A novel evaluation technique for radiochromic films based on cosine-distance analysis: Feasibilities of wide range photon dosimetry and postal intercomparison in lieu of conventional TLD chips

Photon radiation induced decolourisation of radiochromic film changes its optical density. The photon dose is thereby evaluated as a function of optical density. The detection limit of common radiochromic films lies between 1 cGy and 20 Gy. This paper describes a novel evaluation method of radiochromic films using cosine-distance analysis of the RGB-colour vectors instead of conventional optical density measurements using a scanner in transmission mode. This allows extending the detection limit to excess of 40 Gy. •Gaf-EBT3 radiochromic films exposed to photons from 6 MV linac.•RGB colour histogram constructed using ImageJ program.•Cosine-distance (CD) calculated using RGB colour vectors.•Photon doses presented as a function of CD.