Air Kerma Strength Research Articles

Design and evaluation of a HDR skin applicator with flattening filter

The purposes of this study are: (i) to design field flattening filters for the Leipzig applicators of 2 and 3 cm of inner diameter with the source traveling parallel to the applicator contact surface, which are accessories of the microSelectron-HDR afterloader (Nucletron, Veenendaal, The Netherlands). These filters, made of tungsten, aim to flatten the heterogeneous dose distribution obtained with the Leipzig applicators. (ii) To estimate the dose rate distributions for these Leipzig+filter applicators by means of the Monte Carlo (MC) method. (iii) To experimentally verify these distributions for prototypes of these new applicators, and (iv) to obtain the correspondence factors to measure the output of the applicators by the user using an insert into a well chamber. The MC GEANT4 code has been used to design the filters and to obtain the dose rate distributions in liquid water for the two Leipzig+filter applicators. In order to validate this specific application and to guarantee that realistic source-applicator geometry has been considered, an experimental verification procedure was implemented in this study, in accordance with the updated recommendations of the American Association of Physicists in Medicine Task Group No. 43 U1 Report. Thermoluminescent dosimeters, radiochromic film, and a pin-point ionization chamber in a plastic [polymethylmethacrylate (PMMA)] phantom were used to verify the MC results for the two applicators of a microSelectron-HDR afterloader with the mHDR-v2 source. To verify the output of the Leipzig +filter applicators, correspondence factors were deduced for the well chambers HDR100-plus (Standard Imaging, Inc., Middleton, WI) and TM33004 (PTW, Freiburg, Germany) using a specific insert for both applicators. The doses measured in the PMMA phantom agree within experimental uncertainties with the dose obtained by the MC calculations. Percentage depth dose and off-axis profiles were obtained normalized at a depth of 3 mm along the central applicator axis in a cylindrical 20 x 20 cm water phantom. A table of output factors, normalized to 1 U of source air kerma strength at this depth, is presented. Correspondence factors were obtained for the two well chambers considered. The matrix data obtained in the MC simulation with a grid separation of 0.5 mm has been used to build a data set in a convenient format to model these distributions for routine use with a brachytherapy treatment planning system.

An absorbed dose to water standard for HDR brachytherapy sources based on water calorimetry: Numerical and experimental proof‐of‐principle

Water calorimetry is an established technique for absorbed dose to water measurements in external beams. In this paper, the feasibility of direct absorbed dose measurements for high dose rate (HDR) iridium-192 (192Ir) sources using water calorimetry is established. Feasibility is determined primarily by a balance between the need to obtain sufficient signal to perform a reproducible measurement, the effect of heat loss on the measured signal, and the positioning uncertainty affecting the source-detector distance. The heat conduction pattern generated in water by the Nucletron microSelectron-HDR 192Ir brachytherapy source was simulated using COMSOL MULTIPHYSICS software. Source heating due to radiation self-absorption was calculated using EGSnrcMP. A heat-loss correction k(c) was calculated as the ratio of the temperature rise under ideal conditions to temperature rise under realistic conditions. The calorimeter setup used a parallel-plate calorimeter vessel of 79 mm diameter and 1.12 mm thick front and rear glass windows located 24 mm apart. Absorbed dose was measured with two sources with nominal air kerma strengths of 38 000 and 21 000 U, at source-detector separations ranging from 24.7 to 27.6 mm and irradiation times of 36.0 to 80.0 s. The preliminary measured dose rate per unit air kerma strength of (0.502 +/- 0.007) microGy/(s U) compares well with the TG-43 derived 0.505 microGy/(s U). This work shows that combined dose uncertainties of significantly less than 5% can be achieved with only modest modifications of current water calorimetry techniques and instruments. This work forms the basis of a potential future absolute dose to water standard for HDR 192Ir brachytherapy.

