Dependence of radiochromic film optical density post‐exposure kinetics on dose and dose fractionation

Abstract

Radiochromic film (RCF) has been shown to be a precise and accurate secondary planar dosimeter for acute exposure radiation fields. However, its application to low dose-rate brachytherapy has been questioned because of possible dose-rate effects. To address this concern, we have measured the optical density (OD) of Model 55-2 RCF as a function of time (interval between the completion of irradiation and densitometry using a 633 nm laser scanner) following exposure (from less than 1 hour to 90 days) for single and split doses from 1 Gy to 100 Gy. Our work demonstrates that film darkening as a function of post-irradiation time depends significantly on total dose, with films exposed to lower doses developing faster than films given higher doses. At 1 Gy, the OD 90 days after exposure is 200% larger than that measured 1 h after exposure compared to a 20% increase over 90 days for doses larger than 20 Gy. An empirical model with time-independent, fast and slow growth terms was used to fit single exposure data. The dependence of the resulting best-fit parameters on dose was investigated. Splitting the dose into two fractions (20 Gy followed by doses of 1-80 Gy 24 h later) results in modest post-irradiation time-dependent changes in the total optical density (at most 15% at small doses), which dissipates within 20 hours following the second exposure. This experimental finding is consistent with the predictions of a simple cumulative dose superposition model. Overall, both experimental and empirical modeling suggest that dose-rate effects may be relatively small despite the strong dependence of film darkening kinetics on total dose. However, more experimental evaluation of radiochromic film response dependence on dose rate and dose-time-fractionation patterns is needed.

Dosimetry of very high energy electrons (VHEE) for radiotherapy applications: using radiochromic film measurements and Monte Carlo simulations

“…To account for the post-exposure changes all calibration films are scanned 24 ± 2 h after exposure. Under these conditions errors due to time-after-exposure differences can be neglected (Ali et al 2002). It is well known that the response of EBT2 radiochromic film is sensitive to its orientation on the scanner (Paelinck et al 2007).…”

Temperature and hydration effects on absorbance spectra and radiation sensitivity of a radiochromic medium

“…11,12 However, because polymerization proceeds for some time after the radiation pulse, the development of radiochromic media continues even after the source of irradiation is turned off. 11,12,[16][17][18][19] This introduces some dose-rate effects in real-time measurements that need to be taken into consideration. Dose-rate dependent variations introduced during real-time measurements have been quantified for GAFCHROMIC ® EBT film and are small over certain dose and dose rate ranges.…”

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

“…The measured RCF response, OD(D, λ, t), was fitted to an empirical formula similar to the one used in a previous work [13]:…”

“…Following cessation of exposure to ionizing radiation, the optical density of RCF continues to increase for many weeks. The magnitude and speed of post-exposure OD dynamics has a complex dependence on dose [13], densitometry wavelength [14] and temperature [15]. Some evidence suggests that post-exposure OD kinetics could give rise to dose-rate and fractionation dependent RCF responses adding as much as 5% to the uncertainty in absolute dose measurements [5].…”

“…Some evidence suggests that post-exposure OD kinetics could give rise to dose-rate and fractionation dependent RCF responses adding as much as 5% to the uncertainty in absolute dose measurements [5]. In LDR brachytherapy, the uncertainty arises due to the loss of temporal synchronization of the acutely-exposed calibration films and films continuously irradiated by brachytherapy sources for periods of time as long as two weeks [13,16]. The low uncertainties reported in a pervious work [3] required meticulous temporal synchronization of irradiation and densitometry of calibration films and films exposed to unknown radiation fields.…”