Novel radiochromic films for clinical dosimetry

New transparent radiochromic films, GafChromic MD-55 and NMD-55, which turn from colourless to deep blue upon irradiation, have been designed particularly for measuring radiation therapy absorbed doses (1 Gy to 100 Gy). They are also useful for high resolution mapping of dose distributions, radiographic imaging, treatment planning dosimetry, beam penumbra measurements, and interface dosimetry with ionising photons, electrons and protons. The gamma ray responses are linear with dose in terms of increase of optical absorbance at 670, 633, and 600 nm and are independent of absorbed dose rate and relative humidity. The radiochromic images show a slight gradual post-irradiation increase in absorbance especially during the first 24 h. In addition, there is a small but predictable variation of sensitivity with temperature, both during irradiation and during spectrophotometry. The films also have a slight sensitivity to ultraviolet radiation (250 to 350 nm) in direct sunlight. Experiments with X ray beams show no appreciable energy dependence relative to dose in water at photon energies greater than 100 keV, but they have a sensitivity that gives readings of about 60% of the dose in water for photons at 20 to 40 keV.