W.L. McLaughlin

Radiochromic film dosimetry: Recommendations of AAPM Radiation Therapy Committee Task Group 55

Recommendations of the American Association of Physicists in Medicine (AAPM) for the radiochromic film dosimetry are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.

Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry

There is a new radiochromic film, a highly uniform, thin (100-microns) detector whose sensitive layer (6 microns thick) changes from colorless to blue by dye polymerization without processing, upon exposure to ionizing radiation. Because the dose gradients around brachytherapy sources are steep, the high spatial resolution offered by film dosimetry is an advantage over other detectors such as thermoluminescent dosimeters (TLDs). This compares the photon energy dependence of the sensitivities of GafChromic film, silver halide verification film (Kodak X-Omat V Film), and lithium fluoride TLDs (Harshaw), over the photon energy range 28 keV to 1.7 MeV, which is of interest in brachytherapy. Sensitivity of the radiochromic film is observed to decrease by about 30% as effective photon energy decreases from 1710 keV (4-MV x rays) to 28 keV (60-kV x rays, 2-mm A1 filter). In contrast, the sensitivity of verification film increases by 980% and that of LiF TLDs increases by 41%. The variation of the sensitivity of radiochromic film with photon energy is considerably less than that for silver halide film and similar to that for LiF TLDs, but in the opposite direction. Radiochromic film, like LIF TLDs, does not exhibit the drastic sensitivity changes below 127 keV that silver halide film exhibits. Dose distribution in the immediate vicinity of a high activity (370 GBq) brachytherapy 192Ir source has been mapped using radiochromic film and is presented to illustrate the applicability of this new technology to brachytherapy dosimetry.

The use of a radiochromic detector for the determination of stereotactic radiosurgery dose characteristicsa)

The measurement of absorbed dose as well as dose distributions (profiles and isodose curves) for small radiation fields (as encountered in stereotactic surgery) has been difficult due to the usual large detector size or densitometer aperture (> 1 mm) relative to the radiation field (as small as 4 mm). The radiochromic direct-imaging film, when read with a scanning laser microdensitometer (laser beam diameter 0.1 mm), overcomes this difficulty and has advantages over conventional film in providing improved precision, better tissue equivalence, greater dynamic range, higher spatial resolution, and room light handling. As a demonstration of suitability, the calibrated radiochromic film has been used to measure the dose characteristics for the 18-, 14-, 8-, and 4-mm fields from the gamma-ray stereotactic surgery units at Mayo Clinic and the University of Pittsburgh. Intercomparisons of radiochromic film with conventional methods of dosimetry and vendor-supplied computational dose planning system values indicate agreement to within +/- 2%. The dose, dose profiles, and isodose curves obtained with radiochromic film can provide high-spatial-resolution information of value for acceptance testing and quality control of dose measurement and/or calculation.

Temperature and humidity effects on the gamma-ray response and stability of plastic and dyed plastic dosimeters☆

Abstract Several plastics and dyed plastics are widely used for routine dosimetry in radiation processing with gamma radiation. A study was made of the effects of different temperatures and relative humidities during and after irradiation on the dose interpretation of these dosimeters, which are normally calibrated under controlled laboratory conditions. Samples of dosimeter materials investigated were cellulose triacetate, polycarbonate, polyethylene terephthalate, Polyvinyl fluoride, dyed polymethyl methacrylate (“red Perspex”), undyed polymethyl methacrylate (UVT Lucite), dyed Nylon, dyed polyvinyl butyral, dyed polyvinyl pyrrolidone, dyed and undyed polyvinylchloride, and dyed cellophane or Ultraphan. The results show that most of these dosimeters when used under extreme conditions of relative humidity and temperature may give erroneous readings unless suitable corrections are made. There is also indication that the environmental effects on dosimeter response are apt to differ somewhat from one supply of a given material to another.

The response of plastic dosimeters to gamma rays and electrons at high absorbed dose rates

Abstract Several clear plastics and dyed plastics are commercially available for dosimetry in intense radiation fields, particularly for radiation processing applications using gamma rays from intense cobalt-60 sources and electron beams from accelerators running at potentials from 0.1 to 10 MeV. Some of these widely used dosimeters are: colorless cellulose triacetate, polymethylmethacrylate, or polyvinylchloride, showing induced absorption bands in the ultraviolet spectrum; radiochromic dyed plastics, such as nylon, polychlorostyrene, polyvinylbutyral, polyvinylpyrrolidone, showing induced absorption bands in the visible spectrum; blue cellophane, showing a bleaching in the visible absorption spectrum. A major source of error in making dose interpretations by spectrophotometric analysis of these dosimeters comes from dose-rate dependence of response. The present work shows measurements at high dose rates (e.g. 100 and 1050 rad · s -1 with gamma radiation, 10 6 rad · s -1 with a scanned beam of 2-MeV electrons, 10 10 rad · s 1 with a pulsed beam of 10-MeV electrons and 10 11 to 10 14 rad · s -1 with single pulses of 2-MeV electrons). For calibrations with gamma rays, only those plastics containing the radiochromic triphenylmethane cyanides or methoxides, i.e. nylon, polychlorostyrene, polyvinylbutyral, or polyvinylpyrrolidone, can correctly interpret dose determinations from electron beam irradiation. The other systems show a marked rate dependence of response. Of radiochromic dye systems, only polyvinylpyrrolidone as host matrix can be read with precision immediately after electron irradiation, since a slow build-up of the absorption band during the first hours after irradiation occurs in the other radiochromic plastic systems.

