Affiliations: Department of Radiation Oncology, Oklahoma University
Health Sciences Center, Oklahoma City, OK 73104, USA | Department of Biomedical Engineering, Virginia
Commonwealth University, Richmond, Virginia 23284-3067, USA | Department of Radiation Oncology, Virginia
Commonwealth University, Richmond, VA 23298, USA
Note: [] Corresponding author. Imad Ali, Ph.D., Assistant Professor,
Medical Physics, Radiation Oncology Department, Oklahoma University Health
Sciences Center, 825 N.E. 10th Street, OUPB 1430, Oklahoma City, OK 73104, USA.
Tel: +1 405 271 8290; Fax: +1 405 271 9240; E-mail: iali@ouhsc.edu
Abstract: 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� 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�. 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.
