A method for determining tissue importance weighting factors for inverse planning

Full text: Various methods for determining an 'inverse' radiotherapy treatment plan have been developed since the initial concept was first introduced. However for many of these methods a 'trial and error' approach is still required to choose tissue importance weights which result in an acceptable distribution. Although the same dose to the tumour and critical structures may be intended, to achieve the same effect for different patients these weights must vary due to the position and size of the various organs. This study has investigated a method to determine these tissue weights in an iterative fashion. A dose-based inverse planning method was used to generate plans for which the tissue importance weight determination was considered. The inverse plan for an initial set of tissue importance weights was determined and these initial weights then adjusted depending on a set of constraints defined for each tissue type. The inverse plan is then repeated until the constraints are either all met or consistently not met for all tissue types. This ensures that as long as the constraints are set correctly the final plan will achieve the best result possible. This method proved successful in achieving the intended requirements (or as close as possible to these requirements). The constraints used for the different tissues can also be adjusted to consider different effects. For instance either biological or physical parameters or a combination of both could be used. This obviously results in a slightly different endpoint although the actual dose optimisation method is the same in both situations. The most critical factor is found to be defining these endpoints. This tissue importance weight finding technique has proved successful for this optimisation method, resulting in dose distributions meeting or consistently not meeting the intended delivery for all tissue types considered in the optimisation. This method may also prove useful for other methods which determine beam weights. Copyright (2000) Australasian College of Physical Scientists and Engineers in Medicine