Techniques by which the quantitative anatomical data inherent in a CT scan can be directly used in treatment planning are described. The correction algorithms used in the RAD-8 system, based on an effective path length, have been extended to a pixel-by-pixel approach. By calibrating the X-ray transmission CT scanner in terms of electron densities (electron cm-3) inhomogeneity corrections may be made automatically.

A method for performing inhomogeneity corrections using a generalised Batho equation is described. The corrections are performed using the quantitative anatomical data produced by a CT scanner. The generalised Batho equation has been implemented on an EMIPLAN 7000 interactive treatment planning system. Details are given of the acquisition of the required data from the CT scan and from the existing stored beam data of the therapy machines.

In order to assess the role of computed tomography (CT) in radiotherapy treatment planning, tumors were localized by conventional techniques and with CT using an EMI CT5005 scanner. CT scans were obtained under conditions simulating radiotherapy. A comparison between the two localizations was made, and detailed results are given. Forty-seven of 123 patients had their treatment plan altered, implying that, provided information is obtained and used correctly, CT can play a significant role in radiotherapy treatment planning.

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