Monte Carlo characterization of a 32P source for intravascular brachytherapy.

In this work, we have calculated the two-dimensional dose distribution in water for a 32P intravascular brachytherapy source wire using the EGSnrc Monte Carlo code. The beta source (Guidant Vascular Intervention) has a radioactive core with a length of 27 mm and a diameter of 0.24 mm. The dose parameters required by the AAPM TG-60 formalism are discussed and calculated. Dose rate evaluated at the reference point is 0.1311+/-0.0001 Gy min(-1) mCi(-1). For the beta source studied, the dose distribution is uniform along the axial direction z for a given radial position p for - 10 mm< or =z< or =10 mm and p< or =7 mm. In such a dose-uniformity region, the dose field can be characterized by one-dimensional dose distribution, D(p), the dose distribution on the transverse axis. Beyond this region a two-dimensional (2D) description is necessary. However, for the long beta source wire the anisotropy function proposed by the TG-60 formalism becomes indefinable when the radial distance exceeds penetration depth of beta electrons. We have proposed that the anisotropy function be expressed in the cylindrical coordinate system, instead of a polar system, to remedy this deficiency. For practical purposes, the entire 2D dose distribution and the dose parameters calculated in the work are tabulated for ease of use.