Monte Carlo modeling of the influence of strong magnetic fields on the stem-effect in plastic scintillation detectors used in radiotherapy dosimetry.

PURPOSE: To investigate the impact of strong magnetic fields on the stem-effect in plastic scintillation detectors (PSDs) using Monte Carlo methods. METHODS: Prior to building the light guide model, the properties of the Cerenkov process in GEANT4 were investigated by simulating depth-dose and depth-Cerenkov emission profiles in water as functions of Cerenkov process input parameters. In addition, profile simulations were performed for magnetic field strengths ranging from 0 T to 1.5 T. A PMMA light guide was constructed in GEANT4 using data from the manufacturer and literature. Simulations were performed with the model as functions of depth and fiber-beam angle where the simulated stem-effect spectrum and the Cerenkov light ratio (CLR) were scored and compared to measured data in the literature. The light guide optical properties were iteratively adjusted until agreement between the simulated and measured data was achieved. Simulations were performed with the validated model as functions of depth and magnetic field strength and the simulated data were compared to measured data in the literature. The model was also used to evaluate the sensitivity of the CLR to the various optical properties of the light guide in different irradiation conditions. RESULTS: No significant changes in the depth-dose or depth-Cerenkov emission profiles were observed with step-size restrictions imposed by the Cerenkov process input parameters, which was attributed to the condensed history algorithm and transport parameters used in this work. Similar changes in the depth-dose and depth-Cerenkov emission profiles were observed with increasing magnetic field strength, which indicates the Cerenkov process is not adversely impacted by the presence of the magnetic field. Following optimization of the light guide optical properties, agreement within two standard deviations was observed between the simulated and measured optical data for all validation geometries considered. Agreement within one standard deviation was observed between the simulated and measured data for all depths and field strengths ≥0 T whereas discrepancies were observed for magnetic field strengths <-0.35 T. These significant differences were attributed to insufficient measurement data for this irradiation configuration during model validation. Of the light guide optical properties investigated, the fluorescence signal had the greatest impact on the CLR sensitivity to the magnetic field. CONCLUSIONS: No significant change in the Cerenkov emission per dose in water was observed for magnetic field strengths up to 1.5 T. The nominal fiber fluorescence signal was found to have a significant impact on the CLR sensitivity to varying irradiation conditions where changes up to 11.7% were observed whereas the mirror reflectivity and fiber attenuation had a modest impact with maximum CLR changes of 2.6% and 1.2% relative to 0 T, respectively. The results of this work suggest light guides with low fiber fluorescence should be used with PSDs for dosimetry measurements in magnetic fields to minimize the impact of the magnetic field on the CLR correction.

Technical note: characterization of spectral changes with measurement geometry and magnetic field strength in light guides used for scintillation dosimetry.

The purpose of this work was to characterize the stem-effect signal and the Cerenkov light ratio (CLR) in various light guides as functions of measurement geometry and magnetic field strength. Two PMMA-, two silica-, and one polystyrene-based light guides were considered in this work. Spectra measurements were performed as functions of depth, fiber-beam angle, and magnetic field strength using an optical spectrometer. All measurements were performed using a clinical linear accelerator at a nominal photon beam energy of 6 MV. Depths ranging from 1 cm to 10 cm, fiber-beam angles ranging from 90 degrees to 30 degrees, and magnetic field strengths ranging from 0 T to ± 1.40 T were investigated. The CLR was calculated from each spectrum by taking the ratio of the integral signal between 400 nm and 500 nm to the integral signal between 500 nm and 600 nm. A maximum increase of 80.5% in the stem-effect signal was observed in the magnetic field. Variations in spectral shape and, consequently, the CLR were observed for all of the fibers as functions of magnetic field strength and measurement geometry, particularly for wavelengths less than 400 nm. The plastic fibers exhibited decreases in the CLR as a function of magnetic field strength at all depths investigated, whereas the silica fibers exhibited increases in the CLR with decreasing magnetic field strength. A maximum variation of 11.1% in the CLR was observed for the polystyrene fiber due to the magnetic field. The sensitivity of the CLR to the magnetic field decreased as the fiber-beam angle decreased. The measured spectral response, shape, and CLR were found to be sensitive to the applied magnetic field strength and polarity where the variations in response were unique to each fiber.

