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.

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.

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.

Technical note: the calibration of 90Y-labeled SIR-Spheres using a nondestructive spectroscopic assay.

90Y-labeled SIR-Spheres are currently used to treat patients with hepatic metastases secondary to colorectal adenocarcinoma. In general, the prescribed activity is based on empirical data collected during clinical trials. The activity of the source vial is labeled by the manufacturer as 3.0 GBq +/- 10% and is not independently verified by the end user. This technical note shows that the results of a nondestructive spectroscopic assay of a SIR-Spheres sample was 26% higher than the activity stated by the manufacturer. This difference should not impact the current empirical prescription method but may be problematic for patient-specific dosimetry applications, such as image-based dosimetry.

A new internal pair production branching ratio of 90Y: the development of a non-destructive assay for 90Y and 90Sr.

(90)Y is utilized as a therapeutic radioisotope in radiolabeled monoclonal antibodies and in microspheres for targeted radiation therapy of the liver. Currently, the widely used dose calibrator assay of (90)Y can have uncertainties exceeding +/-10%. A non-destructive assay using spectroscopy is possible by reducing the currently published uncertainty (+/-12%) in the internal pair production branching ratio for the 0(+)-0(+) transition of (90)Zr. A high-purity germanium detector was used to determine the branching ratio to be (31.86+/-0.47) x 10(-6).

The US radiation dosimetry standards for 60Co therapy level beams, and the transfer to the AAPM accredited dosimetry calibration laboratories.

This work reports the transfer of the primary standard for air kerma from the National Institute of Standards and Technology (NIST) to the secondary laboratories accredited by the American Association of Physics in Medicine (AAPM). This transfer, performed in August of 2003, was motivated by the recent revision of the NIST air-kerma standards for 60Co gamma-ray beams implemented on July 1, 2003. The revision involved a complete recharacterization of the two NIST therapy-level 60Co gamma-ray beam facilities, resulting in new values for the air-kerma rates disseminated by the NIST. Some of the experimental aspects of the determination of the new air-kerma rates are briefly summarized here; the theoretical aspects have been described in detail by Seltzer and Bergstrom ["Changes in the U.S. primary standards for the air-kerma from gamma-ray beams," J. Res. Natl. Inst. Stand. Technol. 108, 359-381 (2003)]. The standard was transferred to reference-class chambers submitted by each of the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). These secondary-standard instruments were then used to characterize the 60Co gamma-ray beams at the ADCLs. The values of the response (calibration coefficient) of the ADCL secondary-standard ionization chambers are reported and compared to values obtained prior to the change in the NIST air-kerma standards announced on July 1, 2003. The relative change is about 1.1% for all of these chambers, and this value agrees well with the expected change in chambers calibrated at the NIST or at any secondary-standard laboratory traceable to the new NIST standard.

Low dose fraction behavior of high sensitivity radiochromic film.

A high sensitivity (HS) model of radiochromic film is receiving increasing use. The film's linear sensitometric response in the range of 0.5-40 Gy would make this film an ideal candidate for complex dosimetry applications that require tissue equivalence. This study investigates the potential use for clinical dosimetry of typical radiotherapy fractions at relatively low doses (0.5-5 Gy). The experiment involved exposing 25 pre-exposed pieces of HS film to five equal fractions of doses from 0.5 to 5 Gy 24 hours apart. The cumulative dose for each film was carefully monitored and optical density measurements were used as the sole determination of film response to dose. The average behavior of the various fractionation schemes was roughly consistent with previous observations of the MD-55 radiochromic film with about twice the overall sensitivity as expected. However, at low doses and low dose increments, unexpected variations beyond a well-documented low dose nonlinearity were observed. These unexpected variations may indicate complex polymer kinetics at low doses. This type of film would require extra care beyond that described in TG-55 for accurate use at low doses or low dose fraction schemes.

The effect of ambient pressure on well chamber response: experimental results with empirical correction factors.

For some air-communicating well-type chambers used for low-energy brachytherapy source assay, deviations from expected values of measured air kerma strength were observed at low pressures associated with high altitudes. This effect is consistent with an overcompensation by the air density correction to standard atmospheric temperature and pressure (P(TP)). This work demonstrates that the P(TP) correction does not fully compensate for the high altitude pressure effects that are seen with air-communicating chambers at low photon energies in the range of 20-100 keV. Deviations of up to 18% at a pressure corresponding to an approximate elevation of 8500 ft for photon energies of 20 keV are possible. For high-energy photons and for high-energy beta emitters in air-communicating chambers the P(TP) factor is applicable. As expected, the ambient pressure does not significantly affect the response of pressurized well chambers (within 1%) to either low- or high-energy photons. However, when used with beta emitters, pressurized chambers appear to exhibit a slight dependence on the ambient pressure. Using measured data, the response and correction factors were determined for three models of air-communicating well chambers for low-energy photon sources at various pressures corresponding to elevations above sea level. Monte Carlo calculations were also performed which were correlated with the experimental findings. A more complete study of the Monte Carlo calculations is presented in the accompanying paper, "The effect of ambient pressure on well chamber response: Monte Carlo calculated results for the HDR1000 Plus."

