Electron dose distribution due to the material and fabrication of the beam limiting device

Chu, S S; Choi, K H; Choi, B S; Lee, D H

Abstract

Electron beams, because of their sharp dose fall-off and rapid dose build up, have come to be used frequentlyfor radiation therapy, however, have defects that their constriction of isodose curve at depth. These defects ofelectron beams would be some compensated by electrom cones which were made of the most adequate scatteringmaterials according to field sizes and electron energy. We observed the scattering distribution and radiationabsorption of various materials; ie. , lead, brass, acryl, steel, aluminium, copper etc, for 8, 10, 12 Mevelectron beams. By these data, we designed and made of election cones which would be delivered the most usefuldose distribution in tissue according to electron cone would be summarized as follows, 1) Shielding plates,consisted of 1cm Al, 5 mm Pb, 5 mm Al and 1cm Acry., absorbed of 12 MeV electron beam completly and were veryeffective to minimized of bremsstahlung. 2) Election cones which their walls were composed of 1g/cm2 thickness oftransparent materials were convenient to set up and collimate on tumor fields. 3) In small field sizes(10-50cm2),they have favourable dose distribution to use combined wall materials as acryl and Al plates. 4) In medium fieldsizes(50-200cm2), the dose distributions of electron cones, nothing to do with their composed materials, wererelatively satisfactory as 2-5% flatness ratio. 5) Designed the cones of 200cm2 fields above, it has better dosedisibution to make with brass then acryl plates. 6) It was no good to use the lead plate with relatively largebremsstrahlung and materials contained elements to be radioativated. 7) For increasing of surface dose withoutenergy loss, the 40-60 mesh wire netting attached to cone edge are widly used. 8) The materials used in electroncones such as lead, acryl, aluminium, brass etc., were easy to acquire with reasonable price and tractable.