On the accuracy and efficiency of condensed history transport in magnetic fields in GEANT4.

The purpose of this work was to investigate the accuracy and efficiency of electron transport in GEANT4 with and without a magnetic field present. Fano cavity simulations were performed in GEANT4 version 10.02 and 10.04.p01 using two multiple scattering (MSC) algorithms for two slab and one pseudo-ion chamber geometries. An iterative approach was used to optimize the transport parameters to obtain agreement with theory. Similar to previous works, the step lengths had to be severely restricted to obtain agreement with theory when using the Urban MSC model in GEANT4 v10.02. Using the Goudsmit-Saunderson MSC model with the UseSafetyPlus MSC step limitation in GEANT4 v10.04.p01 limited the maximum discrepancy from theory to 0.5%. Minor adjustments to the transport parameters were needed to obtain agreement within 0.16% of theory for all simulation configurations without a magnetic field present. The maximum deviation from theory was within 2% for all simulation configurations in the presence of a magnetic field except for two setups that exhibited discrepancies of up to 10.8%. This anomalous behavior was mitigated by forcing single scattering within the detector gas volume. Further adjustments to the transport parameters resulted in agreement with theory at the 0.21% level. Agreement with theory in the absence of a magnetic field can be obtained without significantly restricting the step size if the Goudsmit-Saunderson MSC model is used with the UseSafetyPlus MSC step limitation in GEANT4 v10.04.p01. The large discrepancies from theory observed for two simulation setups with a magnetic field present were attributed to an issue with energy loss sampling over a step when strict magnetic field transport parameters are used. This problem can be corrected by forcing single scattering within the detector gas volume; however, more work is needed to identify the cause of this anomalous behavior. This work has shown that GEANT4 can perform accurate electron transport with and without a magnetic field present without applying significant step-size reductions.

Spectral characterization of plastic scintillation detector response as a function of magnetic field strength.

The purpose of this work was to characterize intensity and spectral response changes in a plastic scintillation detector (PSD) as a function of magnetic field strength. Spectra measurements as a function of magnetic field strength were performed using an optical spectrometer. The response of both a PSD and PMMA fiber were investigated to isolate the changes in response from the scintillator and the noise signal as a function of magnetic field strength. All irradiations were performed in water at a photon beam energy of 6 MV. Magnetic field strengths of (0, ±0.35, ±0.70, ±1.05, and ±1.40) T were investigated. Four noise subtraction techniques were investigated to evaluate the impact on the resulting noise-subtracted scintillator response with magnetic field strength. The noise subtraction methods included direct spectral subtraction, the spectral method, and variants thereof. The PMMA fiber exhibited changes in response of up to 50% with magnetic field strength due to the directional light emission from [Formula: see text]erenkov radiation. The PSD showed increases in response of up to 10% when not corrected for the noise signal, which agrees with previous investigations of scintillator response in magnetic fields. Decreases in the [Formula: see text]erenkov light ratio with negative field strength were observed with a maximum change at -1.40 T of 3.2% compared to 0 T. The change in the noise-subtracted PSD response as a function of magnetic field strength varied with the noise subtraction technique used. Even after noise subtraction, the PSD exhibited changes in response of up to 5.5% over the four noise subtraction methods investigated.

A round-robin gamma stereotactic radiosurgery dosimetry interinstitution comparison of calibration protocols.

