Isodose Curves in Non-uniform Phantoms

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Abstract
A method of using an isodose plotter to produce isodose charts in a water phantom containing bone, lung-equivalent material, or air cavities is described. For Co60 gamma rays and 4 MV X radiation, standard isodose charts can be modified to fit the experimental results by the use of a simple set of rules to an absolute accuracy of about 2%. Some isodose charts for conventional quality radiation in non-uniform phantoms are also discussed, but here the effects produced by the non-uniformity are more complex than with megavoltage radiation and corrections derived from simple rules are of limited value. Rules for modifying megavoltage isodose charts are used to prepare some treatment plans for real body sections, and compared with conventional treatment plans where it is assumed that the body section consists entirely of soft tissue. It is shown that there is probably no clinically significant difference between conventional and corrected dose distributions, although the absolute tumor doses found by correcting for differential absorption in lung or bone are significantly different from conventionally stated doses.

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