Evaluation of Dosimetric Parameters for Various 192Ir Brachytherapy Sources Under Unbounded Phantom Geometry by Monte Carlo Simulation

As per TG-43 dose calculation formalism, it is essential to obtain various dosimetric parameters such as the air-kerma strength, dose rate constant, radial dose function, and anisotropy function, as they account for accurate determination of dose rate distribution around brachytherapy sources. Most of the available reported Monte Carlo simulations were performed in liquid water phantoms with a bounded region of 30-cm diameter. In this context, an attempt was made to report the dosimetric parameters for various commercially available pulsed-dose rate (PDR) and high-dose rate (HDR) sources under unbounded phantom conditions, as the data may be used as input to treatment planning systems (TPSs) for quality control assistance. The air-kerma strength per unit activity, S k /A, was computed for various Iridium-192 ( 192Ir) sources in dry air medium. The air-kerma strength and dose rate constant for old PDR is (9.77 ± 0.03) 10 −8 U/Bq and 1.124 ± 0.001 cGyh −1U −1; for new PDR, the values are (9.96 ± 0.03) 10 −8 U/Bq and 1.124 ± 0.001 cGyh −1U −1; for old MHDR, the values are (9.80 ± 0.01) 10 −8 U/Bq and 1.115 ± 0.001 cGyh −1U −1; for new MHDR, (9.80 ± 0.01) 10 −8 U/Bq and 1.112 ± 0.001cGyh −1U −1; for old VHDR, the values are (10.32 ± 0.01) 10 −8 U/Bq and 1.035 ± 0.002 cGyh −1U −1; for new VHDR, the values are (10.34 ± 0.02) 10 −8 U/Bq and 1.096 ± 0.001 cGyh −1U −1. The computed radial dose function values and anisotropy function values are also in good agreement with available data.

SU‐FF‐T‐450: Verification of the Source Tech Medical Model STM1251 I‐125 Air‐Kerma Strength in Japan

Purpose: We investigated the difference between measured and manufacturer's stated source strength in a single model STM1251 seeds. Materials and methods: A well‐type ionization chamber with a single seed holder was used to measure source strength of 2412 seeds before implant for 34 patients. The air kerma strength was only used 0.279 U for all cases. The mean source strength used for each patient was compared with the manufacture stated value. The deviation from the measured value was compared with the tolerance level of the recommendation of AAPM TG‐40. Results: Measured source strength was higher than the manufacture stated value with median 1 % (range: −2 to 5 %). Sixteen seeds of all 2412 seeds (0.7 %) were over 5 % different from the manufacture stated value. Standard deviation from the mean value was median 2.2 % (range: 1.1 to 2.5 %) in each patient. Conclusion: A single seed assay was performed for the model STM1251 seed. The manufacture stated strength of the seeds agreed well with the measured value. Our study was helpful as guidance for the permanent prostate implant using the model STM1251 seed.(Submission Withdrawn).

SU-FF-T-147: Determination of TG43 Parameters for Cs-131 Model CS-1R2 Seed Using Radiochromic EBT Film Dosimetry

Purpose: To measure the 2D dose distributions for Cs‐131 seed model CS‐1R2 for distances from 0.06cm to 4cm using radiochromic EBT film dosimetry. TG43 dosimetric parameters were generated. Method and Materials: Each radiochromic film (GAFCHROMIC® EBT lot ♯35076) was in contact with a model CS‐1R2 seed (IsoRay Medical) at the center of a solid water phantom, 30×30×20cm. A multiple film technique was employed. More than 50 films were separately exposed to 10 seeds, with the product of air kerma strength and exposure time between 14 and 800 Uhr. The seed strength ranged from 10 to 4U (NIST traceable) during the experimental runs. For calibration, 15 films (of the same lot) were exposed to 50kV x‐ray (M50) in air at 100cm SSD with doses ranging from 0.2 to 20Gy. Since the EBT film response is almost identical for M50 and M40 x‐rays, the energy response can be considered to be flat in this region and thus also the same for the Cs‐131 energy. All experimental, calibration and background films were scanned (pixel resolution 0.2mm) using a CCD camera‐based densitometer, with red and green light sources. Conversion from net optical density readings to doses were achieved based on the calibration curve established for each light source used in scanning. The 2D dose values in cylindrical coordinates were converted to polar coordinates, and the TG43 parameters were generated. Results: The dose rate constant, radial dose function and 2D anisotropy function were determined, and compared with those reported by other authors. General agreement was found. Conclusion: It is feasible to determine TG43 parameters for Cs‐131 model CS‐1R2 seed in solid water phantom using radiochromic EBT film. This method is superior to TLDdosimetry (1) in providing data with high spatial resolution for distances down to 0.06 cm and (2) within reasonably achievable time frame.

