The use of new GAFCHROMIC® EBT film for 125I seed dosimetry in Solid Water® phantom

Radiochromic film dosimetry has been extensively used for intravascular brachytherapy applications for near field within 1 cm from the sources. With the recent introduction of new model of radiochromic films, GAFCHROMIC (R) EBT, with higher sensitivity than earlier models, it is promising to extend the distances out to 5 cm for low dose rate (LDR) source dosimetry. In this study, the use of new model GAFCHROMIC (R) EBT film for I-125 seed dosimetry in Solid Water (R) was evaluated for radial distances from 0.06 cm out to 5 cm. A multiple film technique was employed for four I-125 seeds (Implant Sciences model 3500) with NIST traceable air kerma strengths. Each experimental film was positioned in contact with a I-125 seed in a Solid Water (R) phantom. The products of the air kerma strength and exposure time ranged from 8 to 3158 U-h, with the initial air kerma strength of 6 U in a series of 25 experiments. A set of 25 calibration films each was sequentially exposed to one I-125 seed at about 0.58 cm distance for doses from 0.1 to 33 Gy. A CCD camera based microdensitometer, with interchangeable green (520 nm) and red (665 nm) light boxes, was used to scan all the films with 0.2 mm pixel resolution. The dose to each I-125 calibration film center was calculated using the air kerma strength of the seed (incorporating decay), exposure time, distance from seed center to film center, and TG43U1S1 recommended dosimetric parameters. Based on the established calibration curve, dose conversion from net optical density was achieved for each light source. The dose rate constant was determined as 0.991 cGy U-1 h(-1) (+/- 6.9%) and 1.014 cGy U-1 h(-1) (+/- 6.8%) from films scanned using green and red light sources, respectively. The difference between these two values was within the uncertainty of the measurement. Radial dose function and 2D anisotropy function were also determined. The results obtained using the two light sources corroborated each other. We found good agreement with the TG43U1S1 recommended values of radial dose function and 2D anisotropy function, to within the uncertainty of the measurement. We also verified the dosimetric parameters in the near field calculated by Rivard using Monte Carlo method. The radial dose function values in Solid Water (R) were lower than those in water recommended by TG43U1S1, by about 2%, 3%, 7%, and 14% at 2, 3, 4, and 5 cm, respectively, partially due to the difference in the phantom material composition. Radiochromic film dosimetry using GAFCHROMIC (R) EBT model is feasible in determining 2D dose distributions around low dose rate I-125 seed. It is a viable alternative to TLD dosimetry for I-125 seed dose characterization. (c) 2008 American Association of Physicists in Medicine.