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The Influence Edge Electrons in Small Fields: Emphasis by the Difference of Copper-Cerrobend Cutout

A Chu

A Chu1*, J Deng1,W Feng2, Z Chen1, M Ahmad1, R Nath3, (1) Yale New Haven Hospital, New Haven, CT, (2) New York Presbyterian Hospital, Tenafly, NJ,(3) Yale Univ School of Medicine, New Haven, CT

SU-E-T-100 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Electron cutout edge scatter is an important factor determining small-field electron dose outputs. The laterally scattered edge electrons presenting wide-spread energy spectrum are usually ignored in Monte Carlo calculation. This report demonstrated the different characteristics in edge electron scatter were highlighted by the copper-cerrobend pair comparisons. Backgournds: There are three possible differences caused by the replacement of copper cutout, i.e. differed by bremsstrahlung, transmission, and edge-electron scatters. Monte Carlo simulation (Ebert et al.) showed the very small amount of bremsstrahlung can get out cutout, and 1.5cm thickness of both can both effectively stop primary electrons (Das et al.), and the photon transmission is negligible (Zhe et al.)

Methods: The chamber-measured comparisons for circular copper- and Cerrobend-cutouts (1.5cm thickness) with diameters (1.0, 2.0, 3.0, 5.0, 7.5, 10.0, and 12.5 cm) were made using electron beams with energies (6, 9, 12, 16, and 20 MeV) from 3 Varian accelerators. To demonstrate the edge-electron near the surface of phantom (0.5cm and 1.0cm solid-water depth), a bottle-shape cutout illustrates the aperture open from narrow width (1cm) and progressively increased to 3cm. The surface edge-electron dose profiles were measured by Gfachromic film with one-scan film dosimetry technique (FilmQAPro/Ashland Inc.) and pp chamber (Markus/PTW).

Results:From the PDD curves collected from chamber-measurement in water tank with all circular cutouts irradiated by all electron energies over 3 LINACs, the beam-quality specifier, R50, was consistently higher by using copper cutout. The PDDs in smaller diameter circular cutout can clearly demonstrated the higher R50 differed by the cerrobend and copper cutouts was due to (edge-scattered) electron. The film profiles showed wide-spread edge-electron energy presented in the filtering effect from the surface at 0.5cm to 1.0cm depth, and different characteristics between copper and cerrobend cutouts.

Conclusion: The different edge-electron characteristics between copper-cerrobend pair may provide the clue for edge-electron modeling.

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