In any trial designed to assess the value of high pressure oxygen in radiotherapy it is essential that overall treatment time, fractionation and the quality of field planning and treatment are as nearly as possible identical for both the hyperbaric and the control series. Therefore, we must strive to achieve a standard of beam direction inside the tank fully comparable in accuracy with present methods outside the tank, since it would be wrong to lower the standard of treatment for those patients treated as controls at atmospheric pressure. This paper describes preliminary attempts to develop a system of beam direction that fills these requirements when used in conjunction with any transparent high pressure oxygen tank, such as the Vickers tank shown in Fig. 5. The restrictions imposed by the presence of the pressure tank largely determine the general form of the beam direction system: (1) Early experience showed that most hyperbaric therapy patients tolerate full immobilisation by conventional plastic she...