Dosage estimation for conformation radiotherapy applied to cervical cancer was discussed. Field size and shape in conformation therapy was freely change-able according to the shape and size of tumor. When the same size and shape of the field for therapy by Co 60 gamma ray was applied to each case, tissue-air ratio TAR was given by an equation,
T=-0.00326
d+1.035, where
T is TAR and
d an average depth.
The concept of tissue-air ratio could not be applied to dose calculation in moving field radiotherapy by 6 MV x-rays, because the quality of x-rays emitted from a linear accelerator is rather different from that of γ-rays of the telecobalt unit. Therefore, the tissue-reference output ratio was defined instead of tissue-air ratio in the form of:
Tissue-reference output ratio=Tissue dose/Output in a field of 10×10cm
The output in a field size of 10×10cm is presented in the monitor chamber which is watched during operation. When the conformation radiotherapy was applied to 20 cases of uterine cancer with fields of the same size and shape, tissue-reference output ratio
R was given by an equation,
R=-0.0026
d+1.048
where
d is the average depth. The difference of calculated dose from measured one using a Mix D phantom was 3%, and the error is negligible in clinical practice.
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