An investigation of some scanner and film characteristics to improve the effective use of radiochromic film for radiotherapy dosimetry

Abstract

Radiochromic film as a radiotherapy dosimeter offers some distinct advantages for IMRT and VMAT plan verification. The main problem with scanning radiochromic film for dosimetry is the systematic artefacts, which result from the film and the flatbed scanners, which are used for digitising the film. The active ingredient of film, LiPCDA, is partly responsible for the orientation effect and the lateral response artefact (LRA) effect. Upon irradiation the active ingredient bonds with other neighbouring monomers to form polymers which increases these effects. The manufacturer of Gafchromic films have been trying to improve the films by changing crystal size and the emulsion. Four different types of films have been characterised to provide quantitative data on their performance and on selection for different applications. Smaller crystal size is shown to reduce the LRA effect and adding bonding inhibitor made LRA independent of dose given to the film.

The construction and components of the digitising flatbed scanners also contribute towards the artefacts, degree of which varies for different scanner makes and models. Epson V700 and Epson V800 were compared and found not interchangeable. The contribution of three scanner components, the lens system, mirror system and scanner bed, to LRA effect and light polarisation were investigated. Firstly, the scanner bed contribution was found to be negligible if the refractive index of the material is similar to that of film. Secondly, lens with bigger focal length was found to be causing less LRA. Thirdly, the mirror system using one mirror or no mirrors reduces the light polarisation from about 10% to about 2% for a film irradiated with 5.6 Gy.

This work has demonstrated that a modified scanner with one mirror or no mirrors and a direct digital camera read-out system with lens with focal length of 50 mm would improve the film dosimetry significantly but would be likely to make the scanner bulkier.