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.

SU-FF-T-383: Stability of the Xoft Axxent® X-Ray Source During Simulated APBI

Purpose: To evaluate the x-ray output stability of the Xoft Axxent® Electronic Brachytherapy System while delivering fractionated doses to a phantom. Method and Materials: A balloon applicator was inflated in a 4.4 cm diameter spherical simulated lumpectomy cavity within an acrylic “breast” on a supine female full-body phantom. Radiation treatments for 5 “patients” were delivered in 10 fractions BID for 5 days using one x-ray source per patient. Per the usual procedure, the air kerma strength for each source was measured prior to each fraction with a calibrated well chamber then dose delivery time was automatically adjusted to account for this source strength. During each fraction, exposure rate was monitored using a calibrated Victoreen Model 451B ion chamber survey meter positioned approximately 10 cm below the phantom. Readings were downloaded at 1 second intervals to a spreadsheet. These data were then analyzed for stability and reproducibility. Results: Fifty fractions were delivered during the five days of simulated treatment. Average treatment time for each fraction was 11 minutes so each source had cumulative operating time of approximately 120 minutes including turn-on and calibration time. Exposure rate measurements increased with decreased source-to-meter distance and with less intervening absorbing material. Exposure rates ranged from 0.1 to 0.55 R/h. The standard deviations from average exposure rates varied from 0.5% to 2.9% with an average over 50 fractions of 0.9%. Conclusions: The Axxent® Electronic Brachytherapy System performed well in five simulated APBI treatments. X-ray source output and system stability were demonstrated to be within 3% for all treatment fractions. Research sponsored by Xoft, Inc.

Neutrons absorbed dose rate calculations for interstitial brachytherapy with 252Cf sources

The AAPM TG-43 protocol was used to evaluate the neutron absorbed dose rate from 252Cf sources for two tissue substitute materials, five normal tissues and six malignant tumours. Three 252Cf source models (AT, VariSource and μSelectron) were simulated using the Monte Carlo MCNPX code in spherical geometry for obtaining the following factors: (a) conversion coefficient of neutron air kerma strength, (b) neutron dose rate constant, (c) neutron radial dose function, (d) geometry function, (e) anisotropy function and (f) neutron absorbed dose rate. For the 252Cf AT source and a Watt fission spectrum, the calculated reference neutron dose rate in water was D ˙ N ( r 0 , θ 0 ) =1.916 cGy h −1 μg −1. The results for the reference neutron dose rate in normal tissues and malignant tumours show how small variations in their elemental composition produce variations in the neutron absorbed dose rate up to 14.9% and 8.4%, respectively, when water and muscle are the reference medium.

Monte Carlo aided design of an improved well-type ionization chamber for low energy brachytherapy sources

