Thomas W. Rusch

Calculated and measured brachytherapy dosimetry parameters in water for the Xoft Axxent X‐Ray Source: An electronic brachytherapy sourcea)

A new x-ray source, the model S700 Axxent™ X-Ray Source (Source), has been developed by Xoft Inc. for electronic brachytherapy. Unlike brachytherapy sources containing radionuclides, this Source may be turned on and off at will and may be operated at variable currents and voltages to change the dose rate and penetration properties. The in-water dosimetry parameters for this electronic brachytherapy source have been determined from measurements and calculations at 40, 45, and 50kV settings. Monte Carlo simulations of radiation transport utilized the MCNP5 code and the EPDL97-based mcplib04 cross-section library. Inter-tube consistency was assessed for 20 different Sources, measured with a PTW 34013 ionization chamber. As the Source is intended to be used for a maximum of ten treatment fractions, tube stability was also assessed. Photon spectra were measured using a high-purity germanium (HPGe) detector, and calculated using MCNP. Parameters used in the two-dimensional (2D) brachytherapy dosimetry formalism were determined. While the Source was characterized as a point due to the small anode size, <1mm, use of the one-dimensional (1D) brachytherapy dosimetry formalism is not recommended due to polar anisotropy. Consequently, 1D brachytherapy dosimetry parameters were not sought. Calculated point-source model radial dose functions at gP(5) were 0.20, 0.24, and 0.29 for the 40, 45, and 50kV voltage settings, respectively. For 1<r<7cm, measured point-source model radial dose functions were typically within 4% of calculated results. Calculated values for F(r,θ) for all operating voltages were within 15% of unity along the distal end (θ=0°), and ranged from F(1cm,160°)=0.2 to F(15cm,175°)=0.4 towards the catheter proximal end. For all three operating voltages using the PTW chamber, measured dependence of output as a function of azimuthal angle, ψ, was typically on average ±3% for 0°⩽ψ⩽360°. Excluding an energy response function, measurements of normalized photon energy spectra were made for three operating voltages, and were typically within 2% agreement with the normalized Monte Carlo calculated spectra. In general, the model S700 Source exhibited depth dose behavior similar to low-energy photon-emitting low dose rate sources I125 and Pd103, yet with capability for variable and much higher dose rates and subsequently adjustable penetration capabilities. This paper presents the calculated and measured in-water brachytherapy dosimetry parameters for the model S700 Source at the aforementioned three operating voltages.

Spectroscopic characterization of a novel electronic brachytherapy system

The Axxent developed by Xoft Inc. is a novel electronic brachytherapy system capable of generating x-rays up to 50 keV. These low energy photon-emitting sources merit attention not only because of their ability to vary the dosimetric properties of the radiation, but also because of the radiobiological effects of low energy x-rays. The objective of this study is to characterize the x-ray source and to model it using the Geant4 Monte Carlo code. Spectral and attenuation curve measurements are performed at various peak voltages and angles and the source is characterized in terms of spectrum and half-value layers (HVLs). Also, the effects of source variation and source aging are quantified. Bremsstrahlung splitting, phase-space scoring and particle-tagging features are implemented in the Geant4 code, which is bench-marked against BEAMnrc simulations. HVLs from spectral measurements, attenuation curve measurements and Geant4 simulations mostly agree within uncertainty. However, there are discrepancies between measurements and simulations for photons emitted on the source transverse plane (90 degrees).

Guidelines by the AAPM and GEC-ESTRO on the use of innovative brachytherapy devices and applications: Report of Task Group 167

Although a multicenter, Phase III, prospective, randomized trial is the gold standard for evidence-based medicine, it is rarely used in the evaluation of innovative devices because of many practical and ethical reasons. It is usually sufficient to compare the dose distributions and dose rates for determining the equivalence of the innovative treatment modality to an existing one. Thus, quantitative evaluation of the dosimetric characteristics of innovative radiotherapy devices or applications is a critical part in which physicists should be actively involved. The physicists role, along with physician colleagues, in this process is highlighted for innovative brachytherapy devices and applications and includes evaluation of (1) dosimetric considerations for clinical implementation (including calibrations, dose calculations, and radiobiological aspects) to comply with existing societal dosimetric prerequisites for sources in routine clinical use, (2) risks and benefits from a regulatory and safety perspective, and (3) resource assessment and preparedness. Further, it is suggested that any developed calibration methods be traceable to a primary standards dosimetry laboratory (PSDL) such as the National Institute of Standards and Technology in the U.S. or to other PSDLs located elsewhere such as in Europe. Clinical users should follow standards as approved by their countrys regulatory agencies that approved such a brachytherapy device. Integration of this system into the medical source calibration infrastructure of secondary standard dosimetry laboratories such as the Accredited Dosimetry Calibration Laboratories in the U.S. is encouraged before a source is introduced into widespread routine clinical use. The American Association of Physicists in Medicine and the Groupe Européen de Curiethérapie-European Society for Radiotherapy and Oncology (GEC-ESTRO) have developed guidelines for the safe and consistent application of brachytherapy using innovative devices and applications. The current report covers regulatory approvals, calibration, dose calculations, radiobiological issues, and overall safety concerns that should be addressed during the commissioning stage preceding clinical use. These guidelines are based on review of requirements of the U.S. Nuclear Regulatory Commission, U.S. Department of Transportation, International Electrotechnical Commission Medical Electrical Equipment Standard 60601, U.S. Food and Drug Administration, European Commission for CE Marking (Conformité Européenne), and institutional review boards and radiation safety committees.

