Physical Factors of Low Voltage “Contact” Roentgen Therapy

Abstract

MOST of the improvements in radiation therapy have been instituted with the aim of increasing the dose to the lesion without causing any harmful effects to the healthy tissue. In irradiating surface conditions and those accessible through the cavities of the body or through operative procedure this object may be attained by using rays at shorter skin-target distance and of longer wave lengths, that is, of less penetrability, thereby reducing the dose to the underlying normal tissue. Until recent years, before the advent of shockproofing, it has not been possible to reduce materially the target-skin distance with safety. The so-called Grenz-ray tube was devised by Bucky to permit operation at from 10 to 12 kv., using an 8 to 10 cm. distance, yet its use is limited to the treatment of only the most superficial layers of skin as the depth dose is only about 15 per cent at 1 mm. depth.2 Furthermore, the necessity of making the x-ray tube wall extremely thin to minimize inherent absorption makes the apparatus very delicate for general use. About six years ago Chaoul and his co-workers (1) devised another method of low voltage irradiation. They used the following factors: approximately 50 kv. constant potential, a filtration equivalent to about 0.2 mm. Cu, and a target distance of from 2 to 6 cm. The short target distance was made possible by the development of a new type of x-ray tube in which a gold-plated nickel target is mounted at the extreme end and serves as both target and filter. Measurements using this type of tube have been described by Mayneord (3) in some detail, and also by Pendergrass, Hodes, and Garrahan (2). More recently another type of low voltage tube with greatly reduced inherent filtration has been devised. Inasmuch as this type has received considerable attention in this country, we have made a study of its physical and technical characteristics. The construction of both types is shown in Figure 1 for comparison. Equipment.—The measurements were carried out on seven constant potential, single valve, low voltage contact therapy units, all of the Philips type. The negative terminal of the generator was grounded, while the other was connected through an insulated, shielded cable to the anode of the x-ray tube. One side of the filament, as well as the outside metal shell of the x-ray tube, was grounded and the x-ray tube was cooled by means of forced air circulation. It will be observed that in contrast to the Chaoul type tube described by the other investigators this x-ray tube has a very low inherent filtrationas its useful beam does not pass through the target. Measuring Instruments.—A 12.5 cm. sphere gap calibrated with one side grounded was used to determine the crest voltage. A Taylor type high voltage resistance unit (4) was used to measure the effective voltage, and the ripple was checked strobographically, using a photo-electric cell densitometer to analyze the film.