SU-E-T-117: A Point Densitometer Scanner for EBT2 Film Dosimetry

To create and characterize a device that can accurately and reproducibly digitize Gafchromic(R) EBT2 film for dosimetry.

We have constructed a point densitometer scanner that allows the acquisition of 1 dimensional optical density profiles of EBT2 dosimetry film at submillimeter resolution. An LED with peak output at 636nm collimated and focused with an aspheric lens was used as a light source. A large area photodiode was used as a detector. A custom transimpedence amplifier was developed to optimize the detector signal response for optical densities corresponding to a clinically relevant dose range (0 - 3Gy). A custom motorized translational stage was developed allowing 1 dimensional scans and manual fine adjustment in the lateral direction. The entire scanner was contained in a light tight enclosure to eliminate the effects of ambient light. Dose profiles were generated from EBT2 films exposed uniformly to varying doses using a superficial unit with the films placed on top of a solid water phantom. Dose profiles were generated from the same films using a flatbed scanner (Epson 10000XL) implementing two and three optical channel corrections for film inhomogeneity and the results were compared.

Initial analysis of the point densitometer shows a beam FWHM of 0.7mm and position error of the translational stage at 0.02mm in the scan direction and 0.2mm in the lateral direction. Reproducibility of the scans has been shown to within 1%. Orientation dependence and measured dose homogeneity have been improved over corrected flatbed digitization techniques.

Preliminary characterization of the point densitometer shows its potential as an improved quality assurance tool for EBT2 film dosimetry.