The effect of spectra on calibration and measurement with mammographic ionization chambers.

Mammographic imaging uses x-ray tubes with molybdenum, rhodium, or tungsten anodes with the produced bremsstrahlung filtered by thin sheets of molybdenum, rhodium, or aluminum. The National Institute of Standards and Technology, the Accredited Dosimetry Calibration Laboratories, and several manufacturers offer calibrations of mammography ionization chambers with reference x-ray beams with different radiation qualities in the range 23-40 kVp. The energy response of ten commercially available chambers was determined for these reference radiation qualities using the Attix variable-length free-air chamber. The evaluated chambers are designed with thin entrance windows of varying thickness and composition. The chambers show variation in their air kerma response as a function of beam radiation quality. This response with beam radiation quality may affect the measurement of clinical beam half value layer (HVL) and the determination of the mean glandular dose. The combined effect of the chamber's energy dependence and HVL measurement affects the mean glandular dose calculation resulting in differences ranging from -1.8% to +2.5%.

Calibration of new high dose rate 192Ir sources.

The increased popularity of high dose rate (HDR) brachytherapy in the treatment of neoplastic disease has resulted in the introduction of new or redesigned 192Ir sources by various manufacturers. In order to provide accurate clinical dosimetry, accurate calibration of these sources is required. In this work, two new sources are calibrated with the standard, NIST traceable, in-air calibration technique. These sources are the Varian VariSource (VS2000) and the newly redesigned Nucletron MicroSelectron source. An uncertainty budget was performed to determine the overall uncertainty of the measurement, and a total relative expanded uncertainty (k = 2) of 2.15% was calculated. The results of the source calibration using the in-air technique and that obtained based upon the 1991 standard calibration all lie well within this uncertainty. In addition to the in-air calibration, contact autoradiographs of the sources were made with International Specialty Products model HD810 radiochromic film to demonstrate the sources relative dose distributions.

Dependence of scatter on atomic number for x rays from tungsten and molybdenum anodes in the mammographic energy range.

A study was done to determine the relative amounts of scatter for the following materials with atomic numbers ranging from Z=6 to Z=82: C, Al, Ti, Fe, Cu, Zn, Zr, Y, Mo, Ta, and Pb. Measurements were performed for each material on two constant potential x-ray units--one fitted with a molybdenum (Mo) anode-Mo filter and the other with a tungsten (W) anode-aluminum (Al) filter (medium filtration) at 30 kVp. Theoretical calculations were also performed for each anode to explain the scatter behavior and to aid in predicting the behavior for materials where measurements were not made. There was good agreement between the theoretical calculations and the experimental data.

Physics of rotating gamma systems for stereotactic radiosurgery.

PURPOSE: The purpose of this study was to evaluate the characteristics of a new rotating gamma system for stereotactic radiosurgery by comparison with a well accepted system. METHODS AND MATERIALS: A novel gamma unit for stereotactic radiosurgery has been developed and distributed to 15 hospitals in China. The unit contains 30 cobalt-60 gamma radiation sources with initial activity of 200 Ci (7.4 x 10(12) Bq) each. The sources are positioned along 30 arcs, and rotate continuously as a group in an axis orthogonal to the patient's body. Measurements have been made on a representative unit installed in the Auhai Radiosurgery Center at the Beijing Navy General Hospital in the People's Republic of China. Ionization chambers calibrated by an American accredited dosimetry calibration laboratory were used for these measurements, as well as radiochromic film and thermoluminescent dosimeters. The unit tested utilizes collimators of nominal diameters of 4, 8, 14, and 18 mm. Radiochromic film samples from a Leksell Model U Gamma Knife were evaluated by the same laboratory and are presented for comparison. The treatment planning system was not evaluated. RESULTS: Radiation-absorbed dose rates and profiles measured for this unit are comparable to those previously measured with the same techniques for the Leksell Model U Gamma Knife units in San Diego and Atlanta. CONCLUSION: This unit is capable of producing well collimated beams of high energy photons, suitable for stereotactic radiosurgery. It has similar physical characteristics to those previously reported for the Leksell Model U Gamma Knife unit.

