Radiochromic film measurements of rounded end multi leaf penumbra

Full text: Multi-leaf penumbral doses have been investigated for 6MV x-rays and a Varian millennium Multi Leaf Collimator (MLC) using Gafchromic MD-55-2, radiochromic film and X-omat V radiographic film. An advantage of Gafchromic film for multi-leaf penumbral dose measurement is the relatively low energy dependence of the film. A comparison of penumbral dose measurements has also ascertained the effects of energy response on radiographic film in this region. Similar 80%/20% penumbral doses have been measured with both types of films. Thus there is a relatively low energy effect on penumbral dose measurements in film dosimetry. The 80% / 20% dose penumbral distances for rounded leaf end multi-leafs for a 10cm x 10cm field at Dmax was found to be 4.6mm and 4.3mm for radiochromic and radiographic film accordingly. This is compared to 2.6mm and 2.6mm for the leaf edge penumbra. Radiochromic film also measured leaf end/interleaf leakage doses in the penumbral region, which was showed to produce an approximate 4% of maximum dose wave across the penumbral region with maximum doses delivered at the MLC leaf interfaces. Copyright (2004) Australasian College of Physical Scientists and Engineers in Medicine