EFFECT OF SHADOW TRAYS ON SURFACE DOSE AND BUILD-UP FOR MEGAVOLTAGE RADIATION

1 January 1973

journal article
Published by American Roentgen Ray Society in American Journal of Roentgenology

Vol. 117 (1) , 168-174
https://doi.org/10.2214/ajr.117.1.168

Abstract

Using LiF powder, the surface dose and build-up were measured in a lucite phantom for several kinds of megavoltage radiation, with the purpose of determining the extent of electron contamination from plastic shadow trays. For the 6 MV. unit, a shadow tray at 29 cm. from the phantom caused considerable contamination of the beam, leading to an increase in surface dose of up to approximately 48 per cent of local dose for large fields. To eliminate this, different kinds of shadow trays were tried, as well as a lead filter under the tray. The last was found to be most efficient. For the 2 MV. unit an inherently good geometry led to low electron contamination, while for the cobalt 60 unit poor geometry led to high contamination.

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