F. Mourtada1,2, C. G. Soares1, S. M. Seltzer
1 - S. H. Lott3, R. Colle1 1National Institute of Standards and Technology Gaithersburg, MD 20899
2 - Department of Radiology, School of Medicine, Johns Hopkins University, Baltimore, MD 20874 3 Guidant Corporation, Houston, Texas 77054
Direct dosimetry measurements in water are described for a catheter-based
32P endovascular brachytherapy wire. The measurements were obtained using
radiochromic dye film, an automated plastic scintillator. The investigated
source has dimensions of 27 mm in length and 0.3-mm in diameter, embedded
in the end of Ni-Ti wire. For the radiochromic film measurements, MD55-2
film was irradiated at distances between 1-5 mm in water, and read out with
a high-resolution scanning densitometer. For the scintillator system, measurements
in water were acquired at distances between 1-6 mm from the center of the
source, along the perpendicular bisector of the source axis. The scintillator
was calibrated in terms of absorbed-dose rate in a reference beta particle
field at multiple depths. Theoretical dosimetry calculations of the catheter-based
32P wire geometry were obtained from Monte Carlo simulations using the Electron
Gamma Shower code (EGS4), the Monte Carlo N-Particle transport code (MCNP4B),
and CYLTRAN from the Integrated Tiger Series codes (ITS v.3). The results
of both measurements and calculations are expressed in absorbed-dose rate
per unit of contained activity. Comparisons indicate that the Monte Carlo
simulations are in excellent agreement (<3%), but due to source non-uniformity,
the plastic scintillator and film dose rates are 20% and 28% lower than
the theoretical estimates. This work is underway to develop a novel approach
to fully characterize the radiation field around a brachytherapy source
in water. The water calibration provides a direct method for producing accurate
dosimetry for vascular irradiation at the clinic.