Biophysical Foundations of Supervoltage Roentgen Therapy
During the past eight years, the routine 200 kilovolt deep therapy roentgen machines have been supplemented by supervoltage roentgen therapy apparatus. Some of these are operated at voltages in excess of one million volts. There are about twenty-five roentgen therapy transformers for voltages of 400 KV in the United States and ten machines operating at approximately one million volts. Most of the latter are rather bulky and expensive, some even requiring special buildings.
Different methods of generating supervoltages are employed as, for example, an induction coil (Memorial Hospital, New York City), cascaded transformers with valve tube rectification for the production of constant or pulsating high potentials (Mercy Hospital, Chicago, 800 KV) (Fig. 1), (Charles T. Miller Hospital, St. Paul, 1200 KV) (Fig. 2), the more recent Sloan resonance oscillator (Crocker Cancer Institute, New York City, 1000 KY) (Fig. 3), and the Van de Graaff static generator (Huntington Memorial Hospital, Boston, 1250 KV) (Fig. 4).
Different types of x-ray tubes also are being tried with these high voltage equipments—-cascaded glass tubes (Fig. 1), Crane-Lauritsen porcelain tubes (Fig. 2), cylindrical porcelain tubes (Fig. 4), or the Sloan tube which is an integral part of the Sloan oscillator (Fig. 3). Only those x-ray tubes for voltages up to 400 KV are sealed off after the gas is evacuated from them. Tubes of higher voltages must be evacuated continuously which necessitates the constant attendance of technically trained personnel as may be appreciated by figure 3 which shows the battery of pumps in connection.