Difference Between CT Images for Radiation Therapy and Those for Diagnostic Purposes
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The CT image must be taken in a position identical to that to be used at time of treatment. The location and extent of organs in the body differ greatly, depending on whether the patient is in a standing or lying position (Takahashi 1969; de Winter 1979; Geise and McCullough 1977; McCullough 1978), and even in the lying position, it is necessary to distinguish between supine or prone. CT or other roentgenograms used for making the treatment plan must be taken in the position identical to that to be used at the time of treatment. For example, in carcinoma of the uterine cervix, the relation of the positions of the lesion and surrounding urinary bladder and rectum at the time of external irradiation differ completely from the tomographed positions in which an adapter of the after-loading tandem and ovoid (TAO-type) have been inserted into the vagina or uterus and the remaining space completely filled with gauze (Fig. IV.2). Thus, two CT examinations for one patient suffering from cancer of the uterus must be conducted, one for external irradiation and the other for intracavitary irradiation. The dose of the lesion and that of the surrounding healthy tissues must be calculated respectively from the two CT images obtained. A 5% Gastrografin is used as contrast medium for the urinary bladder and the large and small intestines. It is necessary to administer the amount of contrast medium as closely as possible to that to be administered at time of treatment.