Activity distribution of a cobalt-60 teletherapy source

In the course of quantifying the effect of radiation source size on the spatial resolution of portal images, a concentric ring structure in the activity distribution of a Cobalt-60 teletherapy source has been observed. The activity distribution was measured using a strip integral technique and confirmed independently by a contact radiograph of an identical but inactive source replica. These two techniques suggested that this concentric ring structure is due to the packing configuration of the small ⁶⁰Co pellets that constitute the source. The source modulation transfer function (MTF) showed that this ring structure has a negligible influence on the spatial resolution of therapy images when compared to the effect of the large size of the ⁶⁰Co source.