Supervoltage Radiotherapy

Abstract

The program of nearly every annual meeting of the several radiological societies of the United States has in recent years included papers and often symposia devoted to supervoltage radiotherapy. The increasing availability and relatively low cost of cobalt 60 has led to an enormous increase in the use of this particular form of supervoltage (roughly equivalent to a 3-million-volt x-ray generator). With few exceptions, those who have presented material have merely given early impressions, since not enough time had yet elapsed for conclusions to be drawn in terms of survival rates. In this issue of Radiology there is included a Symposium on supervoltage therapy. The essayists were chosen from those few in this country who have had many years experience with this form of treatment. Their statements and the conclusions which they reach should be carefully studied. The writer, who served as moderator of the Symposium, has had but short experience with a 2-million-volt x-ray machine and a cobalt teletherapy source, but in three years time has seen enough material to learn that there are some definite advantages in the use of supervoltage and some very serious disadvantages. In fact, advantages with regard to the patient may in some instances constitute a hazard as regards proper treatment of a malignant lesion. It is a matter of general agreement that there is no biological advantage in radiation in the 1 to 3-million-volt range as compared with 200–250 kv therapy. Tumors which are resistant to 250-kv radiations will not be sensitive to radiations of greater energy, at least at the levels now available for therapy. On the other hand, there are some physical advantages, such as increased depth dose, decreased side-scattering, and lessened bone absorption. In lesions surrounded by bone the decreased absorption makes dose calculation more accurate. The increased penetration allows for greater ease in treating deep-seated lesions, and the decreased side-scattering aids in sparing normal tissues. With respect to the patient, supervoltage radiation seems to produce less systemic reaction, probably due to decreased integral dose. There is less radiation sickness, and in general less hematopoietic effect. Because the point of maximum dose lies some millimeters below the surface of the skin, one of the advantages of supervoltage is that the usual skin effects are markedly decreased. This lack of reaction is not without its drawbacks, however, since often with 250 kv the skin reaction is a limiting factor, preventing overdosage of underlying structures. In addition, late fibrosis of subcutaneous tissues can occur where the limiting superficial reaction is lost and overdosage of this tissue is unwittingly given. In general, supcrvoltage therapy is easier to plan than is therapy in the ordinary range. Often fewer portals can be used and dosimetry is thereby simplified.