In recent years there has been a growing interest in all forms of rotational or moving-field therapy, and many different types of therapy machines designed for this kind of treatment have become available. To the medical radiation physicist, the dosimetry of rotation therapy has presented a number of interesting problems, and much useful work has been published on the basic data of dose distribution and dosage calculation. All of the currently popular methods of dose determination—except direct measurement on the patient, which, for good reasons is rarely popular—assume that the absorbing medium is homogeneous and water-equivalent. For many purposes, this may be adequate, but for irradiation of certain parts of the human body, such as the pelvis or the thorax, it is quite apparent that this assumption breaks down badly, especially at conventional x-ray energies. Even with cobalt-60 radiation, discrepancies of 20 to 30 per cent between calculated and measured doses are found in irradiating such heterogeneous tissues. It therefore becomes imperative to be able to assess the magnitude of such discrepancies in some simple fashion. I propose to deal here with this particular aspect of rotation dosimetry. I shall describe our attempts to work out a reasonably simple method which takes account of the inhomogeneities of the medium. At the London Clinic of the Ontario Cancer Foundation we have been using rotation therapy with the Eldorado Cobalt-60 Unit since early 1955. At the outset we decided to adopt the concept of the “tumour-air ratio” (as described by H. E. Johns et al.) as the basis of our rotation dosimetry. The ratio is defined as follows: where Dc = the dose rate at the tumour (centre of rotation) Dca = the dose rate in air at the centre of rotation F = the distance of source to centre of rotation t = the thickness of absorber between source and centre of rotation B = the back-scatter factor for the field size and energy in question P/100 = percentage depth dose at depth t From graphs of Rc versus t, with field size as parameter, one obtains all the necessary information to determine the dose at the centre of rotation; for cobalt-60 therapy irradiation this is often sufficient on the basis of known dose distributions and their lack of sensitivity to changes in the shape and size of the absorber. This holds strictly only for water-equivalent tissue. Our approach to the problem of correcting what may be called the crude tumour-air ratio values for the effects of non-water-equivalent tissues is based on the use of transit-dose values. Our original method, which is now superseded to some extent by simplifications which I shall discuss presently, has been described in Acta Radiologica, January 1956.
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