Author: Rick Holly, Sumudu Katugampola 👨🔬
Affiliation: Gesinger Medical Center, Geisinger Medical Center 🌍
Purpose:
To quantitatively analyze long term variations in optical density of un-irradiated and irradiated EBT3 radio-chromic films and their effects on dose calibration curves in a clinical setting.
Methods:
Two batches of EBT3 films were irradiated about 12 months apart in the dose range 0-800cGy. Both batches were irradiated under similar dosimetric conditions using the same Varian Truebeam linac. Films were scanned using a flatbed scanner (Epson Expression 12000 XL, Japan) in the reflective mode. The first set (Set 1) of films was scanned at 1h, 24h, 6mo, 8mo and 12mo after irradiation. The second set (Set 2) of films, from the same batch as Set 1, was irradiated 12 months later under similar conditions. Films from Set 2 were scanned 24h after irradiation. The scanned films from both sets were analyzed using the commercial software RITG148+ (Radiological Imaging Technology, Inc.). The calibration curves were generated by using RITG148+ and by fitting the raw data to a rational functional form. Calibration curves from Set 1 and that of Set 2 were compared against the 24h curve of Set 1.
Results:
Compared to the calibration curve obtained at 24h post irradiation , the post-irradiation OD change was about 20% or more in the 0-300cGy nominal dose region for the 6mo, 8mo and 12mo calibration curves. The effect observed due to OD change of un-irradiated films, during a period of 12 months, was minimum (< 2.5%) around 180-300cGy, but larger (>10%) in low-dose (0-100cGy) regions.
Conclusion:
The extent of the post-irradiation OD change varies with the irradiated dose and shows signs of saturation after about six months and at around 300cGy. The effect of un-irradiated OD change is minimum in the 180-300cGy range but is larger at lower doses.