Optimal densitometry wavelengths that maximize radiochromic film sensitivity while minimizing OD growth and...

It is well known that optical density (OD) of the radiochromic film (RCF) continues to grow after exposure at rates that have a complex dependence on dose, temperature, and densitometry wavelength. Dose rate and fractionation artifacts associated with variations in OD growth may limit the accuracy achievable by RCF dosimetry in brachytherapy and external beam applications, particularly at low doses (<5 Gy) and low dose rates (<10 cGy/h) where OD growth and sensitivity effects are large. To identify densitometry wavelengths that minimize OD growth artifacts and enhance RCF sensitivity at low doses, we have investigated Model MD-55-2 RCF response as a function of densitometry wavelength, irradiation-to-densitometry time interval, dose and temperature. Using a Perkin Elmer spectrophotometer, the absorption spectrum in the 500-700 nm range was measured for doses ranging from 1-100 Gy, over post-irradiation times from 1 h to 60 days. An empirical model with time-independent, fast and slow growth components was used to fit single exposure data and the dependence of the resulting best-fit parameters on dose and densitometry wavelength was investigated. RCF OD variation with temperature in the range 22-40 degrees was measured. Wavelengths in the 660-690 nm range were found to minimize the dose-dependence of OD post-exposure growth. Densitometry wavelengths in the range of 670-680 nm enhance RCF sensitivity and show small variations in OD with temperature in the range from 22-40 degrees. Compared to 633 nm light, 675 nm densitometry reduces OD growth at 1 Gy from 70% to 10% over a period of nearly 1174.0 h relative to the initial OD measured at 1.7 h post-irradiation. In addition, RCF sensitivity is nearly doubled at this wavelength for all dose levels.