Ytterbium-169: A promising new radionuclide for intravascular brachytherapy

Purpose: To explore the feasibility of ¹⁶⁹Yb (γ, 93 keV) as a new radionuclide for intravascular brachytherapy (IVBT) in terms of dose distribution, penetration power, and radiation safety features as compared with ¹²⁵I and ¹⁹²Ir. Methods: The dose distributions for catheter-based sources, ¹⁶⁹Yb, ¹²⁵I, and ¹⁹²Ir, in homogeneous water and in the presence of calcium and a steel stent have been determined and compared using the Monte Carlo method (MCNP4B2 code). The dose rates of the sources were evaluated from 0.02 to 100 cm. Results: In the short distance range (0.02<r<1.0 cm), the dose distributions in homogeneous water are very similar for the three radionuclides when the dose rates are normalized at 2 mm. Between 1 and 20 cm, the relative dose rates fall off similarly for ¹⁶⁹Yb and ¹⁹²Ir, whereas for ¹²⁵I, it decreases much more rapidly. At a distance further away (r∼100 cm), the dose rate of ¹⁶⁹Yb is about 10 times lower than that of ¹⁹²Ir, indicating the cathlab radiation shielding requirement for ¹⁶⁹Yb is substantially reduced as compared with ¹⁹²Ir. Calcified plaques and stents cause a drastic dose reduction in the arterial wall for ¹²⁵I, but have no effect for ¹⁹²Ir γ-rays. Only slight dose reductions were detected for ¹⁶⁹Yb beyond a layer of 1.0-mm calcium (2-3%), and behind a steel stent strut (5%). Conclusion: ¹⁶⁹Yb is a promising new radionuclide for IVBT. It has a much better penetrating power through calcified plaques and stents compared with the low-energy source ¹²⁵I. It also provides easier radiation protection measures for cardiac cathlab personnel than the high-energy source ¹⁹²Ir, while preserving a favorable dose distribution in tissues surrounding an arterial vessel.