carlo simulations appear to underestimate the dose response of LiF:Mg,Ti when compared to measured response. As a result, the previously published values for the dose rate constants of LDR brachytherapy sources are likely overestimates.

WE‐E‐BRA‐07: Experimental Determination of the LiF:Mg,Ti TLD Energy Response Correction Factor for 125I Brachytherapy Sources

Purpose: To further explore the energy response correction factor for LiF:Mg,Ti thermoluminescent dosimeters for use in experiments with low dose rate brachytherapy sources. Method and Materials: Two distinct experiments were performed to determine the value of the energy response correction factor, E(r), as defined in TG‐43U1, for seeds. The University of Wisconsin Variable Aperture Free Air Chamber was used to determine the air kerma strengths of Best Medical 2301 seeds. A 3×3 array of TLD‐100 chips sealed in a thin plastic bag was attached to the center of the aperture using Kapton tape. was used to irradiate a set of comparison TLDs to a dose to water equivalent to the dose to water given by . Monte Carlo simulations were used in both cases to calculate the dose to water from the measured air kerma. The two experiments had different annealing techniques: one using a standard anneal of one hour at 400°C followed by 24 hours at 80°C, and the other using a standard anneal at the beginning of the sorting process and then only the 80°C anneal between cycles. Results: E(r) was determined to be 1.514 ± 0.019 using the 80°C technique and 1.549 ± 0.025 using the standard technique. These values are significantly higher than the currently applied value of 1.41, but not unexpected. TG‐43U1 noted that the work of Davis et al. also observed an anomalous over‐response relative to theoretical predictions. It is speculated that the differences between these two experimental values may be the result of the annealing techniques and TL readout characteristics, which will be further investigated. Conclusion: It appears that LiF:Mg,Ti TLDs are indeed intrinsically non‐linear in their response to low energy photons. The solid state processes that cause this effect are not taken into account by Monte Carlo.

TU-EE-A1-03: Determining the Air-Kerma Strength of 1cm Coiled Brachytherapy Sources

Purpose: To show the ability of a large-volume free-air chamber to experimentally determine the air-kerma strength of a new 1cm 103Pd coiled brachytherapy sources and show its potential for determining the air-kerma strength of longer (2–6cm) coiled sources. Method and Materials: A Variable Aperture Free-Air Chamber (VAFAC) has been constructed for making air-kerma rate measurements of low-energy photon-emitting brachytherapy sources with photon energies up to 70 keV. The VAFAC has been used to determine the air-kerma strength of 1cm coiled brachytherapy sources. The present US air-kerma strength standard for low dose rate (LDR) brachytherapy sources is the National Institute of Standards and Technology Wide-Angle Free-Air Chamber (NIST WAFAC). The results obtained with the VAFAC were compared to air-kerma strengths determined with an Accredited Dosimetry Calibration Laboratory (ADCL) well ionization chamber (traceable to the NIST WAFAC) and direct measurement in the NIST WAFAC. At present, the WAFAC is unable to measure the air-kerma rate of the coiled sources longer than 1cm due to its geometric limitations. The VAFAC does not have the same geometric limitations as the NIST WAFAC, which extends its capabilities to measure sources up to 6cm in length. Results: It has been shown that air-kerma strengths determined for three 1cm length coils with the VAFAC are within 1% agreement with both an ADCL well ionization chamber and NIST WAFAC values. Conclusion: This work shows that the VAFAC can accurately determine the air-kerma strength of 1cm 103Pd coiled brachytherapy sources. It also shows the ability of the VAFAC to measure sources up to 6cm in length, a feat which was previously impossible due to the limitations of the NIST WAFAC. Conflict of Interest: Research sponsored by RadioMed Corporation.

SU‐FF‐T‐303: Ion Loss Measurements for a Large‐Volume Free‐Air Ionization Chamber

Purpose: To extend the capabilities of large‐volume ionization chambers to higher‐fluence sources by thoroughly evaluating ion loss in preparation for Electronic Brachytherapy air‐kerma strength measurements. Method and Materials: Electronic Brachytherapy was recently approved by the FDA as a low‐energy photon‐emitting source for the treatment of breast tumor cavities after lumpectomy surgery. The dosimetry for these types of treatments is based on well‐established AAPM TG 43 protocols for low‐energy photon‐emitting brachytherapy sources. This task group recommends that source strength for low‐energy brachytherapy sources be measured with a large‐volume free‐air ionization chamber to determine the air‐kerma rate absolutely. Until now, using such a chamber for Electronic Brachytherapy sources has not been possible because of the higher x‐ray energies of Electronic Brachytherapy sources (up to 50 keV) as well as their significantly greater exposure rates. There is convincing evidence that the air‐kerma strength of Electronic Brachytherapy sources may be measured with the Variable‐Aperture Free‐Air Chamber (VAFAC). To this end, a systematic investigation of the mechanisms of ion loss has been completed by varying exposure rates, plate separations, and high‐voltage biases. Results: Measurements of ion‐loss agreed with empirical predictions to within 0.2% using the VAFAC for low‐energy photon‐emitting brachytherapy sources with strengths up to 5000 U, more than 50 times higher than the upper limit of the current U.S. national standard (100 U), and within the estimated source strength of new Electronic Brachytherapy sources. Measurements of two coefficients for conventional ion‐loss equations are and , which are within the range of values published in the literature. Conclusion: The VAFAC's measurement capabilities have been extended to higher‐fluence sources (up to 5000 U) in preparation for air‐kerma strength measurements of Electronic Brachytherapy sources. Measurements of empirical coefficients are within the range of values listed in the literature.

