The dosimetry of eye shields for kilovoltage X-ray beams

The influence of focal spot size, shape, emission profile and position on field coverage in a Gulmay D3300 Kilovoltage X-ray therapy unit

An important characteristic of kilovoltage therapy is the narrow penumbra obtainable with a well designed collimator system. A graphical illustration of applicator geometry is used to show that undesirable penumbral broadening and consequent reduction of field coverage could result if the upper aperture in an applicator is smaller than a critical size or if the applicator is not sufficiently well aligned with the focal spot. This concept is applied in an investigation of the formation of penumbra in the Gulmay D3300, in which the influence of the focal spot size, shape and emission profile, obtained from an image of the focal spot produced using a pin-hole in a sheet of lead, is elucidated. The effective focal spot of the Varian X-ray tube was observed to be rectangular, significantly longer in the front-back direction (6 mm) than in the anode-cathode direction (3.5 mm) and quite non-uniform in emission intensity over its length, with pronounced hot-spots at each end. It is shown that this results in a penumbra which is slightly broadened in the front-back direction when the alignment is perfect, but significantly broadened asymmetrically even when the alignment just meets the manufacturer's stated tolerance. Consequently the alignment, which is performed with an alignment jig supplied by the manufacturer, needs to be very precise to obtain acceptable field coverage, which needs to be checked following an X-ray tube change.