The determination of the air kerma strength of a brachytherapy seed is necessary for effective treatment planning. Well-type ionization chambers are used on site at therapy clinics to determine the air kerma strength of seeds. In this work, an improved well-type ionization chamber for low energy, low dose rate brachytherapy sources is designed using Monte Carlo transport calculations to aid in the design process. The design improvements are the elimination of the air density induced over-response effect seen in other air-communicating chambers for low energy photon sources, and a larger signal strength (response or current) for 103Pd and 125I based seeds. A prototype well chamber based on the Monte Carlo aided design but using graphite coated acrylic walls rather than the design basis air equivalent plastic (C-552) walls was constructed and experimentally evaluated. The prototype chamber produced an 85% stronger signal when measuring a commonly used 103Pd seed and a 26% stronger signal when measuring a commonly used 125I seed when compared to another commonly used well chamber. The normalized PTP corrected chamber response is, at most, 1.3% and 2.4% over unity for air densities/pressures corresponding to an elevation of 3048 m (10000 feet) above sea level for the commonly used 103Pd and 125I based seeds respectively. Comparing calculated and measured chamber responses for common 103Pd and 125I based brachytherapy seeds show agreement within 0.6% and 0.2%, respectively. We conclude that Monte Carlo transport calculations accurately model the response of this new well chamber and in general can be used to predict the response of well chambers. The prototype chamber built in this work responds as predicted by the Monte Carlo calculations.

Dosimetric consideration of individual 125I source strength measurement and a large-scale comparison of that measured with a nominal value in permanent prostate implant brachytherapy

We investigated the difference between measured and manufacturer's nominal source strength in a large sample of a single model of (125)I seeds. Physical characteristics of single seed measurement by the well-type ionization chamber were also investigated to provide dosimetric data. A well-type ionization chamber with a single seed holder was used to measure source strength of all 1935 (125)I seeds implanted in the initial 28 patients in our hospital. Physical characteristics including linearity of readings for different integral time intervals, reproducibility, isotropy, and axial positional sensitivity were assessed. To calculate the source strength, the integral charge during 30 s was measured and converted to air kerma strength. The nominal activity stated by the manufacturer was compared with the measured value. Linearity, reproducibility, and isotropy of the well-type ionization chamber were within 0.2%. Measured source strength was on average 2.1% (range -7.6% to +7.2%), lower than the nominal value. Standard deviation of all measured seeds was 2.0%. The maximum difference between the measured and the manufacturer's nominal source strength in each patient was -3.7%. The standard deviation averaged 1.6%. The nominal source strength of the (125)I seeds agreed well with the measured value. Our study can be helpful as guidance for individual (125)I seed source strength measurement.

A dosimetric comparison of and for HDR brachytherapy of the breast, accounting for the effect of finite patient dimensions and tissue inhomogeneities

Monte Carlo simulation dosimetry is used to compare 169Yb to 192Ir for breast high dose rate (HDR) brachytherapy applications using multiple catheter implants. Results for bare point sources show that while 169Yb delivers a greater dose rate per unit air kerma strength at the radial distance range of interest to brachytherapy in homogeneous water phantoms, it suffers a greater dose rate deficit in missing scatter conditions relative to 192Ir. As a result of these two opposing factors, in the scatter conditions defined by the presence of the lung and the finite patient dimensions in breast brachytherapy the dose distributions calculated in a patient equivalent mathematical phantom by Monte Carlo simulations for the same implant of either 169Yb or 1921r commercially available sources are found comparable. Dose volume histogram results support that 169Yb could be at least as effective as 192Ir delivering the same dose to the lung and slightly reduced dose to the breast skin. The current treatment planning systems' approach of employing dosimetry data precalculated in a homogeneous water phantom of given shape and dimensions, however, is shown to notably overestimate the delivered dose distribution for 169Yb. Especially at the skin and the lung, the treatment planning system dose overestimation is on the order of 15%-30%. These findings do not undermine the potential of 169Yb HDR sources for breast brachytherapy relative to the most commonly used 192Ir HDR sources. They imply, however, that there could be a need for the amendment of dose calculation algorithms employed in clinical treatment planning of particular brachytherapy applications, especially for intermediate photon energy sources such as 169Yb.