PURPOSE: Absorbed dose calibration for gamma stereotactic radiosurgery is challenging due to the unique geometric conditions, dosimetry characteristics, and nonstandard field size of these devices. Members of the American Association of Physicists in Medicine (AAPM) Task Group 178 on Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance have participated in a round-robin exchange of calibrated measurement instrumentation and phantoms exploring two approved and two proposed calibration protocols or formalisms on ten gamma radiosurgery units. The objectives of this study were to benchmark and compare new formalisms to existing calibration methods, while maintaining traceability to U.S. primary dosimetry calibration laboratory standards. METHODS: Nine institutions made measurements using ten gamma stereotactic radiosurgery units in three different 160 mm diameter spherical phantoms [acrylonitrile butadiene styrene (ABS) plastic, Solid Water, and liquid water] and in air using a positioning jig. Two calibrated miniature ionization chambers and one calibrated electrometer were circulated for all measurements. Reference dose-rates at the phantom center were determined using the well-established AAPM TG-21 or TG-51 dose calibration protocols and using two proposed dose calibration protocols/formalisms: an in-air protocol and a formalism proposed by the International Atomic Energy Agency (IAEA) working group for small and nonstandard radiation fields. Each institution's results were normalized to the dose-rate determined at that institution using the TG-21 protocol in the ABS phantom. RESULTS: Percentages of dose-rates within 1.5% of the reference dose-rate (TG-21+ABS phantom) for the eight chamber-protocol-phantom combinations were the following: 88% for TG-21, 70% for TG-51, 93% for the new IAEA nonstandard-field formalism, and 65% for the new in-air protocol. Averages and standard deviations for dose-rates over all measurements relative to the TG-21+ABS dose-rate were 0.999±0.009 (TG-21), 0.991±0.013 (TG-51), 1.000±0.009 (IAEA), and 1.009±0.012 (in-air). There were no statistically significant differences (i.e., p>0.05) between the two ionization chambers for the TG-21 protocol applied to all dosimetry phantoms. The mean results using the TG-51 protocol were notably lower than those for the other dosimetry protocols, with a standard deviation 2-3 times larger. The in-air protocol was not statistically different from TG-21 for the A16 chamber in the liquid water or ABS phantoms (p=0.300 and p=0.135) but was statistically different from TG-21 for the PTW chamber in all phantoms (p=0.006 for Solid Water, 0.014 for liquid water, and 0.020 for ABS). Results of IAEA formalism were statistically different from TG-21 results only for the combination of the A16 chamber with the liquid water phantom (p=0.017). In the latter case, dose-rates measured with the two protocols differed by only 0.4%. For other phantom-ionization-chamber combinations, the new IAEA formalism was not statistically different from TG-21. CONCLUSIONS: Although further investigation is needed to validate the new protocols for other ionization chambers, these results can serve as a reference to quantitatively compare different calibration protocols and ionization chambers if a particular method is chosen by a professional society to serve as a standardized calibration protocol.

A prototype, glassless densitometer traceable to primary optical standards for quantitative radiochromic film dosimetry.

PURPOSE: To evaluate a prototype densitometer traceable to primary optical standards and compare its performance to an EPSON Expression(®) 10000XL flatbed scanner (the Epson) for quantitative radiochromic film (RCF) dosimetry. METHODS: A prototype traceable laser densitometry system (LDS) was developed to mitigate common film scanning artifacts, such as positional scan dependence and high noise in low-dose regions, by performing point-based measurements of RCF suspended in free-space using coherent light. The LDS and the Epson optical absorbance scales were calibrated up to 3 AU, using reference materials calibrated at a primary standards laboratory and a scanner calibration factor (SCF). Calibrated optical density (OD) was determined for 96 Gafchromic(®) EBT3 film segments before and after irradiation to one of 16 dose levels between 0 and 10 Gy, exposed to (60)Co in a polymethyl-methacrylate (PMMA) phantom. The sensitivity was determined at each dose level and at two rotationally orthogonal readout orientations to obtain the sensitometric response of each RCF dosimetry system. LDS rotational scanning dependence was measured at nine angles between 0°and 180°, due to the expected interference between coherent light and polarizing EBT3 material. The response curves were fit to the analytic functions predicted by two physical response models: the two-parameter single-hit model and the four-parameter percolation model. RESULTS: The LDS and the Epson absorbance measurements were linear to primary optical standards to within 0.2% and 0.3% up to 2 and 1 AU, respectively. At higher densities, the LDS had an over-response (2.5% at 3 AU) and the Epson an under-response (3.1% and 9.8% at 2 and 3 AU, respectively). The LDS and the Epson SCF over the applicable range were 0.968% ± 0.2% and 1.561% ± 0.3%, respectively. The positional scan dependence was evaluated on each digitizer and shown to be mitigated on the LDS, as compared to the Epson. Maximum EBT3 rotational dependence was found to have a strong dependence on dose (0.1% and 34% at 30 mGy and 5 Gy, respectively). The preferred EBT3 polymerization axis angle was constant within experimental uncertainties. In its most sensitive orientation, the LDS-measured EBT3 sensitivity was 7.13 × 10(-4) ± 9.2 × 10(-6) AU/mGy, which represented a 4.5 fold increase over the Epson of 1.58 × 10(-4) ± 9.8 × 10(-6) AU/mGy. To first order approximations, EBT3 response was linear up to 500 mGy to within 0.80% and to within 7.5% for the most sensitive LDS and the Epson orientations, respectively. The corresponding single-hit and percolation model relative residual norms were 0.082 and 0.074 for LDS as compared to 0.29 and 0.18 for the Epson, which represented a significant increase in LDS-measured agreement with the simple physical model. Less sensitive LDS and the Epson orientations showed a marked decrease in the physical model agreement, which suggested that suboptimal readout device characteristics may be the origin of the complex sensitometric functional forms currently required for accurate RCF dosimetry. CONCLUSIONS: The prototype densitometer was shown to be superior to a conventional scanner for quantitative RCF dosimetry based on physical models of film response. The Epson was shown to be a reliable tool for routine RCF dosimetry in a clinical setting, yet calibration to primary optical standards did not mitigate the necessity for complex, empirical functional form fitting.

