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An Adjustable Diaphragm for Defining X-ray Treatment Beams

Royal Cancer Hospital, London

This excerpt was created in the absence of an abstract.

As a result of experience with adjustable diaphragms for defining X-ray therapy beams of a number of different types from several countries, we have been able to make comparisons both on the bench and in clinical use and so to develop a more satisfactory apparatus for this purpose than has been available in the past.

An adjustable diaphragm should be interchangeable with ordinary applicators and light enough for a radiographer to make the change unassisted. The projection of the apparatus from the tube head should be as small as possible both to allow room for setting up a patient and to minimise the mechanical moment tending to unbalance the tube head. The projection may be reduced by mounting the separate diaphragms in pairs rather than one above the other, a common arrangement being two pairs of diaphragms mounted perpendicularly and driven by two screws or racks to make them meet at the centre of the field. This arrangement only produces regular quadrilateral fields which can easily be achieved with ordinary treatment cones. We believe that the main advantages of an adjustable diaphragm lie in the production of a wide range of irregular fields, including triangles and pentagons, and in the elimination of expensive applicators for large sized fields where compression is not required. With the assistance of Dr. A. D. O'Connor we designed a device with four separately mounted rotatable slides each carrying a sliding diaphragm.