Radiochromic plastic films for accurate measurement of radiation absorbed dose and dose distributions

Abstract Thin radiochromic dye films are useful for measuring large radiation absorbed doses (105–108 rads) and for high-resolution imaging of dose patterns produced by penetrating radiation beams passing through non-homogeneous media. Certain types of amino-substituted triarylmethane cyanides dissolved in polymeric solutions can be cast into flexible free-standing thin films of uniform thickness and reproducible response to ultraviolet and ionizing radiation. The increase in optical density versus energy deposited by radiation is linear over a wide range of doses and is for practical purposes independent of dose rate (1–1014 rad s−1). Upon irradiation of the film, the profile of the radiation field is registered as a permanent colored image of the dose distribution. Unlike most other types of dyed plastic dose meters, the optical density produced by irradiation is in most cases stable for periods of at least one year. Methods have been developed for casting various types of thin radiochromic plastic films and combinations of plastics (cellulose acetate, polyvinyl butyral, polyvinyl acetate, polyvinyl pyrrolidone and polyvinyl chloride) having radiation absorption characteristics corresponding to those of many polymeric systems in industrial radiation processing. The result is that errors due to energy dependence of response of the radiation sensor are effectively reduced, since the spectral sensitivity of the dose meter matches that of the polymer of interest, over a wide range of photon and electron energies (0.01–10 MeV).

A thin film recording medium for use in food irradiation

Abstract A commercially-available electron recording medium has been examined in terms of response to ionizing radiation in the form of photons emitted from the radionuclides 137Cs and 60Co and accelerated electrons. Operational characteristics of the film have been evaluated using scanning visible spectrophotometry, color photometry, densitometry, and scanning densitometry. The effect of various irradiation parameters (absorbed dose, absorbed dose rate, and irradiation temperature) have been characterized over the absorbed dose range of 0.1 to 5 kGy. Principle attributes of the film appear to be dose-rate independence for a given source, a high degree of uniformity with respect to radiochromic dye coating, and reproducibility of individual film sample measurements. Methods of dosimetry system operation and potential applications are discussed, with emphasis on applications for food irradiation.

Radiochromic dosimetry for validation and commissioning of industrial radiation processes

Abstract Radiochromic dosimeters in various forms, thin films, coated films and papers, optical waveguides, liquid solutions, and small pellets, serve as useful dosimeters for many radiation processing applications. While they cover wide dose ranges (10 0 – 10 6 Gy) as routine dosimeters, if properly calibrated and controlled, they also may be used accurately and reproducibly for dose mapping, dose setting, and process commissioning procedures, as well as for transfer calibration of other routine dosimeters in photon and electron radiation fields.

Examination of gamma-irradiated fruits and vegetables by electron spin resonance spectroscopy

Abstract The ESR spectra of the seeds, pits, shells, and skins of a variety of irradiated fruits and vegetables were measured. All spectra, control and irradiated, contained a single resonance with a g-factor of 2.00. Additional resonances due to Mn2+ were observed for the drupelets of blackberries and red raspberries. An unusual radiation-induced radical was observed for irradiated mango seed; however, the signal decayed completely within a few days. It was concluded that only in a few specialized cases could the ESR resonances observed be suitable for postirradiation monitoring or dosimetry.

The Mechanisms of Removal of Heavy Metals from Water by Ionizing Radiation

Abstract The removal of heavy metal ions from water using electron beam and gamma irradiation has been investigated for the cases of Pb 2+ and Hg 2+ ions. These metal ions are reduced by hydrated electrons and hydrogen atoms to lower or zero valence state and eventually precipitate out of solution. Ethanol is applied as a relatively non-toxic additive to scavenge · OH radicals, to enhance reduction and inhibit oxidation. Mercury can be completely (>99.9%) removed from aqueous solution of 1×10 −3 mol L −1 mercury (II) chloride by using a 3 kGy dose. However, a 40 kGy dose is required to remove 96% of lead ions from a 1×10 −3 mol L −1 of PbCl 2 solution. The effect of dissolved oxygen and carbonate were also investigated. E-beam irradiation of 1×10 −3 mol L −1 lead ions complexed with ethylenediamine tetraacetic acid (EDTA) in deoxygenated as well as air-saturated solutions in the absence of ethanol resulted in removal of about 97% of the lead.