Development and characterization of a prototype selenium-75 high dose rate brachytherapy source

Background

⁷⁵Se ($t_{1/2}$ $\approx$120 days, $E_{\gamma ,\text{avg}}$$\approx$215 keV) offers advantages over ¹⁹²Ir ($t_{1/2}$ $\approx$74 days, $E_{\gamma ,\text{avg}}$$\approx$360 keV) as a high dose rate brachytherapy source due to its lower gamma energy and longer half-life. Despite its widespread use in industrial gamma radiography, a ⁷⁵Se brachytherapy source has yet to be manufactured.

Purpose

A novel ⁷⁵Se-based source design with a vanadium diselenide core, titled the SeCure source, was proposed. This study aimed to evaluate the feasibility of this source design for dosimetry and manufacturability purposes and to develop an activated prototype source.

Methods

The source was modeled and integrated into the Monte Carlo-based treatment planning system RapidBrachyMCTPS, where its TG-43U1 parameters, photon spectrum, and broad beam first half-value layers (HVL₁) and tenth-value layers (TVL₁) in lead, tungsten, and concrete were calculated. A prototype source was manufactured, and the vanadium diselenide content of the capsule was verified with neutron radiography. The source was then activated to a nominal activity of $8.5\pm 0.9$ mCi at the McMaster Nuclear Reactor. The activity was measured with two separate dose calibrators. Gamma spectroscopy was used to characterize any activated radioactive contaminants in the source, and wipe testing was performed to check for any leakage of ⁷⁵Se from the encapsulation.

Results

The SeCure source's TG-43U1 parameters were computed, showing that $2.056\pm 0.003$ times the activity of ⁷⁵Se is required relative to ¹⁹²Ir to achieve the same dose rate in water at (1 cm, 90${}^{\circ }$). The mean spectral energy of the source is $214.695\pm 0.005$ keV, resulting in reduced first half-value and tenth-value layers relative to ¹⁹²Ir in attenuating materials. For example, the ${\mathrm{HVL}}_1$ was reduced from $2.795\pm 0.002$ mm to $1.020\pm 0.001$ mm in lead, from $2.049\pm 0.002$ mm to $0.752\pm 0.001$ mm in tungsten, and from $70.63\pm 0.04$ mm to $61.37\pm 0.03$ mm in concrete. The activated source achieved the desired activity, indicated as $9.2\pm 0.2$ mCi and $8.5\pm 0.9$ mCi at the end of irradiation on the two dose calibrators. All identified radionuclide contaminants decaying below $0.1\%$ of the ⁷⁵Se activity after 5 days post-irradiation. Wipe testing only identified radioactive contaminants present in activated titanium, with only $1.24\pm 0.01\times {10}^{-7}$ mCi of ${}^{24}\mathrm{Na}$ detected 72 h post-irradiation, indicating that the integrity of the encapsulation was maintained.

Conclusions

The SeCure design possesses the dosimetric, spectral, and physical properties necessary for a feasible high dose rate brachytherapy source. Next, manufacturing of a high-activity SeCure source will be pursued.