The film Radiochromic MD-55 (in its third version, MD-V3) It is designed for measurements of absorbed dose for high energy photons. The answer of this film is independent of the higher photon energy 0,2 MV. Its wide dose level (1Gy – 100Gy) It makes it ideal for radiosurgery, or use in Intraoperative radiation therapy.
An active layer of about 16 microns thick, It is covered by a transparent sheet (67 microns) polyester. And these two pieces are glued together to form the film MD-55. The MD-55 film employs the same active material as the HD-810 film and the film HS. In total the total amount of active material sum 32 microns, which varies from batch to batch infinitesimally to maintain a reproducible response.
The MD-55 film can be measured with transmission densitrómetros, film scanners or spectrophotometers. When the active component is exposed to radiation, reacts to form a blue polymer with absorption maxima at 615nm and 675nm. Thus, the response is improved with measures red light. Transmission densitometers for color film within the visible spectrum, They are commonly used in the photographic industry.
But nevertheless, the model densitrómetro 37-443 Nuclear Associates is especially suitable for specific measures in the movie MD-55 since it uses an optimal red light, and is approximately in 660 nm. This corresponds closely with the absorption peak of photopolymer.
The film digitizer Howtek MultiRad 460, available from Associates Nucclear, and CCD100 of Photoelectron Corporation microdensitometer have similar light sources and are therefore optimized for scanning sheets 5″x5″ Film MD-55 and reach a size 8″x10″.
The use of red light sources in the densitrómetros escánereso effectively increases the sensitivity of the film MD-55 up to three times compared to black and white densitrómetros, or He-Ne laser escáneresde.
The escáneresplanos of 48 bits are widely available and are used for multiple tasks currently for photographs or transparencies. When used with the MD-55 film is the best way to scan in trasmisivo. These measured red escánerescomúnmente, green and blue. The response of the film MD-55 is optimal to extract information on the red channel.
Some systems developed for densitrómetros escáneresy blango films and silver halide black, measured in a wavelength band that round the visible spectrum full. This is not optimal for measuring the film MD-55. But nevertheless, an improvement can be obtained when using an orange filter while scanning or measure the film. This will restrict measurement to higher wavelengths where the photopolymer that absorbs 560nm mostly. Practically speaking, the response of the film can be improved between a 50 and a 75% in this way, depending on the characteristics of the instrument. Orange sheets of this filter can be obtained through Nuclear Associates.
|Property||Radiochromic film GAFCHROMIC® MD-55-V2|
|Configuration||Two active flakes polyester, laminated tape|
|Size||5″ x 5″ minimum; other sizes upon request|
|substrata||260 gauge clear transparent polyester|
|Thickness of the layers of active material||Nominally 2 x 16 microns1|
|Lamination||100 gauge polyester with double sided adhesive layers approximately 1 mil thick|
|sensitometric response||net density1from 0.90 to 25Gy and 1.75 the 50Gy|
|energy dependence||<5% difference in net density1exhibition of 50 Gy to 1MeV and 18Mev|
|Fractionated Dose Response||<5% difference in net density1for a single 40Gy dose and five cumulative 8Gy doses at 30min. intervals|
|Response to dose rate||<5% difference in net density1for 10Gy exposures at rates of 3.4Gy/min. and 0.034Gy/min.|
|Light stability||<0.005 change in density per 1000lux-day2,4|
|Stability in dark (pre-exposure)||<0.5×10-3density change/day at 23°C
<0.2×10-3density change/day refrigerated
|Uniformity, of sheets||<8% sensitometric response difference3|
|Uniformity Sheet to Sheet||<5% sensitometric response difference from mean|
|Uniformity Lot to Lot||<10% sensitometric response difference from mean|
|Density growth Post-Exposure||<12% from 1 hr to 1 day after exposure;
<4% 1 day to 4 days after exposure
- The thickness can vary from batch to batch.
- Medido con Nuclear Associates Radiochromic Densitometer Model 37-443. Net density is the change in density due to the absorbed radiation dose.
- 2s.100/density – 49 measurements in a 7×7 grid on a 5″ x 5″ sheet
- Cool white fluorescent light
The MD-55 film has been widely studied and reported by the AAPM TG55 (Niroomand-Rad et al 1998 Radiochromic Film Dosimetry: Recommendation of AAPM TG55 Med. Phys. 25 2093-115). The information presented below includes data from this source, besides the manufacturer itself.
