Feasibility of a Tungsten-181 HDR brachytherapy source: Analysis of isotope production and source dosimetry

Interest in Intensity Modulated Brachytherapy (IMBT) for High Dose Rate Brachytherapy (HDR) treatments has steadily increased in recent years. However, intensity modulation is not best optimized for currently used HDR sources since they emit high energy photons. To that end, the focus on IMBT has moved to middle energy sources, such as Ytterbium-169; yet even Yb-169 emits some high energy photons at a low yield. We present an alternative isotope, Tungsten-181 (T_1/2 = 121 days) that is interesting due to its complete lack of high energy photon emissions. (E_avg = 58.9 keV, E_med = 57.5 keV) making it potentially favorable as high dose rate brachytherapy source from both a medical physics and health physics perspective.

The purpose of this study was to determine the feasibility of using W-181 as an HDR brachytherapy source; in this study we focused on W-181's production, dosimetric properties, and intensity modulation capabilities.

We determined the isotope production kinetics, its Dose Rate Constant, Radial Dose Function, photon self-absorption, and the shielding intensity modulation capabilities for a W-181 pellet source geometry using the MCNP6.2 Computer Simulations Code. All simulations were performed using a personal computer running an AMD Ryzen 5 3600 6-Core Processor 3.59 GHz. The number of histories run for each study were selected to produce relative simulation convergence errors in the MCNP tally output of less than 2%. Dosimetric calculations were made using the MCNP6.2 computer simulations code and activation analyses were determined mathematically using a Catenary kinetics analysis (also known as a Bateman Analysis) of W-181 and Tantlum-182 production from the neutron activation of a pure Tungsten-180 stable target. Since W-181 emits middle energy photons and has a high density, we also assessed the effects of photon self-absorption within a tungsten pellet.

From our analysis, we determined that a 3.5 mm long and 0.6 mm in diameter is feasible for clinical applications. Our activation analyses found that these pellets can achieve W-181 activities up to 7.9Ci and 13Ci using neutron fluence rates of 4E14 cm^-2 s^-1 and 1E15 cm^-2 s^-1 respectively, which then would provide a dose rate of 1.84 ± 0.01 cG y/Ci/min at a depth of 1 cm from the source. Using our resulting Monte Carlo simulated Dose Rate Constant of 1.24 ± 0.02 cGy h^-1∙U^-1, a W-181 source in this geometry would require a source activity upwards of 10Ci for use in HDR treatments. In the intensity modulation analysis, only 0.1 mm of gold shielding was found to reduce a pellet's absorbed dose by over 50% while 0.3 mm of gold shielding, which is thin enough to theoretically fit between an HDR pellet and the inner catheter wall, was found to reduce the pellet's absorbed dose by over 85%.

While W-181 has a lower specific activity than Ir-192 and Yb-169, it shows great promise as an isotope for use in Intensity Modulated Brachytherapy due to its easily shielded photons. We therefore expect that W-181 may lend itself best for use as a multi-pellet configuration in IMBT.