Calibration of multiple LDR brachytherapy sources

A trend is underway toward the use of prepackaged low dose rate brachytherapy sources, which come in the form of strands, coiled line sources, preloaded needles, and sterile cartridge packs. Since the medical physicist is responsible for verification of source strength prior to patient treatment, development of prepackaged source strength verification methods is needed. Existing guidelines are reviewed to establish the situation that medical physicists find with respect to prepackaged sources. This investigation presents an experimental evaluation of the effect of some of these multiseed geometries on source strength measurements. Multiseed strands and coils, whether 125I, 103Pd, or 192Ir can be measured in a chamber with a long, sensitive axial length with a uniform response. Sterile seed cartridge packs can also be measured but require a correction factor to be applied. Sources in needles, however, cannot be measured in the needle since there is too great a variation in needle composition and needle tolerance thickness. Removing these seeds from the needle into a sterile measurement insert, which maintains sterility is a practical source strength verification method, similar to those done for multiple seed configurations in a well chamber with adequate axial uniformity. Values are compared with individual air kerma strength calibrations, and correction factors, are presented' where needed. In each case, care must be taken to maintain sterility as multiple seeds are measured in well chamber inserts.

Calibration procedures for seeds preloaded in cartridges

Radioactive seeds preloaded in sterilized cartridges or needles are commonly obtainable from manufacturers. Under the US regulations for control of radioactive materials, seed users are required to account for all seeds and independently verify their air kerma strength (SK). As a result, the viability of inspection schemes that rely on measurement of aggregate seeds is of interest. In this paper we consider the conditions (if any) under which cartridge inspection can satisfy regulatory requirements and still provide practical benefit (i.e., time savings) against the regular single-seed assay. The standards for comparison are the recommendations of AAPM TG40, AAPM TG56, and ACR's "Standard for the Performance of Manually Loaded Brachytherapy Sources." The practical benefit is judged in comparison to the effort required to apply the 10% assay recommendation of TG40 to seeds in cartridges. Two specific cartridge inspection schemes are considered: (a) measuring the SK of each cartridge in a batch; (b) measuring a single cartridge sampled at random from the batch. Unlike the 10% assay, which is defined (imperfectly, in our view) without reference to the prevalence of in-calibration seeds, the estimation of the relative merits of cartridge inspection methods must necessarily include such information and, as such, is manufacturer specific. In this paper results are provided for Oncura model 6711 125I seeds in shielded and unshielded Mick cartridges. We show that the only practically useful cartridge inspection scheme is the batch scheme applied to unshielded cartridges. The false positive rates associated with the other schemes are such that we expect to open a cartridge (and perform the 10% assay) at least 80% of the time. While anything less than 100% of the time is theoretically an improvement, this neglects the additional effort required to assay the cartridges.

A fibre optic scintillator dosemeter for absorbed dose measurements of low-energy X-ray-emitting brachytherapy sources

A newly developed dosemeter using a 0.5 mm diameter x 0.5 mm thick cylindrical plastic scintillator coupled to the end of a fibre optic cable is capable of measuring the absorbed dose rate in water around low-activity, low-energy X-ray emitters typically used in prostate brachytherapy. Recent tests of this dosemeter showed that it is possible to measure the dose rate as a function of distance in water from 2 to 30 mm of a (103)Pd source of air-kerma strength 3.4 U (1 U = 1 microGy m(2) h(-1)), or 97 MBq (2.6 mCi) apparent activity, with good signal-to-noise ratio. The signal-to-noise ratio is only dependent on the integration time and background subtraction. The detector volume is enclosed in optically opaque, nearly water-equivalent materials so that there is no polar response other than that due to the shape of the scintillator volume chosen, in this case cylindrical. The absorbed dose rate very close to commercial brachytherapy sources can be mapped in an automated water phantom, providing a 3-D dose distribution with sub-millimeter spatial resolution. The sensitive volume of the detector is 0.5 mm from the end of the optically opaque waterproof housing, enabling measurements at very close distances to sources. The sensitive detector electronics allow the measurement of very low dose rates, as exist at centimeter distances from these sources. The detector is also applicable to mapping dose distributions from more complex source geometries such as eye applicators for treating macular degeneration.

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.