Comparison of TG‐43 and TG‐186 in breast irradiation using a low energy electronic brachytherapy source

PURPOSE The recently updated guidelines for dosimetry in brachytherapy in TG-186 have recommended the use of model-based dosimetry calculations as a replacement for TG-43. TG-186 highlights shortcomings in the water-based approach in TG-43, particularly for low energy brachytherapy sources. The Xoft Axxent is a low energy (<50 kV) brachytherapy system used in accelerated partial breast irradiation (APBI). Breast tissue is a heterogeneous tissue in terms of density and composition. Dosimetric calculations of seven APBI patients treated with Axxent were made using a model-based Monte Carlo platform for a number of tissue models and dose reporting methods and compared to TG-43 based plans. METHODS A model of the Axxent source, the S700, was created and validated against experimental data. CT scans of the patients were used to create realistic multi-tissue/heterogeneous models with breast tissue segmented using a published technique. Alternative water models were used to isolate the influence of tissue heterogeneity and backscatter on the dose distribution. Dose calculations were performed using Geant4 according to the original treatment parameters. The effect of the Axxent balloon applicator used in APBI which could not be modeled in the CT-based model, was modeled using a novel technique that utilizes CAD-based geometries. These techniques were validated experimentally. Results were calculated using two dose reporting methods, dose to water (Dw,m) and dose to medium (Dm,m), for the heterogeneous simulations. All results were compared against TG-43-based dose distributions and evaluated using dose ratio maps and DVH metrics. Changes in skin and PTV dose were highlighted. RESULTS All simulated heterogeneous models showed a reduced dose to the DVH metrics that is dependent on the method of dose reporting and patient geometry. Based on a prescription dose of 34 Gy, the average D90 to PTV was reduced by between ~4% and ~40%, depending on the scoring method, compared to the TG-43 result. Peak skin dose is also reduced by 10%-15% due to the absence of backscatter not accounted for in TG-43. The balloon applicator also contributed to the reduced dose. Other ROIs showed a difference depending on the method of dose reporting. CONCLUSIONS TG-186-based calculations produce results that are different from TG-43 for the Axxent source. The differences depend strongly on the method of dose reporting. This study highlights the importance of backscatter to peak skin dose. Tissue heterogeneities, applicator, and patient geometries demonstrate the need for a more robust dose calculation method for low energy brachytherapy sources.

SU-FF-T-58: Dosimetric Study of a New Surface Applicator for the Xoft Axxent System

Purpose: To determine the dosimetricproperties of a new surface applicator designed for the Xoft 50 kVp x‐ray system Method and Materials: A 35mm conical stainless steel applicator with Al flattening filter interfaces to the Xoft 50 kVp x‐ray source to provide therapeutic radiation to surfaces such as skin. Measurements of the dose profile at the surface and at depths to 20 mm were performed using a PTW ionization chamber in a water phantom. The chamber was controlled by a stepper motor and linear stage, and data was read into a controlling computer. In addition, film data was taken both parallel and perpendicular to the surface.Results: Nominal dose profiles and depth dose characteristics were determined from an average of ten sources. Dose profiles across at least 80% of the applicator width were flat to within ± 10% for all sources, and to within 5% on average. Depth dose characteristics are very similar to those published for HDR based surface applicators, with the percent depth dose at 5 and 10 mm at 58% and 36%. Such data can be used for treatment planning purposes. The variation seen among sources determines an error band representative of what will be experienced in clinical use. Conclusion: The FDA approved surface applicator delivers a dose profile that is flat to within ± 10% over a span of 80% of the defined width of the device. It can be used for treatment of skin lesions in lightly shielded rooms due to the low energies employed. There are also intriguing possibilities for use intra‐operatively, since the system can be used in an operating room. Conflict of Interest: Research supported by Xoft, Inc.

