Auxiliary Diaphragm for Radioisotope Teletherapy Units

The properties of the collimating device are of the outmost importance in the clinical application of a radioisotope teletherapy unit. The demand for large fields in stationary field therapy and clearance for moving beam therapy makes it desirable in most units to set the primary diaphragm fairly close to the source. From this point of view the single plane collimator is advantageous, particularly on units using a fixed source position and a shutter mechanism. Other more complicated and spacious collimators have been designed, mainly on considerations regarding the penumbra (JOHNS and MACKAY 1953) and the contamination of the gamma beam with electrons scattered from the diaphragm. It is well known that the penumbra, which of course should be as small as possible, increases with the diaphragm-skin distance (HAYBITTLE 1954). When large source-skin distances (SSD) are used and particularly when radiosensitive organs are adjacent to the tumor-bearing volume it is therefore of importance to reduce the penumbra. This may be achieved by placing so-called penumbra trimmers at a suitable position between the diaphragm and the skin (FLETCHER et coll. 1956). If electrons scattered from the diaphragm reach the skin of the patient the build-up of the absorbed dose in the first millimeters of tissue is diminished