Quantitative measurement of the increase in dose adjacent to voids during irradiation of a lucite phantom by high-energy electron beams.

Abstract

The perturbation of dose distribution caused by small air cavities during electron-beam therapy is a well known phenomenon (1). When a void is present in a volume of material irradiated by highenergy electrons, more electrons will be scattered into the void than will be scattered out. This results in an increased dose in the material near the surface of the void on the side opposite the incident beam. This dose is greater than would be measured if the complete segment of material bounded by the edges of the void in the dimension parallel to the beam and of infinite dimensions perpendicular to the beam were removed to form the void. The increase in dose, which is a complex function of the dimensions of the void, the depth of the void below the surface of the material, and of the energy of the electrons, may result in severe radiation reactions during therapy, particularly in the treatment of tumors of the head and neck. In this investigation a Lucite phantom was constructed which allowed the diameter of the void to be varied from 0 to 10 inches. The buildup layer between the surface of the phantom and the surface of the cavity on the side nearest the incident beam was 1/4 inch. Dose measurements were made in the material at a depth of 1/4 inch from the surface of the void on the side opposite the incident beam. The dimensions of the Lucite phantom were 25 X 30 em in the plane perpendicular to the beam and 25 em parallel to the beam. The electrons were produced by a Brown Boveri Asklepitron 35 betatron. The field was 12 X 14 ern. All measurements were made with a Philips universal dosimeter. The centers of the field, the phantom, the void, and the ionization cavity of the dosimeter were aligned along a single axis. It was shown that a void having a diameter of 10 inches closely approximates a void of infinite diameter. The ratio of the dose measured with a void having a diameter d to the dose measured with a void having a 10-inch diameter when multiplied by 100 was defined as the percentage void dose. The percentage void dose is shown in Figures 1-3 as a function of the diameter of the void for void thicknesses of 1/2, I, and 2 inches, respectively. Each figure includes four curves corresponding to electrons having energies of 10 MeV, 15 MeV, 25 MeV, and 35 MeV. These figures show that for certain critical combinations of energy, void thickness, and void diameter, the percentage void dose curves exhibit sharp peaks, the maximum of which in one case reaches approximately 170 per cent.