SU‐E‐T‐184: Practical Method of Scanner Stability Compensation for Film Dosimetry

Purpose: In high‐accuracy radiation therapy such as intensity‐modulated radiotherapy (IMRT), dosimetric quality assurance is required to ensure the accuracy of the dose delivery. Although film dosimetry, associated with absolute dose measurement, is often used for this purpose, the uncertainty of film dosimetry is strongly influenced by the stability of the scanning device. This paper proposes a practical correction method for compensating scanner instability when making dose response curves. Methods: Gafchromic EBT3 films were irradiated with 6 MV X‐rays to 0, 10, 50, 100, 200 and 300 cGy. All films were scanned 1,000 times for 45 hours with about 2.7 minutes interval by a flat‐bed type image scanner (ES‐10000G, Seiko Epson Corp.). Pixel values for 1,000 images were automatically measured using a macro function of Image J (National Institutes of Health) to analyze the same area for each ROI, stored as the average of 50 x 50 pixel values. The stability of film measurement was assessed by analyzing optical density (OD) and corrected optical density, calculated by summing the OD of unirradiated film and net optical density (ODnet). Results: OD changes were significantly corrected, including post‐exposure density growth, compared to the original OD changes. This means that highly accurate film dosimetry, independent of the condition of the scanner device, may be possible by correcting OD with ODnet. Conclusion: In making dose response curves, dose uncertainty due to scanner device instability can be minimized by correcting OD values using the ODnet value. By using the presented method, a reduction of uncertainty in film dosimetry can be achieved. Furthermore, the procedure is simple and practical for clinical implementation. This research was supported by a grant for Clinical Cancer Research (10103757) from Ministry of Health, Labour and Welfare, Japan.