Some Observations on X-Ray Treatment Cones

Abstract

In recent years, considerable attention has been given to the matter of providing the radiotherapist with more reliable high-tension generating equipment and x-ray tubes, more accurate physical measurements of the x-ray output, and more trustworthy evaluations of skin dose and depth dose data. With respect to treatment cones, however, it would appear that some improvement is still necessary. Manufacturers have made available a large variety of cones and applicators which in many cases are being used in conjunction with x-ray equipment without full realization of their individual limitations. In particular, the practice of supplying accessories which attempt to provide for all types of therapy (superficial, deep, and cavity) with the one high-voltage therapy unit has resulted in the production of some treatment cones which do not conform to sound physical principles. This report is based on experience gained during the routine calibration of all types of x-ray equipment in Australia. With many of the points raised the radiological physicist is already familiar, but it is felt that emphasis should be laid on the need for more careful design of treatment cones. Quimby and Marinelli (1), Jacobson (2), and Silverstone and Wolf (3) have all shown the necessity for the photographic examination of the field of a treatment cone. During the initial calibrations of x-ray equipment by this laboratory a routine check by photographic and ionization means is made of the fields produced by individual cones, and it is our impression that little has been done in recent years to improve the general design of cones. Types of Treatment Cones In general, treatment cones are provided for attachment either to a master cone or directly to the tube housing. Cones are used for the following reasons: (1) To maintain a given focal-skin-distance. (2) To limit the x-ray beam to the particular area which it is desired to irradiate. (3) To aid in the directioning of the x-ray beam. (4) To provide compression when deep-seated lesions are being irradiated. If by the use of compression, the distance between the skin and the lesion can be diminished, an appreciably higher dose can be delivered to the lesion for any given value of the skin dose. Cones can be conveniently classified into two essentially different types, depending on the method by which the area is defined at the cone surface. In the first type (Fig. 1, A) the cone has a large upper aperture and the walls are constructed of radiopaque material (usually lead); this can be called the side-shielded cone. Such cones are supplied with some deep-therapy equipment but are much more generally used in association with superficial and contact therapy apparatus. The second type (Fig. 1, B) has a radiopaque diaphragm at the top with an aperture of the correct dimensions to define the beam at the exit surface of the cone.