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Tungsten-181 is a middle energy radioisotope with an average energy of E avg =64.3keV and a half-life of T 1/2 =140d that holds promise as a new HDR brachytherapy source. This is because, except for its two highest energy photons which have energies of 136 keV (yield: 0.0311%) and 156 keV (yield: 0.0083%), this source is dominated by photons which lie between 56 keV to 67 keV. Such photons easily are absorbed by high Z materials but scatter readily in water. This study will investigate the treatment needs and intensity modulation capabilities of W-181 and compare these results to those of Iridium-192 and Ytterbium-169. Simulations were conducted using the Monte Carlo N-Particle Transport Code (MCNP6.2) to calculate the Dose Rate Constant, Radial Dose Function, photon self-absorption, and treatment activity of a 3.5mm long and 0.6mm diameter pellet encapsulated in stainless steel. We finally evaluated the intensity modulation capabilities of this pellet and compared it to Ir-192 and Yb-169. Our W-181 pellet had a Dose Rate Constant of 1.01 ± 0.01cGy∙h-1∙U-1 and a Radial Dose Function, which was fit to a 5th polynomial function to obtain the following coefficients: a 0 =9.01E-1 a 1 =8.60E-2 a 2 =2.96E-2 a 3 =-1.05E-2 a 4 =1.00E-3 a 5 =-3.00E-5. With respect to shielding and intensity modulation, 0.3mm of gold shielding reduced W-181's absorbed dose by 86%, Yb-169′s dose by 62%, and Ir-192's dose by 15%. While this isotope therefore can be an excellent candidate for IMBT, we also found that it is limited by a high photon self-absorption due to tungsten's very high density (19.3g/cm3) and atomic number (74). Specifically, we found that a W-181 pellet would provide an absorbed dose rate per unit activity of 1.84 ± 0.01cGy∙Ci-1min-1 to a treatment area 1cm from the source as opposed to Ir-192 and Yb-169's 31.0 ± 0.37 cGy∙Ci-1∙min-1 and 8.18 ± 0.11 cGy∙Ci-1∙min-1, respectively. A W-181 therapeutic source therefore would require a higher treatment activity than a Yb-169 or Ir-192 source. The capabilities of W-181 for intensity modulation outperforms that of Ir-192 and even Yb-169. Given these results, W-181 shows promise as a brachytherapy source, especially in multi-pellet configurations. [ABSTRACT FROM AUTHOR]

Purpose: A novel 169Yb low dose rate permanent implant brachytherapy source, the GammaClip™, was developed by Source Production & Equipment Co. (New Orleans, LA) which is designed similar to a surgical staple while delivering therapeutic radiation. In this report, the brachytherapy source was characterized in terms of 'Dose calculation for photon-emitting brachytherapy sources with average energy higher than 50 keV: Report of the AAPM and ESTRO' by Perez-Calatayud et al. [Med. Phys. 39, 2904-2929 (2012)] using the updated AAPM Task Group Report No. 43 formalism. Methods: Monte Carlo calculations were performed using Monte Carlo N-Particle 5, version 1.6 in water and air, the in-air photon spectrum filtered to remove photon energies below 10 keV in accordance with TG-43U1 recommendations and previously reviewed 169Yb energy cutoff levels [D. C. Medich, M. A. Tries, and J. M. Munro, 'Monte Carlo characterization of an Ytterbium-169 high dose rate brachytherapy source with analysis of statistical uncertainty,' Med. Phys. 33, 163-172 (2006)]. TG-43U1 dosimetric data, including SK, D(r,θ), Λ, gL(r), F(r, θ), [lowercase_phi_synonym]an(r), and [lowercase_phi_synonym]an were calculated along with their statistical uncertainties. Since the source is not axially symmetric, an additional set of calculations were performed to assess the resulting axial anisotropy. Results: The brachytherapy source's dose rate constant was calculated to be (1.22 ± 0.03) cGy h-1 U-1. The uncertainty in the dose to water calculations, D(r,θ), was determined to be 2.5%, dominated by the uncertainties in the cross sections. The anisotropy constant, [lowercase_phi_synonym]an, was calculated to be 0.960 ± 0.011 and was obtained by integrating the anisotropy factor between 1 and 10 cm using a weighting factor proportional to r-2. The radial dose function was calculated at distances between 0.5 and 12 cm, with a maximum value of 1.20 at 5.15 ± 0.03 cm. Radial dose values were fit to a fifth order polynomial and dual exponential regression. Since the source is not axially symmetric, angular Monte Carlo calculations were performed at 1 cm which determined that the maximum azimuthal anisotropy was less than 8%. Conclusions: With a higher photon energy, shorter half-life and higher initial dose rate 169Yb is an interesting alternative to 125I for the treatment of nonsmall cell lung cancer. [ABSTRACT FROM AUTHOR]

