GAFCHROMIC® XR Radiochromic Dosimetry Films For Low Energy Photons

Configuration And Specifications

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Description

GAFCHROMIC® XR radiochromic dosimetry films have been developed specifically for the measurement of absorbed dose of low energy photons. In this regard, the sensitivity of these films to photons with energies less than 200keV is substantially greater than previous dosimetry films such as GAFCHROMIC® HD-810 and GAFCHROMIC® MD-55. A principle goal in formulating GAFCHROMIC® XR was to provide dosimetry film media that could be used to measure patient skin dose during fluoroscopically guided medical procedures. For this reason, the product was designed to have minimal dependence on photon energy between about 60keV and 120keV. There are two types of GAFCHROMIC® XR dosimetry media. One, Type R film, is a reflective product and the other, Type T film, is a transmission film. The structures of the two films are shown in Figures A and B.

Figure A: Configuration of GAFCHROMIC® XR Type R dosimetry film

Figure B: Configuration of GAFCHROMIC® XR Type T dosimetry film

In both products the active layer is sandwiched between two sheets of polyester. For the Type T media both polyester substrates are transparent while the Type R film is comprised of one transparent film substrate and one opaque, white film substrate. The transparent polyester substrate used in both types of film contains a yellow dye. One purpose of the yellow dye is to enhance the visual contrast of the chromatic changes that occur when the film is exposed to radiation. A second purpose is to protect the active layer against exposure by UV and blue light and thereby enable the film to be even more tolerant of being handled in the light. The opacity of the white substrate in the Type R media is provided by a baryta filling. The polyester film substrates are 380 gauge, i.e. approximately 97 microns in thickness. Both types of GAFCHROMIC® XR dosimetry media employ the same active component used in the HD-810 and MD-55 films, but include a proprietary high Z material, in addition. The thickness of the active layer in the Type T film is about 18 microns and about 15 microns in the Type R film. These thicknesses may vary from batch-to-batch in order to provide the products with reproducible sensitometric response. Details concerning particular batch numbers will be provided upon request. In contrast to GAFCHROMIC® MD-55, neither of the GAFCHROMIC® XR dosimetry films employs an adhesive laminating film to bond the two coated polyester substrates. Through the use of proprietary ISP technology, the need for the adhesive laminate has been eliminated and the XR media consist solely of the active layer sandwiched between the polyester substrates. This construction of the XR films produces dosimetry media that are much more flexible than GAFCHROMIC® MD-55. Thus GAFCHROMIC® XR dosimetry films can, with care, be bent to a radius as small as 5mm. GAFCHROMIC® XR radiochromic dosimetry films may be measured with densitometers, film scanners or spectrophotometers. Type T film is preferably read in transmitted light, while reflected light measurements are best for theType R film. When the active component in the films is exposed to radiation, it reacts to form a blue colored polymer with absorption maxima at about 615nm and 675nm. Therefore, the response of GAFCHROMIC® XR dosimetry media is enhanced by measurement with red light. Transmission and reflection densitometers (e.g. X-Rite 310 and Gretag-Macbeth D-19C) are available to measure in various parts of the spectrum (e.g. visual, red, green and blue) and are commonly and widely employed in the photographic and printing industries. The Nuclear Associates Radiochromic Densitometer Model 37-443 is especially suited to making spot measurements of GAFCHROMIC® XR Type T film since it employs a red LED light source and filter optimized to the wavelength of the major peak in the spectrum of the photopolymer. The Howtek MultiRad 460 Film Digitizer, available exclusively through Nuclear Associates, and the Photoelectron Corporation CCD100 Microdensitometer both use 660nm LED light sources and are therefore optimized for scanning Type T films. The former will accommodate films up to 14" x 17" in size while the latter can be used with 8" x 10" sizes. The use of the red LED light sources results in about a 3X increase in the sensitivity of the media relative to He-Ne laser scanners, or scanners employing white light sources Low-cost (<$1000) flatbed color scanners are widely available and used in the home and office environments to scan photographic prints and transparencies. With such scanners it is best to work in transmission mode with the Type T films and in reflection mode with the Type R media. These scanners measure the red, green and blue color components, and by using the data from the red color channel the sensitometric response of the XR media will be maximized. Frequently, scanning systems and densitometers developed for conventional silver halide medical x-ray film employ a white light source. The yellow polyester base in GAFCHROMIC® XR films absorbs blue light, enhancing, to a degree, the sensitometric response of the media. However, a greater enhancement can be obtained by using, in addition, a deep orange filter while scanning or measuring. This will effectively restrict the measurement to wavelengths greater than about 560nm, where the photopolymer absorbs most strongly. Practically speaking, the response of the film can be improved by 50-75% in this manner, depending on the characteristics of the instrument. Sheets of the orange color filter are available through Nuclear Associates.

