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British Journal of Radiology (1992) 65, 517-522
© 1992 British Institute of Radiology
doi: 10.1259/0007-1285-65-774-517
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Output factors for blocked fields

E. M. Dean, BSc, MSc * and J. B. Davis, BSc, MSc

Department of Radio-Oncology, University Hospital Zurich, Raemistrasse 100, 8091 Zurich, Switzerland * Department of Radiation Physics, St Bartholomew's Hospital, West Smithfield, London EC1A 7BE, UK

This study investigates the output from blocked treatment fields with 6 MV and 18 MV photons. Square fields, defined collimators, were blocked down to smaller symmetrical square, rectangular and asymmetrical rectangular fields using blocks on tray beneath the collimators. Up to 7.5% difference was found between the output measured for the blocked field and the output calculated using a collimator output factor for the surface field size. A method is described by which output factors for the blocked fields can be measured and used in practice. Three methods of approximating these output factors are also presented.

Key Words: Output factors • Blocks • Scatter

Received for publication June 17, 1991. Revision received October 24, 1991. Accepted for publication December 6, 1991.






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