Purpose: Absorbed dose energy correction factors, used to convert the absorbed dose deposited in a LiF thermoluminescent dosimeter (TLD) into the clinically relevant absorbed dose to water, were obtained for both spherical volumetric sources and for the model 4140 HDR Yb-169 source. These correction factors have a strong energy dependence below 200 keV; therefore, spectral changes were quantified as Yb-169 photons traveled through both source material (Yb2O3) and water with the corresponding absorbed dose energy correction factors, f(r,θ), calculated as a function of location in a phantom. Methods: Using the MCNP5 Monte Carlo radiation transport simulation program, the Yb-169 spectrum emerging from spherical Yb2O3 sources (density 6.9 g/cm3) with radii between 0.2 and 0.9 mm were analyzed and their behavior compared against those for a point-source. The absorbed dose deposited to both LiF and H2O materials was analyzed at phantom depths of 0.1–10 cm for each source radius and the absorbed dose energy correction factor calculated as the ratio of the absorbed dose to water to that of LiF. Absorbed dose energy correction factors for the Model 4140 Yb-169 HDR brachytherapy source similarly were obtained and compared against those calculated for the Model M-19 Ir-192 HDR source. Results: The Yb-169 average spectral energy, emerging from Yb2O3 spherical sources 0.2–0.9 mm in radius, was observed to harden from 7% to 29%; as these photons traveled through the water phantom, the photon average energy softened by as much as 28% at a depth of 10 cm. Spectral softening was dependent on the measurement depth in the phantom. Energy correction factors were found to vary both as a function of source radius and phantom depth by as much as 10% for spherical Yb2O3 sources. The Model 4140 Yb-169 energy correction factors depended on both phantom depth and reference angle and were found to vary by more than 10% between depths of 1 and 10 cm and angles of 0° and 180°. This was in contrast to that of the Model M-19 Ir-192 source which exhibited approximately 3.5%–4.4% variation in its energy correction factors from phantom depths of 0.5–10 cm. The absorbed dose energy correction factor for the Ir-192 source, on the other hand, was independent of angle to within 1%. Conclusions: The application of a single energy correction factor for Yb-169 TLD based dosimetry would introduce a high degree of measurement uncertainty that may not be reasonable for the clinical characterization of a brachytherapy source; rather, an absorbed dose energy correction function will need to be developed for these sources. This correction function should be specific to each source model, type of TLD used, and to the experimental setup to obtain accurate and precise dosimetric measurements. [ABSTRACT FROM AUTHOR]

Abstract: Purpose: To describe a novel source–delivery system for intraoperative brachytherapy in patients with early-stage lung cancer that is readily adaptable to a video-assisted thoracoscopic surgery approach and can be precisely delivered to achieve optimal dose distribution. Methods and Materials: Radioactive ytterbium-169 (169Yb) was sealed within a titanium tube 0.28mm in diameter and then capped and resealed by titanium wires laser welded to the tube to serve as the legs of a tissue-fastening system. Dose simulations were performed using Monte Carlo computer code (Los Alamos National Laboratory, Los Alamos, NM) to mimic the geometric and elemental compositions of the source, fastening apparatus, and surroundings. Results: Five test source capsules were subjected to a tensile load to failure. Failure in each capsule occurred in the wire of the fastener leg; there were no weld failures. Monte Carlo simulations and subsequent dose measurement showed the perturbation by the source legs in the deployed (bent over) position to be small (4–5%) for 169Yb and much less than that for iodine-125 (32%). Conclusion: We have developed a 169Yb brachytherapy source–delivery system that can be used in conjunction with commercially available surgical stapling instruments, facilitates the precise placement of brachytherapy sources relative to the surgical margin, assures the seeds remain fixed in their precise position for the duration of the treatment, overcomes the technical difficulties of manipulating the seeds through the narrow surgical incision associated with video-assisted thoracoscopic surgery, and reduces the radiation dose to the clinicians. [Copyright &y& Elsevier]