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British Journal of Radiology (1979) 52, 398-404
© 1979 British Institute of Radiology
doi: 10.1259/0007-1285-52-617-398
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An intercomparison between two methods of obtaining percentage depth doses for irregular shaped fields and comparison of each method with experimental data for 60Co and 10 MV X rays

D. Wrede, Ph.D. D. Tai, Ph.D. F. Edwards, M.S. C. Coffey, Ph.D. and Kathryn Schroader, M.S.

Department of Medical Physics, King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh, Saudi Arabia

This excerpt was created in the absence of an abstract.

Measurements of central ray tissue-air ratio (TAR) for 60Co for a large number of irregularly shaped fields typically encountered in the cancer clinic were compared with TAR values either calculated by the Clarkson method using scatter air ratios or obtained from tables of square field data using the area/perimeter approach. Irregular field shapes included the "L", pentagonal, rectangular, upper and lower mantle and split fields. Agreement between both calculational methods and direct measurement is within ±2% if careful attention is given to central ray position as affected by the proper use of off-centre ratios in air. In the case of 10 MV X rays, eight randomly chosen irregular fields again yielded an average agreement between both methods of calculation and experimental data of less than 1% and an agreement of less than 0.5% between the A/P and Clarkson calculations.

Irregularly shaped fields are commonly used in radiation therapy in order to provide differential dose between healthy and tumorous tissue. Simple geometrical shapes commonly encountered include "L" shaped fields, split fields, pentagonal shaped fields and mantle fields. Other shapes such as yoke fields, lateral skull ("helmet" shaped) fields as well as other possibilities also commonly occur. Frequently, these fields are used in parallel opposed directions although single field protocols may be used in some instances. In any case, it is necessary to calculate a treatment time in the case of 60Co or monitor units (MU) in the case of 10 MV X rays.

Received for publication May 1, 1978. Revision received October 1, 1978.




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