Radiation dosage planning and dosage calculation.

Abstract

In radiation therapy it is generally appreciated that tumor effect is dependent on tumor dose, and on the total time during which this dose is administered. For a particular patient and series of treatments, determination of tumor dose requires the following procedures: 1. Construction of a life-sized cross-sectional diagram of the individual at the proper level. 2. Indication on this of the exact position, size, and shape of the tumor, in so far as possible, and of the portals of entry of each field. 3. Application of isodose charts for each field to obtain the dosage contribution in various parts of the tumor—not the central point alone —and for any other points of concern, such as skin portals or vital organs. 4. Addition of contributions from all beams to obtain total doses, for all points in question. Frequently before a series of treatments is undertaken, several different dosage schemes need to be studied, to see how the desired tumor dose can best be administered. It is evident that the use of depth-dose tables alone, which supply only the doses along the axes of the beams, can never give really satisfactory tumor dose data. If all the axes pass through the same point, summing up doses from depth dose tables will give the dose at this point, but will tell nothing about dosage at any other place. Few radiologists feel that they have time to do satisfactory dosage planning and calculation on more than a small number of patients. Indeed, this should not be the task of the radiologist alone, but of the radiation physicist in collaboration with the therapist. Departments of radiation physics in the United States are not as rare as they were a few years ago, but they are still the exception, even in hospitals with large x-ray departments. This is partly because of the lack of adequately trained physicists with interest in radiation therapy, and partly because of a reluctance of administrators to add personnel to their departments, or of their failure to realize the value of such personnel. A considerable number of radiologists have circumvented the first difficulty by finding young men or women (college graduates, usually with a major in physics) who are interested in entering this field, taking them into their departments and gradually working out their own procedures. This in-training, supplemented by visits to established radiophysics departments, has produced some very good results. Little formal training in radiation physics is offered in the United States, but it is becoming increasingly possible to arrange for informal “apprenticeships.” Three to six months spent in a busy radio-physics department should provide good basic training; a period less than three months can seldom be considered satisfactory. A person with this minimum training is, of course, not a radiation physicist, but he is well on the way.