Radiochromic film

Radiochromic film is a type of self-developing film typically used in the testing and characterisation of radiographic equipment such as CT scanners and radiotherapy linacs. The film contains a dye which changes colour when exposed to ionising radiation, allowing the level of exposure and beam profile to be characterised.^[1] Unlike x-ray film no developing process is required and results can be obtained almost instantly, while it is insensitive to visible light (making handling easier).^[2]

A piece of radiochromic film that has been exposed to a beam of x-rays (note the dark dot in the middle)

Mechanism

For medical dosimetry "gafchromic dosimetry film (...) is arguably the most widely used commercial product".^[3] Several types of gafchromic film are marketed with differing properties.^[4] One type, MD-55, is made up of layers of polyester substrate with active emulsion layers adhered (approximately 16μm thick).^[2] The active layer consists of polycrystalline, substituted-diacetylene and the colour change occurs due to "progressive 1,4-trans additions as polyconjugations along the ladder-like polymer chains".^[5][3][6]

Usage

Radiochromic films have been in general use since the late 1960s, although the general principle has been known about since the 19th century.^[3][7]

Profiling

Radiochromic film can provide high spatial resolution information about the distribution of radiation. Depending on the scanning technique, sub-millimetre resolution can be achieved.^[4]

Dosimetry

Unlike many other types of radiation detector, radiochromic film can be used for absolute dosimetry where information about absorbed dose is obtained directly.^[3] It is typically scanned, for example using a standard flat bed scanner, to provide accurate quantification of the optical density and therefore degree of exposure. Gafchromic film has been shown to provide measurements accurate to 2% over doses of 0.2–100 Gray (Gy).^[8]