Tina L. Pike

Calibration of GafChromic XR-RV3 radiochromic film for skin dose measurement using standardized x-ray spectra and a commercial flatbed scanner

PURPOSE In this study, newly formulated XR-RV3 GafChromic film was calibrated with National Institute of Standards and Technology (NIST) traceability for measurement of patient skin dose during fluoroscopically guided interventional procedures. METHODS The film was calibrated free-in-air to air kerma levels between 15 and 1100 cGy using four moderately filtered x-ray beam qualities (60, 80, 100, and 120 kVp). The calibration films were scanned with a commercial flatbed document scanner. Film reflective density-to-air kerma calibration curves were constructed for each beam quality, with both the orange and white sides facing the x-ray source. A method to correct for nonuniformity in scanner response (up to 25% depending on position) was developed to enable dose measurement with large films. The response of XR-RV3 film under patient backscattering conditions was examined using on-phantom film exposures and Monte Carlo simulations. RESULTS The response of XR-RV3 film to a given air kerma depended on kVp and film orientation. For a 200 cGy air kerma exposure with the orange side of the film facing the source, the film response increased by 20% from 60 to 120 kVp. At 500 cGy, the increase was 12%. When 500 cGy exposures were performed with the white side facing the x-ray source, the film response increased by 4.0% (60 kVp) to 9.9% (120 kVp) compared to the orange-facing orientation. On-phantom film measurements and Monte Carlo simulations show that using a NIST-traceable free-in-air calibration curve to determine air kerma in the presence of backscatter results in an error from 2% up to 8% depending on beam quality. The combined uncertainty in the air kerma measurement from the calibration curves and scanner nonuniformity correction was +/- 7.1% (95% C.I.). The film showed notable stability. Calibrations of film and scanner separated by 1 yr differed by 1.0%. CONCLUSIONS XR-RV3 radiochromic film response to a given air kerma shows dependence on beam quality and film orientation. The presence of backscatter slightly modifies the x-ray energy spectrum; however, the increase in film response can be attributed primarily to the increase in total photon fluence at the sensitive layer. Film calibration curves created under free-in-air conditions may be used to measure dose from fluoroscopic quality x-ray beams, including patient backscatter with an error less than the uncertainty of the calibration in most cases.

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.

WE‐A‐108‐05: Experimental and Computational Dosimetric Characterization of the Xoft AxxentTM Electronic Brachytherapy Source Within a Titanium Cervical Applicator

PURPOSE To provide a dosimetric characterization of the Xoft Axxent™ source within a titanium cervical applicator using liquid water dosimetry and Monte Carlo simulation methods. METHODS Absorbed dose to water measurements were performed using an annulus of Virtual Water™ (referred to as the Captains Wheel) that is mounted in a water phantom to allow measurements in liquid water. The Captains Wheel permits the precise placement of TLD-100 microcubes around the applicator using aluminum radial gauges of radii 1, 2, 3, and 4 cm. This geometry allows determination of the dose-rate constant, radial dose function, and azimuthal anisotropy. The air-kerma strength was measured using the Attix Free-Air Chamber at the UWADCL and well chamber measurements were completed using two Standard Imaging HDR1000 Plus well chambers. Measurements were performed using two Axxent™ sources calibrated at NIST. Monte Carlo simulations were also performed using MCNP5 based on specifications provided by the manufacturer to calculate the dose-rate constant, radial dose function, and polar anisotropy. MCNP5 was used to generate the source spectrum exiting the titanium applicator for comparison to the experimental results. RESULTS The Monte Carlo dose-rate constant was 3.6% higher than the experimentally determined dose-rate constant. The radial dose value at 2 cm calculated for a 6 mm pullback distance was 3.5% lower when compared to the measured radial dose value at 2 cm. The azimuthal anisotropy measurements were normalized to the smallest TLD reading at a given distance, with a maximum anisotropy value of 1.27 at 2 cm. CONCLUSION TG-43 parameters were experimentally determined and compared to Monte Carlo simulations for the Axxent™ source within a titanium applicator. CONFLICT OF INTEREST Xoft Inc. provided financial support, sources, and applicators. Xoft Inc. provided financial support, sources, and applicators.

WE‐D‐BRB‐05: Dose‐Rate Constant Determination of the Xoft Axxent Electronic Brachytherapy Source Using Spectroscopic Methods

Purpose: To use spectroscopic methods to determine the dose‐rate constant (DRC) of the Xoft Axxent® miniature x‐ray source. Method and Materials: Analytical methods have been used to determine the DRC of brachytherapy sources using spectroscopic methods. The method developed at Yale University (Med. Phys. 28, 86–96, 2001) was used with the Axxent® spectrum to calculate the spectroscopic DRC of the Axxent® source. While this method is useful its limitations are pronounced when applied to sources of considerable diameter, such as the Axxent® source. When source size increases the point‐ and line‐source approximations are insufficient for buildup or attenuation calculations. This investigation modifies the original method to account for source diameter and calculates the DRC with measured and Monte Carlo(MC) generated spectra. The Axxent® spectrum was measured at 1 m in air with a low‐energy germanium detector. The spectrum was corrected for detector response and air attenuation to obtain the emitted spectrum in vacuum. Results: The measured spectrum agreed well with the MC generated spectrum. Slight differences in the spectra can be attributed to the energy resolution of the detector. The spectroscopic DRC calculated with the original method produced DRCs that were up to 23% low when compared to the MC calculated DRC. The modified method corrected the attenuation and buildup calculations to account for source diameter and produced DRC that were within 5% of the MC calculated DRC. Conclusion: It was possible to measure and correct the photonspectrum of the Axxent® source. The original method for analytically determining the spectroscopic DRC works well for sources of very small diameter, but is insufficient when applied to sources of considerable diameter. The method must be modified to correct the attenuation and buildup determination in the monoenergetic DRC determination. Conflict of Interest: This research was partially sponsored by Xoft, Inc.