Determination of the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters for 125I and 103Pd brachytherapy sources relative to 60Co.

PURPOSE: To determine the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters (TLD-100) for (125)I and (103)Pd brachytherapy sources relative to (60)Co. METHODS: LiF:Mg,Ti TLDs were irradiated with low-energy brachytherapy sources and with a (60)Co teletherapy source. The brachytherapy sources measured were the Best 2301 (125)I seed, the OncoSeed 6711 (125)I seed, and the Best 2335 (103)Pd seed. The TLD light output per measured air-kerma strength was determined for the brachytherapy source irradiations, and the TLD light output per air kerma was determined for the (60)Co irradiations. Monte Carlo (MC) simulations were used to calculate the dose-to-TLD rate per air-kerma strength for the brachytherapy source irradiations and the dose to TLD per air kerma for the (60)Co irradiations. The measured and MC-calculated results for all irradiations were used to determine the TLD intrinsic energy dependence for (125)I and (103)Pd relative to (60)Co. RESULTS: The relative TLD intrinsic energy dependences (relative to (60)Co) and associated uncertainties (k = 1) were determined to be 0.883 ± 1.3%, 0.870 ± 1.4%, and 0.871 ± 1.5% for the Best 2301 seed, OncoSeed 6711 seed, and Best 2335 seed, respectively. CONCLUSIONS: The intrinsic energy dependence of TLD-100 is dependent on photon energy, exhibiting changes of 13%-15% for (125)I and (103)Pd sources relative to (60)Co. TLD measurements of absolute dose around (125)I and (103)Pd brachytherapy sources should explicitly account for the relative TLD intrinsic energy dependence in order to improve dosimetric accuracy.

Determination of air-kerma strength for the 192Ir GammaMedplus iX pulsed-dose-rate brachytherapy source.

PURPOSE: Pulsed-dose-rate (PDR) brachytherapy was originally proposed to combine the therapeutic advantages of high-dose-rate (HDR) and low-dose-rate brachytherapy. Though uncommon in the United States, several facilities employ pulsed-dose-rate brachytherapy in Europe and Canada. Currently, there is no air-kerma strength standard for PDR brachytherapy (192)Ir sources traceable to the National Institute of Standards and Technology. Discrepancies in clinical measurements of the air-kerma strength of the PDR brachytherapy sources using HDR source-calibrated well chambers warrant further investigation. METHODS: In this research, the air-kerma strength for an (192)Ir PDR brachytherapy source was compared with the University of Wisconsin Accredited Dosimetry Calibration Laboratory transfer standard well chambers, the seven-distance technique [B. E. Rasmussen et al., "The air-kerma strength standard for 192Ir HDR sources," Med. Phys. 38, 6721-6729 (2011)], and the manufacturer's stated value. Radiochromic film and Monte Carlo techniques were also employed for comparison to the results of the measurements. RESULTS: While the measurements using the seven-distance technique were within + 0.44% from the manufacturer's determination, there was a + 3.10% difference between the transfer standard well chamber measurements and the manufacturer's stated value. Results showed that the PDR brachytherapy source has geometric and thus radiological qualities that exhibit behaviors similar to a point source model in contrast to a conventional line source model. CONCLUSIONS: The resulting effect of the pointlike characteristics of the PDR brachytherapy source likely account for the differences observed between well chamber and in-air measurements.

