Rounded end multi-leaf penumbral measurements with radiochromic film

Multi-leaf penumbral doses have been investigated for 6 MV 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 10 cm x 10 cm field at Dmax were found to be 4.6 mm and 4.3 mm for radiochromic and radiographic film respectively. This is compared to 2.6 mm and 2.6 mm for the leaf edge penumbra. Radiochromic film also measured leaf end/interleaf leakage doses in the penumbral region, which was shown to produce approximately 4% of maximum dose wave across the penumbral region with maximum doses delivered at the MLC leaf interfaces.