TH‐B‐224C‐04: Feasibility of Calibrating Elongated Brachytherapy Sources Using a Well Type Ionization Chamber

Purpose: TG‐43 recommended parameters of a brachytherapy source require calibration of the source using the WAFAC system by the NIST. However, the presently available NIST standard system is limited for calibration of sources with active lengths ⩽1 cm. A new procedure has been introduced for calibration of elongated brachytherapy sources (i.e. active lengths > 1 cm) as an interim solution to the calibration standards by the NIST. This procedure is based on commercially available well type ionization chambers. Materials and Methods: The variation of the source calibrator response as a function of the source position along the longitudinal axis of the chamber was measured to determine the relative correction factor (RCF). Then the NIST calibrated source was used to calibrate the response of the source calibrator. A train of the 1 cm source segments were used to create elongated sources with different active‐lengths ranging from 1 cm to 7 cm. The measured air kerma strength were compared with the total source strengths calculated as the sum of the individual 1 cm source segments utilized to compose the source. Results: The results of these investigation have indicated that the response of the Capintec CRC was nearly constant (within 0.5 %) for distances ranging from 4.9 cm to 14.9 cm from the bottom of the chamber. With this information and a calibration factor of 4.881 Reading/U, air kerma strengths of RadioCoil™ Pd‐103 sources with active length of 1 to 7 cm were measured. The results of these measurements were found to be within ± 0.4% of the values calculated by addition of the 1cm source segments used in creating the sources. Conclusion: A well type chamber with the calibration for the 1 cm long RadioCoil™ 103Pd source segment can be utilized for calibrating of an elongated source with different active lengths sources.

SU‐CC‐ValA‐08: Air‐Kerma Strength Determination of a 169Ytterbium High Dose Rate Brachytherapy Source

Purpose: To provide an accurate determination of the new 169Yb high dose rate (HDR) bracytherapy source in terms of air kerma strength, based on an adaptation of the current, NIST traceable, in air measurement standard in use for HDR sources. Methods and Materials: Several modifications to the seven distance technique, which is the current standard for HDR source strength measurement, were required to adapt it to the 169Yb spectrum. An Exradin A4 spherical chamber was employed, which has a relatively flat chamber response to the range of energies in the 169Yb spectrum, and has been verified to accurately measure the air kerma strength of to within the reported uncertainty of the current standard measurement technique. To convert the electrometer readings to source strength, a chamber coefficient, Nk, was determined by using the NIST calibrated chamber coefficients from several NIST H‐Beams, whose energy spectrums fall strategically within the 169Yb spectrum. Several correction factors must be applied to these electrometer readings, including corrections for temperature and pressure, air attenuation, air scatter, ion recombination, and corrections for the finite size of the chamber. Results: The decay corrected average of fourteen measurement iterations was 8.063 × 10−3 Gy‐m2/hr. Analysis of uncertainty was performed on these experimental 169Yb air kerma measurements using the standard NIST method for evaluating uncertainty. This analysis established an overall k = 2 expanded uncertainty of 2.10%. Conclusion: It is shown that, with a few modifications, the current standard for high dose rate brachytherapy source calibration could be employed to accurately calibrate the new HDR brachytherapy source in terms of air kerma strength. The uncertainties as analyzed fall within those currently used for calibration. Conflict of Interest: Research funding provided by Implant Sciences Corporation.

SU‐FF‐T‐90: An Improved Irradiation Setup for An Accurate Measurement of the Dose Rate Constant of Low‐Energy Brachytherapy Sources Using Micro‐TLD Cubes