Microionization chamber air-kerma calibration coefficients as a function of photon energy for x-ray spectra in the range of 20-250 kVp relative to 60Co.

PURPOSE: To investigate the applicability of a wide range of microionization chambers for reference dosimetry measurements in low- and medium-energy x-ray beams. METHODS: Measurements were performed with six cylindrical microchamber models, as well as one scanning chamber and two Farmer-type chambers for comparison purposes. Air-kerma calibration coefficients were determined at the University of Wisconsin Accredited Dosimetry Calibration Laboratory for each chamber for a range of low- and medium-energy x-ray beams (20-250 kVp), with effective energies ranging from 11.5 keV to 145 keV, and a (60)Co beam. A low-Z proof-of-concept microchamber was developed and calibrated with and without a high-Z silver epoxy on the collecting electrode. RESULTS: All chambers composed of low-Z materials (Z ≤ 13), including the Farmer-type chambers, the scanning chamber, and the PTW TN31014 and the proof-of-concept microchambers, exhibited air-kerma calibration coefficients with little dependence on the quality of the beam. These chambers typically exhibited variations in calibration coefficients of less than 3% with the beam quality, for medium energy beams. However, variations in air-kerma calibration coefficients of greater than 50% were measured over the range of medium-energy x-ray beams for each of the microchambers containing high-Z collecting electrodes (Z > 13). For these high-Z chambers, which include the Exradin A14SL and A16 chambers, the PTW TN31006 chamber, the IBA CC01 chamber, and the proof-of-concept chamber containing silver, the average variation in air-kerma calibration coefficients between any two calibration beams was nearly 25% over the entire range of beam qualities investigated. CONCLUSIONS: Due to the strong energy dependence observed with microchambers containing high-Z components, these chambers may not be suitable dosimeters for kilovoltage x-ray applications, as they do not meet the TG-61 requirements. It is recommended that only microchambers containing low-Z materials (Z ≤ 13) be considered for air-kerma calibrations for reference dosimetry in low- and medium-energy x-ray beams.

Tomotherapy dose distribution verification using MAGIC-f polymer gel dosimetry.

PURPOSE: This paper presents the application of MAGIC-f gel in a three-dimensional dose distribution measurement and its ability to accurately measure the dose distribution from a tomotherapy unit. METHODS: A prostate intensity-modulated radiation therapy (IMRT) irradiation was simulated in the gel phantom and the treatment was delivered by a TomoTherapy equipment. Dose distribution was evaluated by the R2 distribution measured in magnetic resonance imaging. RESULTS: A high similarity was found by overlapping of isodoses of the dose distribution measured with the gel and expected by the treatment planning system (TPS). Another analysis was done by comparing the relative absorbed dose profiles in the measured and in the expected dose distributions extracted along indicated lines of the volume and the results were also in agreement. The gamma index analysis was also applied to the data and a high pass rate was achieved (88.4% for analysis using 3%∕3 mm and of 96.5% using 4%∕4 mm). The real three-dimensional analysis compared the dose-volume histograms measured for the planning volumes and expected by the treatment planning, being the results also in good agreement by the overlapping of the curves. CONCLUSIONS: These results show that MAGIC-f gel is a promise for tridimensional dose distribution measurements.

SU-E-T-137: The Response of TLD-100 in Mixed Fields of Photons and Electrons.