The information in the figure shown below, is for film exposure MD-55 to Co60. Density measurements were made with a radiocrómico densitrómetro Nuclear Associates Model 37-443. The net density is the density change due to the exposure dose, that is to say, subtraction between density after exposure less base and fog. The response of the film as gafchromic MD55 with this type of densitometer is linear up to 50 Gy.
Several measures were taken to determine the effect of dose fractionation on film response MD-55. The initial densities of two film samples were measured. Each film sample was measured five times.
Exposure the films were of 40Gy X-rays filtered with 120kVp aluminum 2mm. For a sample, the total dose was fractionated in 5 each increases 8Gy 30 minutes. The other sample received 40 Gy once, lasting 5 minutes. Samples were remeasured 24 hours later exposure, each sample in five separate locations leida.
Density differences by subtracting were calculated density before exposure and after.
Since the total number of exposures for the two examples were different, the net density was normalized to the corresponding dose of 40Gy absorvidad. Changes in density are shown below. within experimental error, only the results for exhibitions, and fractionated are indistinguishable and demonstrate that the effects of dose fractionation are negligible.
|Dose Fractionation of GAFCHROMICMD-55 Dosimetry Film
|Total Dose, Gy||Number of Dose Fractions||Number of Measurements||Net Density Change|
|40||5 – @30 minute intervals||15||1.15|
Table MD-1: Effect of dose fractionation on the response of GAFCHROMIC® MD-55 dosimetry film, measured with Nuclear Associates Radiochromic Densitometer 37-443
The effect of dose rate in the response of the MD-55 film was measured and found that the film is independent of the dose rate to the range of 0,034Gy / min to 3,4Gy / min. Each dose rate three samples were exposed film, for a total of 10 Gy.
The net density was measured for each example 5 sometimes with a Densitrómetro Radiocrómico Nuclear Associates Model 37-443. The net density is the density change due to absorbed dose.
Net density values were normalized to a dose of 10Gy absorbed.
In the table below, They are the average values for 15 measures a dose rates. The net density differs in only one 2% average. These differences are within experimental error, and these results indicate that the response of the film is independent of dose rate.
|DOSE RATE DEPENDENCE OF GAFCHROMICMD55
Lot# J1426-MD55: 10Gy total dose
Nuclear Associates Radiochromic Densitometer 37-443
The active component in GAFCHROMIC® dosimetry films is a radiation sensitive monomer. Upon exposure to radiation, the active component polymerizes to form a dye polymer. The polymerization has been investigated by McLaughlin, et al (ACS Symposium Series, «Irradiation of Polymers, Fundamentals and Technological Applications», Chapter 11, American Chemical Society 1996). This work showed that after flash photolysis the reaction has an incubation period of at least 1 microsecond. After pulsed electron beam radiolysis, the polymerization proceeds with first order kinetics and a rate constant of about 103 sec-1 In the first minutes after exposure, the post-exposure density growth effect manifests itself as a significant increase in optical absorption. This corresponds to an increasing concentration of polymer within the active layer. However, the rate of change of absorption diminishes rapidly with time. Thus the optical absorption asymptotes to a practically constant value about 2 days after exposure.
If measurements are to be made within a few hours of the exposure, a practical and effective technique to eliminate error due to the effects of post-exposure density growth is to make the density or optical absorption measurements at a consistent time after exposure. Alternatively, errors caused by mistiming of the measurements can be practically eliminated if such measurements are delayed until 24hours, or more, after the exposure.
The data in Figures MD-3 and MD-4 show the post-exposure density growth of GAFCHROMIC® MD-55 radiochromic dosimetry film. In Figure MD-3 the densities of several film samples exposed to different absorbed doses of x-rays are plotted versus the time after exposure. This reveals that the rate of change of density decreases continuously and rapidly with time after exposure, becoming very slow within about 24 hours.
n Figure MD-4, the density data for each individual exposure has been normalized to the value of the density at 24 hours after exposure. This figure reveals that post-exposure density growth, relative to the density at 24 hours, is essentially independent of exposure dose. The density changes about 10% in the period between 1 hour after exposure and 24 hours after exposure, but the rate diminishes and the density changes by less than 2% over the next 96 hours.
EBT film reveals real time.
