Advanced Physics for Brachytherapy 2018

Radiochromic films: influence factors

• There is post-irradiation signal growth that depends on t (log, 5% per decade) and T. A k t,T (t,T,D) correction must be applied OR films are kept @ stable T (store exposed and unexposed films at ambient temperature ~22 C or less) for (at least 8h) 24h before scanning. • Background signal from an un-irradiated control film of the same batch and size, handled in the same way as the exp. films must be subtracted pixel-by- pixel to account for base OD and absorbance changes due to environmental conditions (T, visible light, humidity, scanning light, etc.) and obtain net OD change • Film non-uniformity correction, k nu (x,y), is important. A double exposure technique with pixel-by-pixel subtraction must be employed. • Alternatively, a triple channel technique has been developed (Micke et al, Multichannel film dosimetry with nonuniformity correction, Med. Phys. 38(5) 2523, 2011) and commercially available (FilmQA Pro software, Ashland-former ISP). • There is also reader non-uniformity and films should always be read at the same scanner bed location to avoid application of a k pos(x,y) correction • Film response is dose rate independent! • Films can be used in water (~1mm/h water penetration)!