There are numerous dosimeters, some of which are in common use.
Film dosimetry
Film may be photographic/radiographic or radiochromic.
Principles of photographic film
Photographic film is made up of a thin polyester sheet coated on one or both sides with a silver bromide (Ag+Br-) emulsion. When exposed to ionising radiation, an electron may be released by the bromide atom and be captured by the silver atom. The neutral silver atom may then fix itself to the polyester sheet, sometimes dragging nearby silver atoms with it. When developed, the silver bromide emulsion is removed but the silver atoms remain on the sheet, giving a dark appearance. The amount of darkness corresponds to the dose received by the film. Film dosimetry is limited by several problems, most notably the need to develop the films and the inconsistent methods of doing this. It is most useful in relative dosimetry.
Principles of radiochromic film
Radiochromic film is made with the opposite structure to photographic film. Two sheets of polyester contain an emulsion containing a dye molecule. When exposed to ionising radiation, the dye polymerises and turns blue. The amount of polymerised dye is directly related to the amount ionising radiation. Unlike photographic film, radiochromic film does not require development or other processing, and it can also be used for absolute dosimetry.
Radiochromic film is much more expensive than photographic film. It also requires larger doses to produce readable results.
Radiochromic film was developed for the food irradiation industry, which normally gives doses in excess of 30,000 Gy. This explains why larger doses are needed to see visible results.
Use of film dosimetry
Both photographic and radiochromic film can be used for relative dosimetry. Film can be placed between slab phantoms to measurement of beam profiles or isodose charts. It is also used for quality assurance tests to confirm isocentre position.
Radiochromic film could be used for absolute dosimetry if required.
Scintillation dosimeters
Principles of scintillation dosimeters
Some compounds may release light photons when exposed to ionising radiation. The release of light corresponds to the amount of ionising radiation present. A scintillation dosimeter functions by capturing this light with fibreoptic cabling and transporting it to a photomultiplier. This device translates the light signal into a dose. A second fibreoptic cable functions as a 'control', as megavoltage photons may create Cerenkov radiation that could confuse results.
Scintillation dosimeters are small, tissue equivalent, direction independent, dose rate independent and energy independent. They require attachment to fibreoptic cabling and are relatively expensive.
Use of scintillation dosimeters
Scintillation dosimeters are not in common use due to their cost, but their small size makes them ideal for measuring dose in regions of steep dose gradients (penumbra). They are also useful in brachytherapy due to the dose rate and directional independence.
Chemical dosimeters
Liquid chemical dosimetry - physical principles
The most commonly used liquid dosimeter is Fricke's solution, a suspension of Fe2+ in sulphuric acid and water. When exposed to ionising radiation, release of electrons from Fe2+ leads to formation of Fe3+. The quantity of Fe3+ can be measured chemically and correlates with the exposed dose. Fricke's solution can be placed in small, sealed containers for dosimetry purposes.
Gel dosimetry - physical principles
Instead of a liquid suspension, gel dosimetry suspends Fe2+ in a gel. The same transformation to Fe2+ occurs when exposed to ionising radiation. The presence of Fe3+ can be detected in a magnetic resonance imaging device, which correlates with absorbed dose. Gel can be shaped into numerous forms for dosimetric purposes.
Newer gel dosimetry suspensions use polymers (similar to the method of radiochromic film, above) that can be read with other devices without the need for MRI.
Use of chemical dosimeters
Chemical dosimeters are used in national standards laboratories as their results are highly quantifiable.
Gel dosimetry is proving useful for beams with very sharp dose gradients, such as stereotactic radiosurgery.