PURPOSE: Thermoluminescent dosimeters are used routinely for dosimetric measurements of photon and electron fields. However, no work has been published characterizing TLDs for use in combined photon and electron fields. This work investigates the response of TLD-100 (LiF:Mg,Ti) in mixed fields of photon and electron beam qualities. METHODS: TLDs were irradiated in a 6 MV photon beam, 6 MeV electron beam, and a NIST traceable cobalt-60 beam. TLDs were also irradiated in a mixed field of the electron and photon beams. All irradiations were normalized to absorbed dose to water as defined in the AAPM TG-51 report. The average response per dose (nC/Gy) for each linac beam quality was normalized to the average response per dose of the TLDs irradiated by the cobalt-60 standard.Irradiations were performed in a water tank and a Virtual Water™ phantom. Two TLD dose calibration curves for determining absorbed dose to water were generated using photon and electron field TLD response data. These individual beam quality dose calibration curves were applied to the TLDs irradiated in the mixed field. RESULTS: The TLD response in the mixed field was less sensitive than the response in the photon field and more sensitive than the response in the electron field. TLD determination of dose in the mixed field using the dose calibration curve generated by TLDs irradiated by photons resulted in an underestimation of the delivered dose, while the use of a dose calibration curve generated using electrons resulted in an overestimation of the delivered dose. CONCLUSIONS: The relative response of TLD-100 in mixed fields fell consistently between the photon nd electron relative responses. When using TLD-100 in mixed fields, the user must account for this intermediate response to avoid an over- or underestimation of the dose due to calibration in a single photon or electron field.

SU-E-T-431: Investigation of BrachyVision™ Acuros™ Using Varian Surface Applicators.

PURPOSE: Conical brachytherapy surface applicators with diameters ranging from 10 mm to 45 mm have been developed by Varian Medical Systems, Inc. These applicators are designed to be used with the GammaMedplus iX and VariSource iX high-dose rate Ir-192 afterloaders, allowing for conformal dose delivery for the treatment of surface lesions. Treatment plans for these applicators are created in BrachyVision Acuros. Few studies have been completed with Acuros in clinical situations. The purpose of this work is to perform a comparison of the Acuros-calculated dose distributions with those calculated using Monte Carlo and measured with various detectors. METHODS: Surface applicator treatment plans for each source/applicator combination were created using Acuros and a virtual water phantom. Simulations to characterize the dose distributions were completed using MCNP5 based on specifications provided by the manufacturer. A collision kerma tally was used to determine the dose distributions at the surface and at depth in water. Experimental verification of the depth-dose and surface dose distributions was completed using an ionization chamber, and TLDs and film, respectively. Acuros-calculated depth-dose and isodose values were compared to the Monte Carlo and experimental values. RESULTS: Assessment of the surface dose distributions shows a peak at the center of the applicator with rapid fall off to the edges. The TPS-calculatedpercentage depth-dose curves were within 3.7% of the MC and 5.8% of the measured for the 30 mm applicator and were within 4.4% of the MC and measured for the 35 mm applicator. CONCLUSIONS: BrachyVision Acuros is capable of calculating the depth-dose and surface dose distributions for the simple water phantom case with surface applicators. Investigation of additional treatment geometries and applicators is ongoing. Conflict ofInterest: Varian Medical Systems, Inc. provided financial support, software, sources, and applicators. Varian Medical Systems, Inc. provided financial support, software, sources, and applicators.

SU-D-BRCD-03: Spectroscopic Characterization of a 6 MV Linear Accelerator Field Using Compton Spectrometry Measurements and Monte Carlo Techniques.

PURPOSE: Effective treatment planning for radiotherapy is dependent on accurate spectra determination; however, direct measurements of spectra arecomplicated by fluence rates that exceed detection system limitations. This work demonstrates the potential to use a Compton scattering technique to measure the spectrum of a 6MV linac. These spectra are further characterized using Monte Carlo (MC) simulations. METHODS: A high-purity germanium detector was used to measure the photon spectrum scattered at35 degrees from the central axis of 3cmx3cm and lOcmxlOcm 6MV fields from a Varian linac. Photons were scattered using aluminum rods positioned at isocenter, and were admitted to the detector through a 30cm-long collimating aperture. The measured Compton-scattered spectra were corrected for background. An MC model of the linac was developed in MCNP5 to calculate central- and off-axis spectra. The model geometry was verified by comparisons with percentage depth-dose and profilemeasurements. The spectroscopic effect of the mean energy, radius, and divergence of the electron beam incident on the target was tested for twofield sizes. RESULTS: The count rate of the scattered beam increased with field size and scattering rod diameter. Preliminary measurements indicate that the spectrum was shifted to lower energies using this technique; however, the signal-to-noise ratio was poor due to leakage and room scatter. MC simulations demonstrate that the central- and off-axis spectra were sensitive to changes in mean electron energy; however, changes in beam diameter and angular divergence did not substantially affect either the central- or off-axis spectra. CONCLUSIONS: This work demonstrates that the spectrum from a 6MV linac can be measured using Compton spectrometry. Further work is required to increase the signal-to-noise ratio and correct fordetector response. MC simulations indicate that the spectra were sensitive to variations in the parameters used to define the primary electron beam incident on the target.

