Delineation of potential hot spots for hyperthermia treatment planning optimisation.

The optimal feed parameters of the generators for a complex-phased hyperthermia array system consisting of 4, 8 or even more applicators cannot be found using only the expertise of the treatment staff or using the limited amount of field and temperature data obtained during treatment. A number of strategies have been proposed to help us with the task to optimise the hyperthermia treatment, including several strategies specifically addressing the occurrence of hot spots. Each of the latter strategies strongly relies on the specification of the potential hot spots. This specification is either based on anatomy or the selection of an arbitrary number of potential hot spots. Therefore it is not guaranteed that all potential hot spots are included. This paper introduces a procedure for the delineation and visualisation of potential (SAR) hot spots. The potential hot spots are delineated by selecting those points for which the maximal SAR exceeds a specific SAR selection level. This SAR selection level is defined relative to the highest achievable SAR in the target volume for a certain fixed heating power. A larger number of potential hot spots and hot spots of larger size are delineated if the selection level is decreased. Although the procedure still includes an arbitrary selection criterion, i.e. the selection level, the selection is solely based on calculated EM-field data. As a result all potential hot spots can be delineated a priori. Three different objective functions are applied to maximise the SAR in the target. The first only maximises the SAR in the target volume for a given system power output. The other two intrinsically set a constraint on the set of potential hot spots as a whole. Additionally the SAR in each delineated potential hot spot separately can be constrained. In two patient cases the SAR in potential hot spots can be kept below the selection value applied for delineation of the potential hot spots. If assessed in terms of constraining the SAR value below the selection level while maximising target heating efficiency the combination of an objective function only maximising the SAR in the target with a separate constraint on each potential hot spots appears to be the most efficient.