Prototype electron phantom for radiographic and radiochromic film dosimetry

Chad Joseph Robertson, Louisiana State University and Agricultural and Mechanical College


Robertson, Chad Joseph, B.S. Louisiana Tech University, 2006 Master of Science, Spring Commencement, 2006 Major: Medical Physics and Health Physics Prototype Electron Phantom for Radiographic and Radiochromic Film Dosimetry Thesis directed by Professor Kenneth R. Hogstrom Pages in thesis, 133. Words in abstract, 350 ABSTRACT Purpose: The purpose of this work is to develop a solid electron beam film phantom for use with radiographic film (RGF) and radiochromic film (RCF) to measure relative dose distributions in a principal plane containing the central axis for 6¨C20MeV electron beams. It was hypothesized that relative dose distributions measured using film will agree with corresponding diode measurements within ¡À2% of the central-axis maximum dose or ¡À1mm distance-to-agreement (DTA). Method and Materials: Three prototype film phantoms were designed at Mary Bird Perkins Cancer Center and constructed by Gammex-RMI, Inc. Relative dose measurements, planar (2D) dose distributions containing central-axis, were acquired in the phantom using both Kodak-XV RGF and GafChromic-EBT RCF. Correspondingly, diode measurements were acquired utilizing a Scanditronix-Wellhofer 2D-water phantom. For prototype 3, dose distributions were measured at 100-cm SSD using a 15x15-cm2 field-size at 6, 9, 12, 16, and 20 MeV, as well as 2x2-cm2 and 4x4-cm2 field-sizes at 9 and 16 MeV. Relative dose differences were evaluated with respect to regional criteria of acceptability: (1) high dose, low dose-gradient region (¡Ü 2 % dose), (2) high dose-gradient region (¡Ü 2 mm DTA), and (3) low-dose, low dose-gradient region (¡Ü 2 % dose). Results: RGF depth-dose measurements agreed with diode measurements within all criteria for all measurements conditions. 2D dose distributions were in agreement with over 98% of measured dose points agreeing within ¡À2% dose or ¡À1mm DTA for all energies (6¨C20MeV, 15x15-cm2). RCF depth-dose measurements agreed for all measurement conditions in all regions excluding the build-up region (<1¨C2cm depth), where measurements were approximately 3¨C4% low. 2D dose distributions reflected differences seen in the depth-doses with 90% of data points within criteria. Conclusion: With appropriate modifications, the prototype 3 phantom is capable of accurately measuring relative electron dose distributions using RGF sufficiently for clinical use. RCF measurements acquired in the same phantom consistently underestimated diode measurements by 3¨C4% at depths <2-cm. The cause of this systematic error, believed to be a combination of film-edge misalignment and RCF depth-dependency, must be resolved before prototype phantom 3 with appropriate modifications would be acceptable for clinical use.