Consideration of dose distributions within a target volume in a conformation technique

In radiotherapy, the deliverly of an accurate absorbed dose to target volume in a patient is fundamental strategy. The ICRU Report 29 recommended to achieve the accuracy of +-5% to tumor points for certain type of tumor. Ideally the accuracy should be accomplished all over the target volume rather than a tumor point. The purpose of this paper is to describe an analysis of dose distributions in a target volume. The dose distribution obtained with a conformation technique were used and compared with one of the other technique. Newly developed software on a commercial RTP computer system (CMS, Modulex) calculated dose spectrum, which is similar to the histogram in a target area shown on the ICRU Report 29, in any regions predefined at planning procedures. The extent of dose cocentration to the target volume, the uniformity of doses in the target volume and doses given to normal tissues and to particular organs were obtained by utilizing this dose spectrum. Selected clinical examples were the treatment of cancer of the hypohysis, the maxillary, the esophagus and the pancreas. The conformation technique showed better results than the others with regard to the dose concentration in all cases. As for the dose homogeneity, no differences between two irradiation techniques were observed. Planning procedures in a conformation radiotherapy is not complicate as compared to another techniques, therefore the conformation technique is suitable method for fitting high dose region to the target volume. Use of rotational blocks with the conformation therapy, which are used for selective protection to organs at risk, accomplished the purpose of dose reduction at the organs, however the dose uniformity in a target volume becomes slightly worse. Carefull considerations will be required to avoid the poor dose uniformity in a target volume when a technique for selective protection is planned with another techniques. (author)