SU‐FF‐T‐380: Radiological Dependence of Electronic Brachytherapy Simulation On Input Parameters

Purpose: In comparison to 125I or 192Ir, characterization of dose rate distributions from electronic brachytherapy is subject to the additional challenge of unforeseen photon energy spectra. Towards simulating photon energy spectra and resultant dose rate distribution, Monte Carlo investigators first generate electrons which bombard the x‐ray tube anode and subsequently create photons via bremsstrahlung. Modeling techniques for this endeavor are largely unexplored. Therefore, sensitivities of spectra and dose rate distributions were assessed through varying modeling parameters for the Xoft Axxent x‐ray source. Materials & Methods: MCNP5 was used to simulate photonspectra and dose rate distributions, with comparisons to experimental measurements (PTW model 34013 chamber in liquid water) for 1<r⩽7 cm and 0°⩽θ⩽150° with simulations covering 0.3 ⩽r⩽ 15 cm and all available angles. The following source modeling parameters were evaluated for impact on in‐water spectra and dose:electron beam radius (R), electron beam annularity (R′) like a doughnut, and anode film thickness (t). Since simulations of electron:photon transport are inefficient in comparison to Monte Carlo modeling of radionuclides, MCNP variance reduction techniques such as cell importances (IMP), electron cutoff energies (PHYS:E), high‐energy biasing of bremsstrahlung spectrum (BBREM), and bremsstrahlung photon multiplicity (BNUM) were assessed. Results: Due to the complex anode shape, F(r,θ) was highly‐dependent on R, varying a factor of 2 when changing R from 0 to 0.084 cm. This effect was more pronounced when varying R′ due to less radial volume averaging. Through comparison with experimental measurements, the optimal electron beam shape had the largest spot size which could fit within the anode and no annularity; it was a uniform pencil beam. Altering MCNP variance reduction techniques did not significantly alter results, but greatly hastened simulation efficiency. Conflict of Interest: Research was sponsored in part by Xoft, Inc.

SU‐GG‐T‐36: Film Based Treatment Plan Validation for a New Vaginal Applicator Using the Xoft Axxent™ 50 KVp Miniature X‐Ray Source

Purpose: Compare delivered to planned dose for the Xoft Axxent™ vaginal applicator and 50 kVp x‐ray source using radiochromic film. Method and Materials: A 25mm diameter vaginal applicator (FDA clearance pending) was used to deliver a simulated treatment in a water phantom. The treatment was planned with Varian BrachyVision™, using the Xoft 50 kVp source TG‐43 parameters. The prescription dose was 7 Gy at 5mm from the applicator surface. The applicator and a 5″ square of GAFChromic EBT film were held in a Solid Water™ frame in a water phantom. The film plane was parallel to the long axis. The exposed film was scanned and processed to create a calibrated dose profile. The BrachyVision isodose‐line plot was transformed into an image with identical size and pixel density to the film then combined with the film image to create a new image with dose exposure values only along the planned isodose contours. These contours were analyzed to determine the variation in actual delivered dose along them. Results: Visual comparison of isodose contours and film image showed qualitatively good agreement of the delivered treatment with the plan. Further image processing quantified the agreement. An ad hoc film calibration was employed to estimate dose values along planned isodose contours, with emphasis on the prescription dose of 7 Gy. Thus absolute dose values averaged along a given contour were only approximately correct but the more germane variation of dose along each contour was found to be less than 8% (2 sigma) for dose contours from 1.75 to 8.75 Gy. Conclusion: Dose measured by film exposure in a plane parallel to the applicator axis was found to be constant along plan isodose contours with SD less than 8% (2 sigma). Conflict of Interest: Research supported by Xoft, Inc.

SU-G-TeP2-02: Characterization of An Improved X-Ray Source for Use with the Xoft Surface Applicator

PURPOSE To measure the depth dose distribution and half value layers of an updated x-ray source using the Xoft Axxent Surface Applicator. METHODS Modifications were made to the existing Axxent HDR X-ray source to increase the dose rate and improve the source life time. Measurements were made comparing dose distributions of this improved x-ray source and the existing Axxent HDR X-ray source. All sources were spatially characterized in a water tank prior to testing and each source passed all acceptance testing prior to measurement. Depth dose measurements were made using nine x-ray sources of each type through a 10 mm Surface Applicator using an ionization chamber at depths of 2, 5, and 10 mm in a Gammex Solid Water phantom, corrected for the ionization chamber collecting volume centroid and the use of a water-like phantom. Half value layers (HVLs) were measured for the family of surface applicators using five improved design sources. RESULTS The average normalized surface dose rate for the updated x-ray source is 1.65 Gy/min, in agreement with the existing x-ray source to within 2%. The average normalized dose rate at the other measured depths agrees to within 1%, with dose rates of 1.31 Gy/min, 0.90 Gy/min, and 0.52 Gy/min at 2 mm, 5 mm, and 10 mm respectively. First and second HVLs agree between the two x-ray source types to within standard errors for all Surface Applicators. First HVLs range from 1.45 to 1.58 mm Al and second HVLs range from 2.20 to 2.52 mm Al. CONCLUSION Depth dose distributions were measured for the existing Axxent X-ray source and an updated source through the Surface Applicator and shown to be equivalent. Half value layers for the new x-ray source are in agreement with those for the existing source. Funding provided by Xoft, a subsidiary of iCAD.