Specifications Of GAFCHROMIC® XR Radiochromic Dosimetry Films

The following table lists typical performance data, based on evaluation of prototype batches of GAFCHROMIC® XR Type R and Type T dosimetry films. It is ISP's intention to produce subsequent batches of film to meet these performance standards. However, since these are new products with limited history we do not warranty that future batches will meet this level of performance in all respects. The performance characteristics of individual batches of film will be available upon request.

Property GAFCHROMIC® XR Type R Film GAFCHROMIC® XR Type T Film Configuration Active layer between polyester substrates Active layer between polyester substrates Size 14" x 17"; other sizes upon request 5" x 5" min., other sizes upon request Substrate 1 380 gauge transparent yellow polyester 380 gauge transparent yellow polyester Substrate 2 380 gauge opaque white polyester 380 gauge transparent yellow polyester Active layer thicknesses ~15microns1 ~18microns1 Sensitometric response Net density2 of 0.26, 0.58, 1.27 and 1.58 at absorbed doses4 of 0.75Gy, 2Gy, 6Gy and 10Gy respectively Net densities3 of 0.40, 0.80,1.75 and 3.00 at absorbed doses4 of 1Gy, 2Gy, 5Gy and 10Gy respectively Energy dependency <6% difference in net density5 for 5Gy exposures between 80keV - 120keV <6% difference in net density6 for 5Gy exposures between 80keV - 120keV Dose fractionation response <5% difference in net density5 between a single 5Gy dose and four cumulative 1.25Gy doses at 30min. intervals <5% difference in net density6 between a single 5Gy dose and four cumulative 1.25Gy doses at 30min. intervals Dose rate response <5% difference in net density5 between exposures at rates from 2Gy/min. to 0.02Gy/min. <5% difference in net density6 between exposures at rates from 3.4Gy/min. to 0.034Gy/min. Stability in light <0.005 density change per 1000lux-day2,7 <0.005 density change per 1000lux-day3,7 Stability in dark (pre-exposure) <0.5x10-3 density change/day at 23°C <0.5x10-3 density change/day at 23°C Uniformity, cross web <5%8 <5%8 Uniformity, down web <3%9 <3%9 Sheet-to-sheet uniformity <5% sensitometric response difference from mean <5% sensitometric response difference from mean Batch-to-batch uniformity <15% sensitometric response difference from mean <15% sensitometric response difference from mean Post exposure density growth <10% from 1 hr to 1 day after exposure; <4% 1 day to 4 days after exposure <10% from 1 hr to 1 day after exposure; <4% 1 day to 4 days after exposure

1. Actual thickness may vary slightly from batch-to-batch in order to match sensitivity specification. 2. Gretag-Macbeth D19C densitometer, cyan color channel - net density is the change in density due to the absorbed radiation dose 3. Nuclear Associates Radiochromic Densitometer 37-443 - net density is the change in density due to the absorbed radiation dose 4. 120kVp x-rays, 2mm aluminum filtration 5. Measured with Gretag-Macbeth D19C densitometer, cyan color channel. Net density is the change in density due to the absorbed radiation dose 6. Measured with Nuclear Associates 37-443 densitometer. Net density is the change in density due to the absorbed radiation dose 7. Cool white fluorescent light 8. 2s.100/density - 34 measurements at 0.5" intervals 9. 2s.100/density - 24 measurements at 0.5" intervals

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