Dose distribution in grid therapy with 15- to 33-Mev electrons.

Abstract

The physical characteristics of high-energy electron beams which make them attractive for therapeutic application have been described (1, 2) and the following advantages have been emphasized: the wide range of penetration, which is mainly dependent on electron energy; the ease of obtaining practical field sizes; the low absorption in bone which is characteristic of high-energy radiation; the possibility of reducing the integral dose by using only one or two fields. The absence of a sizable build-up under the surface in electron therapy leads to a relatively high skin dose in contrast to kilocurie cobalt teletherapy and megavoltage x-ray beams. Although, in our experience, skin reactions normally do not limit the ability to deliver the prescribed tumor dose, they may be a source of discomfort to the patient which one might want to eliminate. The use of grids in connection with high-energy electrons, to achieve protection of the skin and healthy tissue overlying the tumor, has been reported previously (1, 3, 4, 5).Contrary to the situation prevalent when grids are used with x-ray beams, a completely uniform distribution over the entire area of the beam is obtained at a relatively shallow depth, due to scattering. This report describes a study of the effect of two grid parameters on the dose distribution: 1. Variation of grid ratio: 60% closed/40% open, 50% closed/50% open, 40% closed/60% open, all for an 8-mm. diameter hole size. 2. Variation of hole size: 5-mm. hole, 8-mm. hole, 1.l-cm, hole, all for a 60% closed/40% open grid ratio. The grids were made by drilling a hexagonal array in a lead sheet 1.25 cm. thick. The experimental results were obtained by measuring the density of film embedded in a unit density Presdwood phantom exposed through various grids to electrons of 15, 25, and 33 Mev. A study of the central- axis depth-dose distributions leads to the following conclusions: 1. The depth of the non-uniform dose distribution in the phantom increases with hole size; however, the grid ratio, within the limits investigated, has little effect. 2. A build-up of the dose occurs behind the closed areas of the grid, leading to a fairly fiat maximum in the dose distribution. The depth at which this maximum occurs increases with hole size, and its width is greatest with a 40% closed/ 60% open grid ratio. Thus it appears that there is no optimum value for the grid ratio and hole size, but that a definite combination should be selected, based upon the depth and thickness of the volume to be irradiated.