IV 不均等物質による線量分布の変化(放射線治療時における組織内線量分布)(第27回総会)

Abstract

The presence of tissues of different composition or density, such as bone, lung and air cavity, influences the dose distribution. The distortion of dose distribution depends on the quality of thd radiation, the size of the field, and the size and composition of heterogeneities. For cobalt 60 gamma rays and 4 MV x rays, corrections are negligible for the reduction of the dose due to the interposition of bone and due to the lack of dose build-up beyond an air cavity in cavity in clinical practice. In this parer, the methods of calculation of the dose distribution in the treatment of thoracic lesions with cobalt 60 gamma rays and 4.3 MV x rays used in the Cancer Institute Hospital are described. (1) From transverse tomographs and radiographic transit dose measurements, it is possible to determine the thickness and density of the lung and the water equivalent thickness of body to be traversed by each pencil of the beam considered. (2) When the density and thickness of lung in the pass of the beam are determnied, radiation dosage can readily be calculated at any point in the thoracic section by the following correction method. The correction factor (that is, actual/water phantom dose) can be expressed as [numerical formual] where S is the scatter correction factor, μe is the effective absorption coefficient in water, l is the passed thickness of the air-filled lung, and ρ is the average density of the air-filled lung. In general, the scatter correction factor S may be expressed as [numerical formual] where t is the depth in water-equivalent tissue beyond the lung. When t =0,k=C and when t>0,k is unity. B and C are given by B=0.138-0.243ρ+0.105ρ^2 C=0.975+0.016ρ-0.086ρ^2 The effective absorption coefficient can be expressed as μe=0.0656-0.011 logA where A is the field size at the surface. (3) For a mediastinal lesion treated with any angle of rotation and for a tumor locatea in the lung treated with rotation greater than 240 degrees, the effect of lung on the overall treatment plan may be neglected for clinical purposes, provided the absolute dose at the center of rotation is corrected for the presence of lung tissue.