Principal Component Analysis of EBT2 Radiochromic Film for Multichannel Film Dosimetry

Abstract

Radiochromic\rfilm with a dye incorporated into the radiation sensitive layer [Gafchromic\rEBT2, Ashland, Inc.] may be digitized by a color transparency scanner,\rdigitally processed, and calibrated so\rthat a digital image in units of radiation absorbed dose is obtained. A\rtransformation from raw scanner values to dose values was developed based upon\ra principal component analysis of the optical densities of the red, green and\rblue channels of the color image of a dose of 0.942 Gy delivered by a\rSr-90/Y-90 disk-shaped source. In the order of increasing eigenvalue, the three\reigenimages of the principal component analysis contained, by visual\rinspection, 1) mainly noise; 2) mainly a\rpattern of irregular streaks; and 3) most\rof the expected dose information along with some of the same background\rstreaking that predominated in the second eigenimage. The combination of the\rsecond and third eigenimages that minimized the background streaking was converted\rinto a transformation of the red, green and blue channels’ optical densities and applied to films with\ra range of doses from 0 to 63.7 Gy. The curve of dose vs. processed optical\rdensity was fit by a two-phase association curve. This processing was applied\rto a film exposed from its edge by a different Y-90 source in a configuration\rthat was modeled by Monte Carlo simulation. The depth-dose curves of the\rmeasurement and simulation agree closely, suggesting that this approach is a valid method of processing EBT2\rradiochromic film into maps of radiation absorbed dose.