Purpose: Guide wires with high torquability and steerability are commonly used to navigate through a tortuous and/or branching arterial tree in a catheter-based intravascular brachytherapy procedure. The dosimetric effects due to the presence of metallic guide wires have not been addressed. This work investigates these dose effects for the three most commonly used beta and gamma sources (90Sr, 32P, and 192Ir).
Methods and materials: The EGS4 Monte Carlo codes were used to calculate the dose distributions for the 90Sr(NOVOSTE), 32P (Guidant), and 192Ir (BEST Ind.) with and without a guide wire in place. Energy spectra for particles exiting the sources were calculated from the full phase-space data obtained from the Monte Carlo simulations of the source constructions. Guide wires of various thicknesses and compositions were studied.
Results: The dose perturbations due to the presence of guide wires were found to be far more significant for the 90Sr/90Y and 32P beta sources than those for the 192Ir gamma source. Because of the attenuation by the guide wires, a dose reduction of up to 60% behind a guide wire was observed for the beta sources, whereas the dose perturbation was found to be negligible for the gamma source. For a beta source, the dose perturbations depend on the thickness and the material of the guide wire. When the region behind a guide wire is part of an intravascular brachytherapy target, the presence of the guide wire results in a significant underdosing for beta sources. The underdosed region can extend a few mm behind the guide wire and up to 1 mm in other directions.
Conclusion: Significant dose perturbations by the presence of a metallic guide wire have been found in catheter-based intravascular brachytherapy using beta sources. The dose effects should be considered in the dose prescription and/or in analyzing the treatment outcome for beta sources. Such precautions are not necessary if using a gamma source.