Intensity modulated Australian radiation therapy (IMART)

Full text: Forward and inverse planned intensity modulated radiotherapy (IMRT) methods will be discussed. IMRT consists of providing a number of differently weighted infield beam segments with the dose from each segment being defined by the multi leaf collimator (MLC) shape and beam weighting. Inverse planned distributions are achieved by the radiotherapy planning computer (RTPC) using an iterative optimisation method to reach a dose solution. With the pinnacle RTPC the number of infield segments to achieve an approximation to the continuous segmented solution can be chosen then the dose is forward computed using these discrete segments. Quality assurance consists of placing the same segmented fields onto a water phantom on the RTPC. Then dose profiles taken at dmax are compared with film maps taken in the same real geometry on the linear accelerator. The film is scanned using a Vidar film scanner and density to dose conversion is achieved by fitting an exponential function to normalisation films. The film dose profiles are then compared with the RTPC dose profiles. A 3% match between film and RTPC has been achieved with the dose profiles provided care is taken with the leaf delivery sequence. There are some leaf delivery non idealities which the dosimetry films show. For example the matchline effect on the Varian 120 leaf MLC has been measured as a 31% hot line which occurs if one leaf drives to where the other leaf was. This effect is mainly due to the rounded leaf ends causing penumbra asymmetry whereby the top of the penumbral profile is slightly different to the bottom. This effect can be reduced by using software offsets of the leaf positions or by simply having a small number of segments with minimal leaf overlap. Copyright (2000) Australasian College of Physical Scientists and Engineers in Medicine