II.A点線量と線量分布計算法(高線量率腔内治療時の技術的諸問題)

抄録

Recently remotely controlled high dose rate brachytherapy apparatus is diffusing rapidly. We can irradiate several hundreds rad in a few minutes by appling this equipment. Therefore a slight error of irradiation time produce decrease of cure rate or increase of radiation injury. So it is necessary to compute the dose and dose distribution with accuracy and speedy before treatment. In this paper, we investigated technical problems in the calculation method. 1. There are two method to compute a position of source. The first method is two perpendicular radiography. In this case, we must set X-ray beam angle carefully to get a accurate data. The second is stereoradiography. There is a possibility that error of Z direction rise. Therefore we must measure the place as precisely as possible. 2. The situation of point A is fixed by rotation of co-ordinates or construction. 3. The formulas of dose calculation have been reported by many authers. But we can obtain similar numerical value by using Misberger's, Ponnunni's, Morimoto's formula. 4. Compensation of capsule absorption can be computed using energy absorption coefficient. But gynecological disease, error is so small that it is not necessary to correct capsule absorption. (However, it is influenced by shape of source capsule.) 5. In order to obtain the dose at point A, the simple method is being used. We can compute the point A dose from tandem source using distance between ring and tandem source, and tandem intensity. The dose at point A from ovoid source is calculated using a chart method or distance between point A and ovoid source. We could obtain the dose at point A within 10% error by using these method. 6. The value of dose rate is most essential quantity in dose calculation. So we must measure it accurately. 7. In over-ride system, tandem source moves slowly. Therefore it is necessary to compensate moving time. 8. Moving distance of tandem source ought to be lesser than 1.5cm. Further more, we examined the relation among intensity of sources, ovoid space and distance between tandem and ovoid source.