Purpose: To develop an improved irradiation setup for the measurement of dose‐rate‐constant for interstitial brachytherapy sources using micro‐TLD cubes by placing a TLD cube at the mid‐point of two sources of nearly equal air kerma strength separated by a distance of 2 cm in a water equivalent solid phantom. Method and Materials: The impact of geometric uncertainties associated with micro‐TLD cubes on measuring the dose‐rate‐constant of low‐energy brachytherapy sources was investigated quantitatively. Integral dose deposited to the volume occupied by a TLD cube in water was calculated by using the Berger formula for the conventional one‐source and the proposed two‐source irradiation setups. The effects of TLD size, shape‐asymmetry (due to mechanical damage and manufacturing defect), and positioning uncertainty on the dose measured by TLD was quantified by comparing the integral TLD dose to expected point‐dose at the measurement point. Results: For a perfect TLD cube centered at the reference point, the integral dose to TLD was nearly identical to the expected point‐dose, independent of TLD orientations. Displacements of TLD by 0.1 to 0.5 mm in the source‐TLD direction resulted in dose errors of 2.3 to 10% in the one‐source setup and 0.1 to 1% in the two‐source setup. The dose errors were < 1% when the displacements of < 0.5 mm was in the perpendicular direction for both setups. The shape asymmetry measured by volume deficit produced similar dose errors, from 2.5% to over 9%, in both setups for volume deficits of 3 to 10%. Conclusion: To keep geometry‐induced uncertainty in dose‐rate‐constant measurement < 2%, the TLD cube must have 1) nearly perfect symmetry with volume deficits < 2%, 2) positional uncertainty < 0.1 mm in source‐TLD direction and < 0.5 mm in perpendicular direction. The requirement for TLD positioning is greatly reduced in the proposed two‐source experimental setup.

SU-CC-ValA-02: More Accurate Determinations of Air-Kerma Strength for Brachytherapy Sources

Purpose: To determine more accurately the air‐kerma strength of low‐energy photon‐emitting brachytherapy sources. Method and Materials: Air‐kerma strength is the agreed upon metric for most brachytherapy sources and is defined as the product of the kerma rate in vacuo at distance d on the transverse axis of a seed (polar angle of 90 degrees), multiplied by the square of this distance d 2. This work introduces new methods to evaluate how anisotropy affects air‐kerma strength. First, NaI detector measurements yield in‐air anisotropy data that were previously reported only with Monte Carlo transport methods. In addition, multiple aperture sizes are used to evaluate large‐angle free‐air ionization chamber measurements. Lastly, a new method of Monte Carlo transport, which incorporates the distance‐dependent geometry effects of certain brachytherapy seeds, is used to determine more accurately the air‐kerma rate on the transverse axis of the source. These methods may be used to evaluate air‐kerma strength to a higher degree of accuracy. Results: Well‐collimated NaI detector measurements yield precise in‐air anisotropy measurements that may be compared directly with Monte‐Carlo transport simulations. Measurements of two seed types show deviations near the transverse axis of the source of at least 5%. Monte‐Carlo determined point‐detector simulations yield more accurate estimations of air‐kerma strength and show that air‐kerma strength is not constant for all distances in vacuo, as would be predicted by its definition. Conclusion: This work shows that with the combination of these new measurement techniques, air‐kerma strength may be evaluated to a higher degree of accuracy for low‐energy photon‐emitting brachytherapy sources. New experimental data on two seeds' in‐air anisotropy is also presented.

Calibration of a TLD-100 powder dosimetric system to verify the absorbed dose to water imparted by 137Cs sources in low dose rate brachytherapy at the oncology unit in the Hospital General de Mexico

A thermoluminescence dosimetry (TLD) system was characterised at SSDL-ININ to verify the air-kerma strength (S(K)) and dose-to-water (D(W)) values for (137)Cs sources used in low dose rate (LDR) brachytherapy treatments at the Hospital General de Mexico (HGM). It consists of a Harshaw 3500 reader and a set of TLD-100 powder capsules. The samples of TLD-100 powder were calibrated in terms of D(W) vs. nC or nC mg(-1), and their dose response curves were corrected for supralinearity. The D(W) was calculated using the AAPM TG-43 formalism using S(K) for a CDCSM4 (137)Cs reference source. The S(K) value was obtained by using a NE 2611 chamber, and with two well chambers. The angular anisotropy factor was measured with the NE 2611 chamber for this source. The HGM irradiated TLD-100 powder capsules to a reference dose D(W) of 2 Gy with their (137)Cs sources. The percent deviations between the imparted and reference doses were 1.2% < or = Delta < or = 6.5%, which are consistent with the combined uncertainties: 5.6% < or = u(c) < or = 9.8% for D(W).