AbstractID: 7954 Title: A Study of the Errors in Delivering IMRT with DMLC
Dynamic multi-leaf collimator (DMLC) is a general way to deliver Intensity Modulated
Radiation Therapy (IMRT). Practical techniques are the continuous beam and motion
method (sliding window) and the discrete skip and shoot. In the sliding window
technique, the beam remains on while the MLC moves from one user-defined shape to
the next with instantaneous MLC shape determined by linear interpolation between the
two shapes, the instantaneous accumulated dose being the interpolation index. The leaves
are mechanically driven, they are forced to arrive at their interpolated positions at each
regular time interval to within a user-defined leaf dynamic position tolerance. This
tolerance determines how the actual leaf motion matches the intended, which in turns
determines the deviation of the actual delivered intensity from the desired pattern. This
study quantifies the dependence of the treatment delivery on this tolerance in these
aspects: the treatment time, the statistical distribution of leaf position deviation, and the
actual dose distribution. The dose deviation is studied in two ways: one measured with
film dosimetry, the other with model calculation. Knowing the statistics of the leaf
deviation, the expected dose error can be calculated using the average leaf gaps or the
leaf motion patterns. By varying the tolerance from smallest to largest values, the
treatment time changes 50%, and the leaf deviations several fold, and the dose
differences exceeding 10%. Based on acceptable delivered dose deviations, a judicious
choice of the leaf tolerance is important for each clinic and possibly each treatment site.