SU-F-BRCD-02: Activity Assay of a Y-90 Microsphere Sample Using a Coincidence Detection System.

PURPOSE: This work takes advantage of a newly-determined, low-uncertainty branching ratio of the internal pair production component of Y-90 decay to spectroscopically assay the activity of a Y-90 microsphere sample with a coincidence detection system (CDS). METHODS: The CDS pairs a HPGe detector with a large NaI detector. The system is able to electronically filter the bremsstrahlung continuum from the photon spectrum by gating the energy signal from the HPGe with the coincidence signal. This reduces the uncertainty in the spectra measurement compared to measurements with a single HPGe detector. A series of pulsers were used to correct for counting losses. A geometric characterization was completed to find the optimal source position for measurements. An efficiency calibration of the CDS was completed using a Na-22 standard source. To validate the measurement accuracy of the CDS, the activity of a Y-90 standard activity solution from the NIST SRM program was determined and compared to the value given by NIST. The CDS was then used to determine the activity of a Y-90 microsphere sample. This value was compared to the 3.0 GBq value given by the manufacturer. RESULTS: The activity determined with the CDS was within 2.6% from the given activity of the NIST SRM source, which is within the expanded uncertainty associated with the CDS measurement. The activity of the Y-90 microsphere sample was determined to be 3.72 GBq +/- 1.9%. This is 19% higher than the manufacturer-stated activity of 3 GBq. This is outside the manufacturer-stated uncertainty of +/-10%. CONCLUSIONS: The ability of CDS to determine the activity of a Y-90 source has been validated with comparison to a NIST Y-90 standard activity solution. The use of the CDS has been extended to determine the activity of a Y-90 microsphere sample.

SU-E-T-15: Small and Nonstandard Photon Field Dosimetry Characterization Using Monte Carlo Methods.

PURPOSE: To quantify uncertainty reduction in small photon field dosimetry through characterization of ionization chambers and calibration conditions using detailed Monte Carlo methods benchmarked against NIST-traceable measurements. METHODS: Phase space profiles were obtained using detailed EGSnrc Monte Carlo models for a Varian 6 MV photon linear accelerator, and a NIST-traceable cobalt-60 teletherapy unit. The responses of a farmer-type ionization chamber, two micro-ionization chambers, and one scanning-type ionization chamber were simulated in multiple calibration conditions. Calibration conditions included static field sizes ranging from (10×10) cm squared to (0.5×0.5) cm squared for the 6 MV and cobalt-60 beam qualities and an additional dynamic IMRT plan for the 6 MV beam quality. Calibration conditions also consisted of ionization chambers placed in a standard water phantom and in a specially designed acrylic phantom. Tolerance limits on the calibration conditions were investigated. All models were benchmarked against measured beam quality data, including ionization chamber beam quality correction factors for the standard absorbed dose to water cobalt-60 calibration coefficient. RESULTS: The majority of the simulated small field response values fell within the uncertainty of the measured values. A database was created for several proposed small field calibration conditions to provide comparisons with the Co-60 standard reference conditions. The database includes the small field calibration conditions' beam quality correction factors, tolerances, and dose calibration uncertainties. CONCLUSIONS: The characterization of multiple calibration conditions provided an improved understanding of how the cobalt-60 ionization chamber absorbed dose to water calibration coefficient from an ADCL can optimally be applied to small and nonstandard field calibrations to reduce the associated dose uncertainty. The developed methodology will contribute to future research of other small field radiotherapy modalities. The resulting database provides support for future recommendations on the implementation of small and non-standard field calibration protocols.

Photon beam audits for radiation therapy clinics: a pilot mailed dosemeter study in Turkey.