Aromatic hydrocarbon receptor polymorphism: development of new methods to correlate genotype with phenotype.

Differential CYP1A1 inducibility, reflecting variations in aromatic hydrocarbon receptor (AHR) affinity among inbred mouse strains, is an important determinant of environmental toxicity. We took advantage of the Ahr polymorphism in C57BL/6 and DBA/2 mice to develop an oligonucleotide-hybridization screening approach for the rapid identification of DNA sequence differences between Ahr alleles. Oligonucleotides containing single-base changes at polymorphic sites were immobilized on a solid support and hybridized with C57BL/6 or DBA/2 AHR cDNA radiolabeled probes. The observed hybridization patterns demonstrate that this approach can be used to detect nucleotide differences in the Ahr coding region with very high accuracy. In parallel experiments, we used a yeast two-hybrid system to assess phenotypic differences in AHR function. AHR activation, as measured by beta-galactosidase reporter activity in Saccharomyces cerevisiae strain SFY526, was determined following treatment with varying doses of the AHR ligand beta-naphthoflavone (BNF). We found that the C57BL/6 AHR has about a 15-fold higher affinity for BNF than the DBA/2 AHR, in much better agreement with results reported for whole-animal studies than the values observed by in vitro ligand-binding assays. Using C57BL/6 and DBA/2 AHR chimeric proteins, we also confirmed the previously reported observation that an A375V change is principally responsible for the high- to low-affinity AHR phenotype. There has been no straightforward method to reliably and reproducibly phenotype large numbers of humans for CYP1A1 inducibility or AHR affinity. Screening human AHR cDNAs by oligonucleotide-hybridization and yeast two-hybrid methodologies will be invaluable for the rapid and unequivocal determination of changes in DNA sequence and receptor-ligand affinities associated with human AHR polymorphisms.

Human Ah receptor (AHR) gene: localization to 7p15 and suggestive correlation of polymorphism with CYP1A1 inducibility.

The mammalian aromatic hydrocarbon receptor (AHR) is a ubiquitous ligand-activated transcription factor. AHR ligands include 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin), benzo[a]pyrene, and polychlorinated and polybrominated biphenyls; the endogenous ligand is not yet known. Following ligand binding, the AHR transcriptionally activates genes encoding drug-metabolizing enzymes important in both the metabolic potentiation of substrates to genotoxic reactive intermediates and ultimate carcinogens, and the detoxification of toxic or carcinogenic drugs and other environmental pollutants. AHR-mediated gene expression is also involved in many critical life processes (e.g. cell type-specific differentiation, cell division, apoptosis) by signal transduction mechanisms. Similar to mice, human populations exhibit a > 20-fold range of the CYP1A1 inducibility/AHR affinity phenotype. In the present study, we localized the human AHR gene to chromosome 7p15, using fluorescence in situ hybridization (FISH). Performing linkage analysis in a three-generation family, we show with good probability that the high CYP1A1 inducibility phenotype segregates with the 7p15 region. Sequencing 93 nt (31 amino acids) of the human AHR gene's exon 9, which is the region correlated with the mouse A375V polymorphism responsible for the major portion of high vs low CYP1A1 inducibility/AHR affinity, we found no nucleotide differences; Val-381 was present in all five individuals examined (four related and one unrelated), two of whom show "high' and three of whom show "low' CYP1A1 inducibility. These data indicate that the "high' and "low' CYP1A1 inducibility trait, in the population studied, cannot be explained by a difference among these 31 amino acids in exon 9 of the AHR gene.

A comparison of methods for calibrating parallel-plate chambers/.

All dosimetry protocols for calibrating the output of electron beams recommend the use of parallel-plate ionization chambers, but the method of determining their value of Ngas is a matter of concern. The AAPM Protocol (TG 21) recommends a direct comparison with a calibrated cylindrical chamber in phantom at dmax with the highest available electron energy beam. This must be done by the user. Since all calibration laboratories traditionally use 60Co for megavoltage chamber calibrations, two alternate procedures based on exposures in-air, or in-phantom, have been proposed. All methods use correction factors in the data reduction. To verify the consistency of the three methods, we have measured Ngas using each of these techniques for six of the most commonly used and commercially-available parallel-plate ionization chambers. The paired cylindrical and parallel-plate ionization chambers, and phantom materials/buildup caps were matched to the wall composition of the plane chambers, as recommended in TG 39. A 22 MeV electron beam was used for the electron irradiations. The ionization chambers were then taken to an Accredited Dosimetry Calibration Laboratory (ADCL), where 60Co calibrations were performed. The results demonstrate that, by using the appropriate correction factors for the chambers described in this work, all three methods yield values for Ngas that are within 1% of each other.