Experimental validation of dose calculation algorithms for the GliaSite (TM) RTS, a Novel I-125 liquid-filled balloon brachytherapy applicator

This paper compares experimentally measured and calculated dose-rate distri butions for a novel I-125 liquid-filled brachytherapy balloon applicator (t he GliaSite RTS), designed for the treatment of malignant brain-tumor resec tion-cavity margins. This work is intended to comply with the American Asso ciation of Physicists in Medicine (AAPM) Radiation Therapy Committee's reco mmendations [Med. Phys. 25, 2269-2270 (1998)] for dosimetric characterizati on of low-energy photon interstitial brachytherapy sources. Absolute low do se-rate radiochromic film (RCF) dosimetry measurements were performed in co ronal planes about the applicator. The applicator was placed in a solid wat er phantom, machined to conform to the inflated applicator's surface. The r esults were used to validate the accuracy of Monte Carlo photon transport ( MCPT) simulations and a point-source dose-kernel algorithm in predicting do se to water. The absolute activity of the I-125 solution was determined by intercomparing a National Institute of Standards and Technology (NIST) I-12 5 standard with a known mass of radiotherapy solution (Iotrex(TM)) in an id entical vial and geometry. For the two films not in contact with applicator , the average agreement between RCF and MCPT (specified as the mean absolut e deviation in successive 4 mm rings) was found to be within +/-5% at dista nces 0.2-25 mm from the film centers. For the two films touching the cathet er, the mean agreement was +/-14.5% and 7.5% near the balloon surface but i mproving to 7.5% and 6% by 3.5 mm from the surface. These errors, as large as 20% in isolated pixels, are likely due to trim damage, I-125 contaminati on, and poor conformance with the balloon. At larger distances where the ra diation doses were very low, the observed discrepancies were significantly larger than expected. We hypothesize that they are due to a dose-rate depen dence of the RCF response. A 1%-10% average difference between a simple one -dimensional path-length semiempirical dose-kernel model and the MCPT calcu lations was observed over clinically relevant distances. (C) 2001 American Association of Physicists in Medicine.