A thermoluminescent dosemeter (TLD) mailed dose audit programme was performed at five radiotherapy clinics in Turkey. The intercomparison was organised by the University of Wisconsin Radiation Calibration Laboratory (UWRCL), which was responsible for the technical aspects of the study including reference irradiations, distribution, collection and evaluation. The purpose of these audits was to perform an independent dosimetry check of the radiation beams using TLDs sent by mail. Acrylic holders, each with five TLD chips inside and instructions for their irradiation to specified absorbed dose to water of 2 Gy, were mailed to all participating clinics. TLD irradiations were performed with a 6 MV linear accelerator and (60)Co photon beams. The deviations from the TL readings of UWRCL were calculated. Discrepancies inside the limits of ±5 % between the participant-stated dose, and the TLD-measured dose were considered acceptable. One out of 10 beams checked was outside this limit, with a difference of 5.8 %.

Experimental and Monte Carlo determination of the TG-43 dosimetric parameters for the model 9011 THINSeed brachytherapy source.

PURPOSE: AAPM TG-43 brachytherapy dosimetry parameters for a new, smaller diameter 1251 brachytherapy source (THINSeed, model 9011) were determined using LiF:Mg,Ti thermoluminescent dosimeter (TLD-100) microcubes and Monte Carlo simulations. METHODS: Two polymethyl methacrylate phantoms were machined to hold TLD-100 microcubes at specific locations for the experimental determination of the radial dose function, dose-rate constant, and anisotropy functions of the new source. The TG-43 parameters were also calculated using Monte Carlo simulations. For comparison, the model 6711 source was also investigated. RESULTS: Experimental results for both models 9011 and 6711 sources showed good agreement with Monte Carlo values, as well as with previously published values. CONCLUSIONS: The TG-43 parameters for the new source model are similar to those of model 6711; however, they represent two separate sources and TG-43 parameters used in treatment planning must be source specific.

Air-kerma strength determination of a 169Yb high dose rate brachytherapy source.

The increased demand for high dose rate (HDR) brachytherapy as an alternative to external beam radiotherapy has led to the introduction of a HDR brachytherapy isotope 169Yb. This source offers a dose rate similar to 192Ir HDR sources, at about one fourth the effective photon energy. This work presents the calibration of this source in terms of air-kerma strength, based on an adaptation of the current, National Institute of Standards and Technology traceable, in air measurement technique currently used for 192Ir HDR sources. Several additional measurement correction factors were required, including corrections for air scatter, air attenuation, and ion recombination. A new method 169Yb is introduced for determining the ion chamber calibration coefficient Nk(169Yb). An uncertainty analysis was also performed, indicating an overall measurement expanded uncertainty in the air-kerma strength (k=2) of 2.2%.

LiF:Mg,Ti TLD response as a function of photon energy for moderately filtered x-ray spectra in the range of 20-250 kVp relative to 60Co.

The response of LiF:Mg,Ti thermoluminescent dosimeters (TLDs) as a function of photon energy was determined using irradiations with moderately filtered x-ray beams in the energy range of 20-250 kVp relative to the response to irradiations with 60Co photons. To determine if the relative light output from LiF:Mg,Ti TLDs per unit air kerma as a function of photon energy can be predicted using calculations such as Monte Carlo (MC) simulations, measurements from the x-ray beam irradiations were compared with MC calculated results, similar to the methodology used by Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)]. TLDs were irradiated in photon beams with well-known air kerma rates using the National Institute of Standards and Technology traceable M-series x-ray beams in the range of 20-250 kVp. For each x-ray beam, several sets of TLDs were irradiated for times corresponding to different air kerma levels to take into account any dose nonlinearity. TLD light output was then compared to that from several sets of TLDs irradiated at similar corresponding air kerma levels using a 60Co irradiator. The MC code MCNP5 was used to account for photon scatter and attenuation in the holder and TLDs and was used to calculate the predicted relative TLD light output per unit air kerma for irradiations with each of the experimentally used photon beams. The measured relative TLD response as a function of photon energy differed by up to 13% from the MC calculations. We conclude that MC calculations do not accurately predict the relative response of TLDs as a function of photon energy, consistent with the conclusions of Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)]. This is likely due to complications in the solid state physics of the thermoluminescence process that are not incorporated into the simulation.