The use of new GAFCHROMIC ® EBT film for 125 I seed dosimetry in Solid Water ® phantom Sou-Tung Chiu-Tsao Quality MediPhys LLC, 17 Jade Lane, Denville, New Jersey 07834 David Medich University of Massachusetts Lowell, Lowell, Massachusetts 01854 John Munro III Source Production & Equipment Co., Inc., 113 Teal Street, St. Rose, Louisiana 70087 Received 4 February 2008; revised 12 May 2008; accepted for publication 17 June 2008; published 24 July 2008 Radiochromic film dosimetry has been extensively used for intravascular brachytherapy applica- tions for near field within 1 cm from the sources. With the recent introduction of new model of radiochromic films, GAFCHROMIC ® EBT, with higher sensitivity than earlier models, it is prom- ising to extend the distances out to 5 cm for low dose rate LDR source dosimetry. In this study, the use of new model GAFCHROMIC ® EBT film for 125 I seed dosimetry in Solid Water ® was evaluated for radial distances from 0.06 cm out to 5 cm. A multiple film technique was employed for four 125 I seeds Implant Sciences model 3500 with NIST traceable air kerma strengths. Each experimental film was positioned in contact with a 125 I seed in a Solid Water ® 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 125 I 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 125 I 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 measure- ment. 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 ® 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 ® EBT model is feasible in determining 2D dose distributions around low dose rate 125 I seed. It is a viable alternative to TLD dosimetry for 125 I seed dose characterization. © 2008 American Association of Physicists in Medicine. DOI: 10.1118/1.2955746 Key words: radiochromic film dosimetry, near field, 125 I seed, TG-43 parameters, brachytherapy I. INTRODUCTION Since the introduction of radiochromic films to the radiation therapy community, earlier models like GAFCHROMIC ® HD810, MD55, and HS have been used extensively in two- dimensional 2D dose measurement for conventional and intravascular brachytherapy sources, as well as external beam and radiosurgery treatment. 1–17 However, due to the low sensitivity of these earlier models, accurate dosimetry was limited to near field within 1 cm from brachytherapy sources. 4–8 A new model of radiochromic film, GAFCHROMIC ® EBT, became available in 2004. Advantages of EBT film over earlier models include 1 higher sensitivity 18,19 by about an order of magnitude, 2 weak energy dependence over a wide range of energies from megavoltage down to kilovoltage radiation sources, 20,21 3 improved film unifor- 3787 3787 Med. Phys. 35 „8…, August 2008 0094-2405/2008/35„8…/3787/13/$23.00 © 2008 Am. Assoc. Phys. Med.