Two Years' Experience with 400 kv. Constant Potential Deep Therapy

Abstract

THE introduction of deep therapy machines running at voltages higher than the conventional 200 kv. has brought about a varied reaction among radiation therapists, a reaction ranging from enthusiasm to apathy and to complete disapproval. It is not the purpose of this paper to argue the merits of higher voltages. At the time of writing, the author's experience extends a little over two years. This experience is presented and certain conclusions are drawn which at the present time seem conservatively justified. The machine I have used is a 400 kv. constant potential deep therapy using a Coolidge tube with a built-in filter of 2 mm. of copper. In addition to this, a Thoraeus filter (0.93 mm. of tin plus 0.27 mm. Cu) plus bakelite and an iometer chamber have been used constantly and exclusively, giving a filter having the equivalent of 6.5 mm. of copper. The machine has run at all times at 400 kv. constant potential, 5 ma. and 80 cm. distance. Under such conditions the half value layer is 5.4 mm. copper and the intensity is 14 r per minute (Glasser). The absorption curve (Glasser) in tin is as follows: These factors have remained constant since the machine was installed in February, 1935, in the St. John's Hospital, of Cleveland, Ohio. At the time of installation the protracted method of application either as Coutard described or as some form of “modification” seemed to be the vogue. The author had attempted early to follow the basic principle of Coutard, and even though the technic was not the same, it seemed, nevertheless, that clinical results were definitely improved. With this experience with 200 kv. in neck tumors, it seemed quite logical to predict that a better result would be obtained in any malignant tumors no matter what the location, and hence from the start all patients treated with a higher voltage were given small daily doses over a long period of time. The immediate results in the first few cases treated substantiated this idea and encouraged the continuance of small daily doses. In other words, not only has our physical factor remained the same for two years for the 400 kv. constant potential machine, but the method of application, for all practical purposes, has also remained the same. Even though the time factor of two years is relatively short, the experience is valuable because of the absence of variation in these two important factors. The daily dosage is low—seldom over 140 r to anyone field. The fields are laid out with two definite ideas in mind: one, to cross-fire the lesion wherever possible, and two, to give daily dosage to the tumor without using the same field or fields each day. This idea can be easily worked out in all the common sites; in the pelvic cases, for instance, by using two fields front and two fields back, treating two fields every other day. The reactions other than in the skin are surprisingly minimal. The majority of patients go through the course with no serious complaints.