Central axis depth doses for a constant source-tumour distance

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Abstract
Some aspects of central axis depth dose calculations for treatments with a constant source-tumor distance are considered. Errors of about 2% for 60Co gamma rays, may be introduced when using tissue-air ratios and tissue-phantom ratios in centain central axis dose calculations. By introducing a modified definition of tissue-phantom ratio, these errors are eliminated. Measured values of modified tissue-phantom ratios are presented for 60Co radiation for square fields ranging from 5 x 5 cm to 20 x 20 cm and for 6 MV (million volts) X-rays for fields ranging from 4 x 4 cm to 25 x 25 Cm. Both sets of values extend to a depth of 30 cm. From these and other measured values, tables of central axis depth doses have been calculated for SSDs from 60 to 80 cm in 1 cm intervals for 60Co gamma rays and for SSDs from 75 to 100 cm in 1 cm intervals for 6 MV X-rays. By comparison between these calculated depth doses and those measured, the methods of calculation described in this paper are shown to be accurate to within 1% of the local dose.

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