SU-E-T-301: Spectral Comparison of the Xoft and Zeiss 50 KVp X-Ray Systems

PURPOSE To compare x-ray spectra of the 50 kVp Xoft Axxent™ and Zeiss INTRABEAM™ x-ray sources after filtration by saline-filled balloons applicators or spherical polymer applicators, respectively. METHODS Measurements were made for 3.5, 4.0 and 5.0 cm diameter applicators using an AmpTek model XR-100T-CdTe cadmium telluride spectrometer with 100 μm diameter collimating aperture and model PX4 digital pulse processor. Spectra were then corrected for escape processes using AmpTek XRF-FP Escape software. Both Axxent and INTRABEAM sources were operated at 50 kV and 40 μA to eliminate pulse saturation. The balloon or spherical applicator was placed in a centering fixture in contact with the collimator cap. The distance through the collimator housing from the applicator surface to the spectrometers beryllium entrance window was nominally 52mm. Approximately 500,000 counts were collected for each spectrum. RESULTS Measured spectra in all cases had a broad Bremsstrahlung continuum with subtle differences in characteristic low energy x-rays lines from the different materials used for the anode thin films and substrates. After corrections for escape events average energies were calculated for spectra emerging from applicators. The average energies were 28.2 ±0.3 keV, 29.0±0.7 keV, and 31.7±0.9 keV for the 3.5, 4.0 and 5.0 cm diameter applicators, respectively. Differences in average energies ranged from 2.0 to 5.6% for these diameters. The mean energies of the spectra are more dependent on balloon size than on the delivery system used. CONCLUSION Energy spectra at the surfaces of 3.5, 4.0 and 5.0 cm diameter applicators were measured for the Axxent and INTRABEAM x-ray systems were using a Cd-Te spectrometer. The average energies of the two x-ray systems for comparable applicator sizes were within 5.6%, and as little as 0.6 keV difference for the smaller applicator size. Research sponsored by Xoft, a subsidiary of ICAD.

SU‐E‐T‐454: Comparative Dosimetry of the Xoft Cervical Applicator and HDR Ir‐192 Henschke Applicator

PURPOSE To evaluate HDR dosimetry for the Xoft Axxent ™cervical tandem and ovoid (T&O) applicator using the 50-kV electronic brachytherapy (eBx) source. The titanium wall of this Henschke-style applicator filters lower energies which flattens the radial dose function. Dose distributions around the T&O for 50-kV-Ti were investigated with comparison to Ir-192. METHODS TG-43u parameters were derived from film measurements of the 50-kV-Ti. To account for varying filtration effects on anisotropy from tip to barrel in the tandem, a mixed source model using the anisotropy at two dwell positions was implemented. Stylized geometry of cervical PTV, rectum and bladder were created on a CT scan of a gelatin phantom with the 15-degree T&O. Plans for each source were optimized on BrachyVision™ using identical constraints and normalization. The GMP-192 source was used for the Ir-192 plans. RESULTS Both plans were normalized at 6-Gy (D95) and have classic pear-shaped doses. Doses at Points A-H are within 5% for each plan, but are 20% lower for 50-kV-Ti at Point B. Along the tandem length peak Ir-192 doses are 50% higher. The Ir-192 dose dip at the tandem tip is absent for 50-kV-Ti. PTV DVHs are similar except for the effect of the peak tandem surface doses. PTV coverage (D95) for both is 6 Gy (+/- 1%) and the median dose for both was 950 cGy (+/- 2%). The peak dose region (D5) is 14% lower for 50-kV-Ti (18.1 Gy vs 20.7 Gy). D5 of rectum and bladder are 13% and 20% respectively for 50-kV-Ti and Ir-192. Median doses to critical structures are 18-20% lower for 50-kV-Ti. CONCLUSION PTV doses for Ir-192 HDR and 50-kV-Ti eBx have similar target coverage, except at the tandem surface where doses were lower for eBx. Critical organs dose outside the PTV will be lower for eBx. Funding for this study was providing by Xoft, Inc - an iCad subsidiary.