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Use of Megavoltage Therapy in Cancer: End-Results

Radiology1962Vol. 79(2), pp. 181–189

Citations Over TimeTop 23% of 1962 papers

Justin J. Stein, Richard E. Ottoman, Edward A. Langdon, William A. Gore

Abstract

Although megavoltage radiotherapy has been available for many years, it has come into relatively widespread use only during the past decade. The increasing popularity of cobalt 60 has stimulated interest in this type of therapy. Megavoltage equipment in this country consists largely of telecobalt-60 sources, one- and two-million-electron volt resonant type generators (G. E.), the two-millionvolt Van de Graaff, the betatron, and the linear accelerator. There is also one 70-Mev synchrotron in medical use. In England and Scotland, where there has been a great deal of interest in linear accelerators, the trend is toward the use of 4-to 6-Mev linear accelerators. Cesium-137 teletherapy units are not considered in this discussion, since they are equivalent to only 660 kv of energy. Because of their low output and the large penumbra, it is not believed that they will ever become very popular in this country in spite of the long half-life of the cesium source. It is frequently stated that we have progressed about as far as possible with radiotherapy in the treatment of cancer. If one asks the question, “Are we doing as well as we can with what we have to treat cancer?” the answer is a definite “No.” There is much to learn about radiobiology, radiation physics, the life history of cancer, and the indications and contraindications for the combined use of radiotherapy with chemotherapy or surgery. Unfortunately, some of the deficiencies which result in the radiation therapy failures are as follows: improper positioning of patients without port films for accurate localization of the tumor; under-dosing the peripheral edges of the tumor; failure to maintain the nutrition of and to provide proper general care of the patient; and the unwillingness of some radiotherapists to take the calculated risk of a small percentage of complications when an adequate dose for a localized tumor is indicated. These are but a few of the many factors involved in the inadequate treatment of the cancer patient. In cancer of the lung, for example, when a treatment field is outlined on the posterior chest by visual means and anatomic landmarks only, the chance of not including a portion of the tumor in the field of irradiation is markedly increased. The taking of port films eliminates guesswork and provides for accurate inclusion of the total lesion in the treatment field. When two opposing fields are used, and only the tumor dose for the center of the field is given, a deficiency of 25 per cent or more in the amount of radiation received at the periphery may result. For example, with our technic—cobalt 60, 75 cm. SSD, 15 × 15-cm. field—when the dose given to the center of the tumor is 100 r, only about 75 r or less is received at the lateral edges of the field. In other words, if maximum and minimum tumor doses are not stated, the peripheral portions of the tumor may be under-treated. When a single field of 6 × 6 cm. is used, the desired dose is delivered to the middle 2 cm. only, with a rapid fall-off. This inequality in dosage can be prevented by using multiple fields with reference to appropriate isodose curves, by using wedge filters, or when small fields are used with cobalt 60, by opening the diaphragm to an appropriately larger size and protecting the area not to be irradiated.

Citations Over TimeTop 23% of 1962 papers

Abstract

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