Automation of Radiation Treatment Planning – VI. A General Field Equation to Calculate Percent Depth Dose in the Irradiated Volume of a Cobalt 60 Beam

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Abstract
Data on dose values were collected throughout a water phantom at different source-surface distances [SSD] for different field sizes and for beams entering the phantom at different angles. The basic form of the model which was derived originally for 80 cm SSD held quite well in principle for any SSD. The general equation used 2 sets of constants, 1 set for depth less than 2 in. and the other for depth of more than 2 in. This division was introduced to adjust for electron build-up over the initial depth. While the relations described in the mathematical model hold for both sections of the field, a single set of constants fitted to all the data did not yield an accuracy of dose determination comparable to dividing the field in 2. The equations were next modified to give precise per cent of depth dose estimates for any off-axis points and make proper corrections for surface curvature. The final equations determine the per cent depth dose in the irradiated target volume of a 60Co beam corrected for angle of entry, SSD and curvature of the irradiated medium with an accuracy that falls well within error of measurement.