An Indicator and Recorder for Checking Diaphragm Settings on a Cobalt 60 Unit

Abstract

An applicator on a conventional X-ray set is used both to limit the field size and to assist in setting up to the patient. Since the applicator is a detachable and therefore more obvious component, there is little chance of error and use of the wrong field size. With high-energy units, however, it is usual to adjust the field size by variable diaphragms at a distance from the patient and hidden within the cover of the machine. The field size set is not obvious, and it is possible for the operator to forget to adjust the diaphragms or adjust them wrongly (particularly, for example, to set 8 × 10 cm instead of 10 × 8 cm). These errors are less likely where the field size is shown on the patient by an area of light from a built-in light source. This facility is not available on the unit at Oxford (constructed by Messrs. Husband & Co., Sheffield), and even if it were the frequent use of wedge filters or metal compensators (Ellis, Lewis, Oliver and Hall, 1959) would mean that such a light beam would not be visible on the skin, at any rate in the final stages of the set up. However, some check on the setting of the diaphragms appeared to be desirable. In multifield systems the same field settings may be used for several consecutive treatments, and it would therefore be inconvenient to reset the diaphragms each time.