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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)!