Uniformity in the directions of the filaments deposited film
Four 5″ x 5″ sheets of GAFCHROMIC® MD-55 dosimetry film were stacked on top of one another and exposed to an absorbed dose of about 40Gy at the University of Wisconsin. The field size of the beam was approximately 6″ square with a flatness of about 2%. The film sheets were marked to indicate the directions orthogonal (cross-web) and parallel (down-web) to the coating direction. Prior to the exposures the optical densities of each sheet had been measured in forty-nine locations arranged in a regularly spaced 7×7 array. The densitometer was a Nuclear Associates Radiochromic Densitometer, Model 37-443.
Seven days after exposure the densities of all samples were re-measured (49 locations per sheet) and the change in the density at each of the measurement points was calculated. Then, for each sheet, the average value and standard deviation was calculated in the down-web and cross-web directions, as well as an overall average and standard deviation for that sheet. Finally the cross-web, down-web and sheet uniformities were calculated as defined by two times the standard deviation divided by the average, expressed as a percentage. With uniformity expressed in this manner, ninety-six percent (96%) of the measurements on a uniformly exposed sheet would fall within ±2s of the average.
The results in Table MD-3 show that the cross-web uniformity, calculated as the average of all measurement sets, was 6.6% and values of the twenty-eight sets of measurements ranged from 1.9% to 11.0%. Similarly the down-web uniformity is 4.1% with the individual values ranging from 1.8% to 6.4%. The overall uniformity of the four sheets, cross-web and down-web combined, was 6.6% with the values for individual sheets ranging from 3.7% to 8.9%.
|UNIFORMITY OF GAFCHROMICMD-55 DOSIMETRY FILM
|Direction||Number of measurements||Uniformity (2X std. dev./average)||Range of values|
|cross-web||28 x 7||6.6%||1.9% – 11.0%|
|down-web||28 x 7||4.1%||1.8% – 6.4%|
|whole sheet||4 x 49||6.6%||3.7% – 8.9%|
Table MD-3: Uniformity of GAFCHROMIC® MD-55 dosimetry film, exposed to 25Gy and measured with Nuclear Associates Radiochromic Densitometer 37-443
Sensitivity to white light
Numerous tests and observations have clearly established that while the active component in GAFCHROMIC® dosimetry films is not particularly sensitive to visible light, it is comparatively more sensitive to short wavelength light than to long wavelength light. The interior environment in buildings is predominantly illuminated with incandescent or cool white fluorescent light bulbs. The latter produce a higher proportion of blue light and the former a higher proportion of red light. Therefore, in measuring the white light sensitivity of GAFCHROMIC® MD-55 dosimetry film, tests were performed in the more demanding condition by exposing the film to the light from cool white fluorescent bulbs.
Offices and laboratories are commonly illuminated by cool white fluorescent light bulbs. The intensity of the illumination on working surfaces such as desktops and laboratory benches was measured in a representative number of offices and laboratories. It was found that the light intensity was in the range from about 600lux to 1000lux. Therefore, for the purpose of the evaluation of white light sensitivity of GAFCHROMIC® dosimetry films it was assumed that «standard» indoor illumination intensity is 1000lux.
A light table comprised of cool white fluorescent light bulbs illuminating an opal glass viewing surface was used as a test fixture. The intensity of light at the surface of the glass was measured at about 2900 lux. Samples of GAFCHROMIC® MD-55 dosimetry film about 1″x1″ in size were cut and the Status Red densities of the samples were measured with an X-Rite 310T densitometer. The samples were then placed on the surface of the light table and covered with a black sheet to shield them from room light. The temperature of the samples was 23±2°C during the test period. At various intervals up to 26 days the densities of the samples were re-measured. The density change values were calculated and normalized to a light intensity of 1000lux and plotted against the exposure in lux-days. An exposure of 1000lux-days represents the quantity of visible light that a film sample would receive were it to be exposed to the illumination in the «standard» indoor environment for 24 hours.
The data have been plotted in Figure MD-5. The trend of the data points suggests that the rate of change of density diminishes with exposure time. This behavior has been consistently seen in previous measurements of the white light sensitivity of GAFCHROMIC® dosimetry films. However, for simplification, it has been assumed that the change in density is linear with time. A linear fit of the data shows that the trendline has a slope of 0.00086 density units per 1000lux-days of exposure, i.e. the amount of exposure if the «standard» interior illumination of 1000lux intensity was applied for an entire 24-hour period. This low white light sensitivity indicates that GAFCHROMIC® MD-55 dosimetry film can be handled in normal room light for at least several hours without noticeable effects. However, it also suggests that the film should not be left exposed to room light indefinitely, but rather should be kept in the dark when it is not being handled.