Physical considerations of treatment planning.

Abstract

A discussion is presented of the special problems encountered in the use of a pressure vessel in oxygen barotherapy which are not encountered in ordinary megavoltage treatments. The pressure vessel is constructed of a light alloy, with Perspex portals through which the x-ray beams can be directed. It was designed specifically for use with the Metropolitan Vickers Linear Accelerator. The longitudinal axis of the pressure vessel must always lie perpendicular to the plane of rotation of the accelerator x-ray beam; this means that all x-ray fields must be in a transverse plane. In selecting the field distribution, the angles have to be chosen so that the x-ray beams avoid the metal members between adjacent Perspex pontals. All treatments are planned for a focus-tumor distance of 100 cm, so that when the tumor is positioned at the isocenter of the linear accelerator, ali fields pass through the tumor whatever the angulation of the beam. Shielding has been found necessary in many cases to provide odd-shaped fields giving protection to eyes, spinal cord, or other vital structures. For this, a special shielding device was designed which fits into the end of the accelerator housing. The device is a Perspex plate with radialmore » slots which open out to a larger diameter near the periphery for loading shielding blocks. The distance between the Perspex shielding plate and the end of the accelerator head is oniy 2.9 cm. Since the shielding device could not be allowed to protrude beyond the end of the accelerator head, the shielding blocks had to be made of a high density material and a readily available Cu-W alloy was chosen; 2.9 cm of this alloy permits the transmission of only 8% of the direct 4-Mev x-ray beam. The presence of a Perspex portal in proximity to the skin results in a loss of the so-called skin-sparing effect of m megavoltage x radiation. Secondary electrons from the Perspex span the gap between the Perspex and the skin and raise the skin dose, the increase depending upon how close the Perspex is to the skin. The effect of the proximity of the Perspex on the skin dose was determined over a full range of field sizes and a full range of Perspexskin distances. Considering a field size of 100 cm/sup 2/, the normal skin dose of 19% of peak dose is raised to 27, 40, 59, and 85% for Perspex-skin distances of 20, 15, 10, and 5 cm, respectively, and this factor has to be taken into account when determining skin dose in oxygen barotherapy. Since treatment in the pressure vessel generally results in Perspex-skin distances of 15 to 10 cm, the skin dose for a 100-cm/sup 2/ field will be between 40 and 59% of peak dose and the dose at 1/2-mm depth, which may be responsible for skin reaction, will be between 63 and 76% of peak dose. (BBB)« less