Large-volume ionization chamber with variable apertures for air-kerma measurements of low-energy radiation sources

This report describes the design and initial measurement results of a large-volume ionization chamber designed to realize the air kerma from therapeutic radiation sources emitting photons with energies up to about 70 keV. The measured ionization current is used to provide absolute source calibrations by the most widely used brachytherapy source strength metric called air-kerma strength. The variable-aperture free-air chamber (VAFAC), named because of its variable aperture stand, will provide insight into the angular dependence of air-kerma strength measurements. Another unique feature of this ionization chamber is its seed holder design that reduces unwanted scatter, ensures vertical positioning, and rotates the seed with a positional accuracy of 0.3°. Benchmark experiments show that the VAFAC agrees well with existing National Institute of Standards and Technology standards and is capable of measuring clinical-strength seeds with repeatabilities of less than 2% at the 1σ level (k=1).

WE‐C‐T‐617‐05: The Effects of Aperture Size On Low‐Energy Brachytherapy Sk Measurements

Purpose: To experimentally determine the angular dependence of and interstitial brachytherapy seed air‐kerma strength measurements with a large‐volume ionization chamber and compare with Monte Carlo calculations. Method and Materials: A Variable Aperture Free Air Chamber (VAFAC) has been constructed for making air‐kerma rate measurements of low‐energy photon emitting sources with photon energies up to 70 keV. Its underlying principle is that the air‐kerma rate at a given point is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating in the entrance electrode. A unique feature of the VAFAC is its variable aperture stand, which accommodates five apertures with half angles ranging from 4 to 18 degrees. The US primary air‐kerma strength standard is the National Institute of Standards and Technology Wide Angle Free Air Chamber (NIST WAFAC), which has a single fixed aperture size corresponding to a half‐angle of 7.6 degrees. Relative air‐kerma rate measurements have been completed for one seed type with all five aperture sizes. Additionally, MCNP5 particle transport has been used to theoretically determine the expected relative ionization with these aperture sizes. Results: It is shown that the new VAFAC is capable of making air‐kerma rate measurements for low‐energy brachytherapy sources with a noise floor of 1–2 fA. The results from the first seed type indicate that varying the angle from 4 to 18 degrees yields up to a 5% change in the final air‐kerma strength determination. Conclusion: This work shows the capability of the VAFAC to experimentally measure the effects of and interstitial brachytherapy seed in‐air anisotropy on a large angular measurement.

MO-E-T-618-07: Toward An Energy-Based Dosimetry

Purpose: To present a methodology for energy‐based dosimetry for brachytherapy sources Method and Materials: Methodology for brachytherapydosimetry is based on TG 43 formulism. For low ‐dose rate low‐energy brachytherapy sources, TG‐43 involves a measurement of the air‐kerma strength traceable to NIST using the Wide Angle Free Air Chamber (WAFAC). A Variable Aperture Free Air Chamber (VAFAC) has been constructed for making air‐kerma rate measurements with various solid angles. Both instruments provide a determination of the air‐kerma strength. In addition, spectra of these seeds can be measured both in air and liquid water by an intrinsic germanium spectrometer. We have also recently measured the total energy contained and emitted from various sources using a novel cryogeniccalorimeter. This provides an absolute measurement of the power of the source. Each of these experimental processes is presented in other papers. This talk is an overview of how these measurements can be combined to yield an energy‐based dosimetry process. These measurements provide a good basis for Monte Carlo calculations. The energy standard would be used for calculation of dose or by extension, energy deposited. However, with extension to energy‐based dosimetry, a modification of treatment planning systems for a gradual shift from TG 43 formulism will be necessary. An energy‐based dosimetric formulism involves knowing the contained energy or the total emitted energy of a brachytherapy source. A review of the present dosimetry with an extension to energy‐based dosimetry will be presented for discussion. Results: It is shown that energy‐based dosimetry is feasible. The experimental parameters necessary are presented and are being measured presently. The results are very promising. Conclusion: This dosimetry schema is feasible and may be the dosimetry of the future. Moreover, this methodology provides an efficient platform upon which to base Monte Carlodosimetry calculations.