SU-E-T-186: Evaluation of Optical Density Growth and Sensitivity of EBT1 and EBT2 Gafchromic Films on the Dosimetry for IMRT Quality Assurance

Abstract

Purpose: To investigate the effects of optical density (OD) sensitivity and growth with time post‐irradiation on intensity‐modulated radiation therapy(IMRT)dosimetry with gafchromic films (EBT1 and EBT2). Method and Materials:EBT1 and EBT2 films were used to perform IMRTquality assurance, which were exposed to IMRT plans created in treatment planning system using 6 MV photon beam from a Trilogy Varian linac.Calibrationfilms were exposed to graded doses from 78–399cGy in order to convert OD to dose. The films were scanned using a flat panel scanner several times starting from 4 to 480 hours post‐irradiation. Gray scale and red colors for both EBT1 and EBT2 films were used to evaluate film sensitivity on IMRT analysis. The IMRTfilms were analyzed with calibrationfilms that were scanned synchronously and at different times post‐irradiation to quantify the effect of OD growth.Dose difference and gamma analysis of the dose distributions measured by film and calculated by treatment planning system were used to evaluate the effects of OD growth and sensitivity. Result: OD variations show that EBT1 grows slower, however, reaching higher values (14%) than EBT2 (13%) after long time post‐irradiation. The gray color values grow higher (13–14%) than red values (6%) for both EBT1&2 films after 480 hours. The results of dose difference and gamma parameter with more than 85% and 97% passing rates, respectively, using acceptance criteria (5%, 5mm) were better for films analyzed with synchronized calibration and red color values than non‐synchronized calibration and grayscale. Conclusion: The measurement of dose distributions agrees well with those calculated by treatment planning for synchronized IMRT and calibrationfilms where variation due to OD growth is minimized. Red color values provide better gamma for IMRT analysis with EBT1&2 films because of lower OD growth and high sensitivity than gray color values.