1992 Volume 4 Issue 1 Pages 15-23
Proton beams have the physical characteristics that provide dose distribution superior to that of X-rays, allowing precise delivery of high radiation doses to the tumor while avoiding irradiation of adjacent normal tissue. Because of this quality, in carrying out proton radiotherapy, the special effort in obtaining a high precision of field placement is warranted. Accordingly, we have developed a localize-and-verify system consisting of a fluoroscopy and digital image processing unit. With this system field localizations have been performed for each treatment, and if there were any difference in position between the current field and the field previously determined, it was corrected and the distance correction was measured. In this study an accuracy of the field localizations was evaluated in 788 localization attempts for 52 patients (53 tumors sites). The average distance correction for all localization attempts was 3.5 mm. Field positions were corrected by<3mm in 60.7%, 3-5 mm in 16.2%, and > 5 mm in 23.1% of the radiation treatments. Various risk factors were evaluated that may influence the accuracy or reproducibility of field localization. Among these, the most significant factors were the site of irradiation and patients age; the frequency and amount of corrections increased with increasing age and in movable tumors of the digestive tract. The sex, use of an immobilization device, and beam directions appeared to be less important. From these results, it is strongly emphasized that in certain tumor sites the daily localization could only offer precise deposition of high doses to the target volume. This can also be applied to conventional irradiation, especially when attempting definitive irradiation with curative intent.
