Characterization and real-time optical measurements of the ionizing radiation dose response for a new radiochromic medium Alexandra Rink, I. Alex Vitkin, and David A. Jaffray Princess Margaret Hospital/Ontario Cancer Institute, Department of Medical Biophysics and Radiation Oncology, University of Toronto, Toronto, Ontario Canada Received 3 March 2005; revised 18 May 2005; accepted for publication 24 May 2005; published 18 July 2005 A new radiochromic film, GafChromic EBT, was investigated for use in a real-time radiation dosimetry system. It was found to be approximately eight times more sensitive to ionizing radiation dose, exhibited less postexposure development and achieved stable readout faster than one of its predecessors, GafChromic MD-55. A clear distinction in change in optical density between expo- sure and postexposure was observed, but the measurements obtained during exposure were not linear with time or dose. This could not be explained by a shift in wavelength of maximum change in absorbance, as it was stable at 636 nm during the entire exposure range up to 9.52 Gy. Increasing the spectral window of interest over which calculations were performed did little to correct the nonlinearity. The radiochromic film exhibited small dose rate dependence in real-time measurements, with an increase in standard deviation of change in optical density measurements from 0.9% to 1.8% over a sixfold variation in dose rate. Overall, GafChromic EBT has increased sensitivity and decreased postexposure darkening, and this bodes well for its potential role as a radiation dosimeter, including real-time applications. © 2005 American Association of Physicists in Medicine. DOI: 10.1118/1.1951447 I. INTRODUCTION Several radiochromic films, manufactured by International Specialty Products ISP, Wayne, NJ for dosimetry purposes, have been studied for more than a decade, 1–3 often for two- dimensional 2D radiation dose verification, 4,5 and thorough reviews have been published. 6,7 In general, a dose of 1 Gy or more is recommended for accurate dose measurement, 8 pre- venting use of these films for low-dose studies. Radiochro- mic substances have also recently been investigated in real- time dosimetry systems. 7 In these studies, GafChromic® MD-55 performed reasonably well whereas some issues re- mained unresolved. GafChromic® EBT film ISP was cre- ated for use in external beam dose verification, and is adver- tised by the manufacturer to be more sensitive than both GafChromic® HS and MD-55 films referred to as EBT and MD-55 from here on, respectively. This makes it potentially useful for low dose verification, such as doses delivered in each intensity-modulated radiation segment. It is also pro- posed as an improvement for some of the real-time dosime- try issues, including stability of wavelength of maximum ab- sorbance  max , 9 and decreased extent of postexposure darkening. The radiochromic material used in EBT film appears to be a reasonable candidate for use as an optical media for real- time in vivo dosimetry, as per criteria previously listed. 7 It addresses the requirement for a small radiation sensitive vol- ume with improved water-equivalency compared to its pre- decessor MD-55, increased sensitivity to low doses, and faster polymerization kinetics, resulting in a stable response shortly after exposure ISP product information. To deter- mine the suitability of EBT to real-time in vivo dosimetry and verify some of the above-mentioned claims, an experi- mental setup and method previously used in real-time inves- tigations of MD-55 film was employed. 7 In this technical report investigations of linearity, stability, and sensitivity are described. II. METHODS AND MATERIALS The experimental setup described previously 7 was modi- fied by removing the beam splitter Fig. 1a and using a light emitting diode HLMP-ED25-TW00, Agilent Technolo- gies, Palo Alto, CA with a 633 nm emission peak Fig. 1b, chosen to interrogate the greater of the two absorbance peaks of the EBT film. The EBT film used in these investi- gations Lot No. 34098-8  2W consisted of five layers Fig. 2, ISP product information where the clear polyester was assumed to be Mylar™ as employed in the MD-55 film. 2 The system operated with 13.55± 0.01 mA power supply driving light output from the light emitting diode. A dark spectrum I D  and a reference spectrum I R  were collected prior to each radiation exposure. The I R at a given wave- length is proportional to the radiant power of the light trans- mitted through the unirradiated piece of EBT film measured over 0.33 mm 2 . 7 Irradiation of the film was initiated shortly typically 3s after starting the collection of the sample spectra I s . The spectrometer integration time ranged from 8 to 10 ms, depending on the study performed, and each spectrum was recorded. The change in absorbance A, at any measurement interval, for each wavelength was calculated using A log 10  I R  - I D  I S  - I D   . 1 The EBT film likely undergoes interactions with interrogat- ing light other than absorption. However, the optical scatter- 2510 2510 Med. Phys. 32 „8…, August 2005 0094-2405/2005/32„8…/2510/7/$22.50 © 2005 Am. Assoc. Phys. Med.