On most radiotherapy treatment units, a light field indicates on the patient's skin where the treatment field will irradiate the patient. It was the aim of the present study to investigate the perception of the light field edge by different operators on different surfaces under different lighting conditions. Ten radiation therapists and physicists were asked to mark the light field edge of an 8 x 10-cm2 radiation field from a linear accelerator on prepacked radiographic film. Each operator marked the field 4 times each with the room light turned on and dimmed as usual for patient setup. Two operators marked the field on 5 different surfaces (film envelope, brown solid water, clear plastic used for the manufacturing of immobilization shells, black rubber, and Orfit patient immobilization material). The interoperator reproducibility (+/- 0.39 mm, ISD) was larger than the intraoperator reproducibility (+/- 0.27 mm). The light field was judged consistently to be 0.6 mm smaller in the light room than under dimmed light conditions and the physicists judged the field to be approximately 0.4 mm smaller compared to the radiation therapists' judgement. Compared to the yellow film wrapping, the light field on solid water, black rubber, and the clear plastic were judged to be 0.6 mm smaller by both operators. The same observation was made using a slotted block tray, which also gave the worst reproducibility of perceived field edges. While these systematic errors are relatively small and difficult to correct for, it appears to be important to be at least aware of them, in particular if the light field is used to junction radiation fields with steep penumbras, as commonly done in megavoltage treatments of head & neck and breast cancer.
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