PO125: Feasibility of a Tungsten-181 Source for Use in High Dose-Rate Brachytherapy.

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]