Program Information

Impact of Region of Interest Size and Location in Gafchromic Film Dosimetry

P Alaei

N Gholizadeh Sendani , C Ferreira , P Alaei*, University of Minnesota, Minneapolis, MN

Presentations

SU-I-GPD-T-538 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate dependency of measured dose on size and location of region of interest (ROI) in Gafchromic EBT3 film for different field sizes.

Methods: Gafchromic EBT3 film (20.3×25.4 cm2) was used in this study. All films were handled as outlined in the AAPM TG-55 report. The film sheets were cut to pieces of 3×4 cm2 and were irradiated perpendicularly using the 6MV beam from a Varian linac at 10cm depth (100cm SSD) of a 30×30x20 cm3 solid water phantom for a range of field sizes (6×6 to 100×100 mm2).To create a calibration curve, dose values of 0 to 350cGy were used. The measurements were repeated three times for each dose. Films were scanned using an Epson scanner 24h after irradiation and images were read by Image-J software in red color channel. The net optical density was converted to dose by calibration curve fitting. To find appropriate location of ROI in scanned films, two methods were used. First, the ROI was placed in center of image visually. Second, the profile of pixel value versus distance was plotted and the center of profile was used for drawing ROI. Each scanned film was read using both methods and for three ROI sizes (1, 2, and 4 mm).

Results: Comparing the three ROIs using both methods shows that there is less than 1% difference in dose measurements for field sizes above 10×10 mm2. Differences of up to 7% are observed for smaller field sizes with largest differences for the 6x6 mm2 one.

Conclusion: There is no dependency of measured dose on ROI size for field sizes above 10×10 mm2. For smaller field sizes, the profile-based method can improve reproducibility and help find the best location of measurement over the visual one.


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