(Dr. Chris Wang, advisor)

"Verification of TLD/MCNP Depth Dose Distribution of a ¹⁰³ Pd IVBT Source Using Radiochromic Film"

Abstract

A dosimetry system is being developed to determine the depth-dose distribution of a palladium-103 (¹⁰³ Pd) intravascular brachytherapy (IVBT) source, which is used to treat restenosis after percutaneous transluminal coronary angioplasty (PTCA) and restenosis in pereferial arteries.  The system determines the depth-dose distribution via three steps: (1) exposing a set of GafChromic films (MD-55-2), sandwiched in a Solid Water phantom and at various distances (2, 5, 10, 15, and 20 mm) from the center line, (2) reading the optical densities (OD) of the exposed films, and (3) converting ODs to absorbed doses (D).  The major part of the project is to establish the correlation of OD vs. D_water so that step (3) above can be implemented accurately.

 The absorbed doses at various distances from source to centerline were determined both by experiments and by calculations.  The experimental setup includes exposing a large number of thermoluminescent dosimeters (TLD-100 microcubes) embedded at various distances from the source centerline in a Water Solid phantom.  The size of a TLD-100 microcube is 1mm x 1mm x 1mm.  The response of the TLDs for low energy photons from a ¹⁰³ Pd was previously calibrated with ion chambers.  The calculations of absorbed doses in the phantom were performed using MCNP code.  An excellent agreement (within 5%) has been achieved between the TLD results and the MCNP results.  This agreement allows us to trust the absorbed dose of GafChromic film (D_film ) and of water (D_water) calculated by MCNP.  The MCNP-calculated depth-dose distribution is then used to establish the correlation of OD vs. D_water for the GafChromic films.

 Because there are several experimental and computational steps involved in this dosimetry system, the error propagation can be significant.  We estimate the uncertainty of the absorbed dose determined by this system to be less than 18%.  The major contributor to the uncertainty is the uniformity of the source.