SU‐GG‐T‐276: Dosimetric Characterization of a Set of Surface Applicators from Xoft Inc

Purpose: A set of conical surface applicators with diameters ranging from 10 mm to 50 mm have been developed by Xoft Inc. These applicators are designed to be used with the Axxent® eBx System, allowing for conformal dose delivery for the treatment of surface lesions. Current methods of output verification for these applicators are based on the AAPMs TG‐61 protocol. Certain components of TG‐61 may need modification for applicators like those under investigation. The goal of this work is an accurate experimental determination of the output of each applicator with comparison to Monte Carlo determined values. Methods and Materials: Air‐kerma rate measurements along the central axis of each applicator were performed using an Attix free‐air chamber, and at the applicator window with two parallel plate chamber models. Each applicator was simulated using MCNP5 and a collision kerma tally was used to determine the air‐kerma rate along the central axis and at the applicator window. Results: Initial measurements of the air‐kerma rate at the applicator window with the parallel plate chambers agreed within the expected uncertainty. Extrapolation of the air‐kerma rate measured with the FAC back to the window surface agreed with the parallel plate chambers within 15% for all but the 50 mm diameter applicator. Monte Carlo calculated values agree with the parallel plate chambers for all applicators except the 50 mm diameter as well. Conclusions: Current forms of output verification for these applicators are based on TG‐61, which was designed for low‐energy external beams, not miniature X‐ray sources in an applicator. Initial work has shown that output verification may be performed using calibrated ionization chambers, however some aspects of TG‐61 need modification for use with applicators like those under investigation here. Conflict of Interest: Xoft Inc. provided sources and applicators.

SU‐FF‐T‐273: Energy Dependence of TLD‐100 Chips and Microcubes for the Xoft Axxent® 50 KV Miniature X‐Ray Source and Cs‐137 Relative to Co‐60

Purpose: To use experimental methods to measure the overall energy dependence of LiF:Mg,Ti thermoluminescent dosimeters(TLDs) for the Xoft Axxent® miniature x‐ray source (mean energy ∼27 keV) and 137Cs (662 keV) relative to 60Co (1250 keV), to calculate the absorbed‐dose energy dependence with the Monte Carlo(MC) code MCNP5, and to determine the intrinsic energy dependence of the TLDs by dividing the measured overall energy dependence by the absorbed‐dose energy dependence. Method and Materials:TLDs were irradiated to a known air kerma with all three sources. The air kerma rate of the miniature x‐ray source was measured using an Attix free‐air chamber. Each irradiation was performed for TLDs of two different sizes, microcubes (1 mm3) and chips (3×3×1 mm3). MC simulations were performed for each setup to calculate dose to the TLDs including photonscatter from the holders. MC methods were also used to calculate the air kerma. Results: The overall measured energy response was greater than the MC calculated absorbed‐dose energy dependence for both sources and both chip sizes. The difference between the measured and calculated energy dependence for the miniature x‐ray source was greater than for 137Cs, which means that the intrinsic energy dependence of LiF:Mg,Ti TLD chips and microcubes relative to 60Co is more pronounced at lower energies. Conclusion: This experiment provides further evidence that TLDs have intrinsic energy dependence in addition to the absorbed‐dose energy dependence that can be calculated using Monte Carlo methods.MC corrections alone are not able to calculate the overall energy dependence of TLDs. Therefore, appropriate energy dependence corrections must be measured and applied to TLD measurements when calibration irradiations are performed at a different energy. Conflict of Interest: This research was partially sponsored by Xoft, Inc.

WE‐E‐AUD‐02: Dose Rate Measurements of An Electronic Brachytherapy Source Using Thermoluminescent Dosimeters in Water

Purpose: To measure the dose rate of an electronic brachytherapy source using thermoluminescent dosimeters(TLDs) in water.Method and Materials:Dose rates from several Xoft Axxent™ electronic brachytherapy sources were measured using TLD microcubes (1 mm × 1 mm × 1 mm). The sources were operated at 50 kV and at a beam current of 100 μA. Measurements were done in liquid water to avoid the large conversion from water mimicking plastics to liquid water at low energies. A Virtual Water™ apparatus was designed to position the TLD microcubes in water specifically so that no Virtual Water™ was between the sources and microcubes during irradiation. The design allowed for the placement of 12 TLDs in 30° increments around the source on the transverse axis. A positioning disk was designed to accurately position the center of the TLDs at 3 cm from the center of the source and to position the center of the sources at the same height as the center of the TLDs. Air kerma rate measurements were done with a well‐type ionization chamber fitted with a custom‐built aluminum source holder. The measured air kerma rates were used to calculate the desired irradiation time to deliver an estimated 70 and 130 cGy dose to water at 3 cm from the sources. Results: The dose rates measured with the TLD microcubes were lower than the Monte Carlo predicted values. Air kerma rates measured were also determined to be lower than the Monte Carlo predicted values, but the measured ratio of dose rate to air kerma rate agreed with the Monte Carlo predicted values. Conclusion:TLDs can be used in water to determine dose rates of electronic brachytherapy sources, but further work is necessary to confirm measured values. Conflict of Interest: Funding for this research was provided by Xoft, Inc.