Post-irradiation colouration of Gafchromic EBT radiochromic film.

Gafchromic EBT (International Specialty Products, NJ, USA), radiochromic film is one of the newest radiation-induced auto-developing x-ray analysis films available for therapeutic radiation dosimetry in radiotherapy applications. Part of any radiochromic film product which undergoes a polymerization reaction for automatic darkening is an associated post-irradiation colouration whereby the film continues to darken after irradiation has ceased. The Gafchromic EBT film has been shown to produce an approximate 6% to 9% increase in post-irradiation optical density within the first 12 h of irradiation within the 1 Gy to 5 Gy dose range. This is compared to approximately 13%, 15% and 19% for MD-55-2, XR type T and HS radiochromic film, respectively. It is also shown that the EBT film's post-irradiation growth stabilizes to within 1% within the first 6 h. Thus EBT provides a reduced post-irradiation growth effect. However, to increase the accuracy of the film analysis, it is recommended that films be left for a significant period (at least 6 h) before the analysis is performed to provide a high level of accuracy. Also, calibration films must be read out with the same post-irradiation time to further enhance the accuracy of dosimetry.