OPTIMIZATION OF THE DOSE DELIVERY IN A FEW FIELD TECHNIQUES USING RADIOBIOLOGICAL OBJECTIVE FUNCTIONS

A method for finding optimal primary fluence profiles for multiple fie ld external beam radiation therapy techniques has been developed using a radiobiologically based objective function that quantifies the prob ability of achieving complication-free tumor control, P+. The objectiv e function P+ has the valuable property of giving the highest possible dose to the tumor without causing severe damage to normal tissues at risk. This radiobiologically based objective function selects suitable dose levels but also takes into account the dose homogeneity in the t arget volume to the extent that it is not causing an excessive risk of local recurrence or damage to surrounding normal tissues. The biologi cal parameters used can either be patient specific, as determined by a predictive assay on biopsy specimens, or taken from a library of radi obiological parameter values characteristic for different tissue types of a reference patient. In its present form the method can be used to determine the optimum incident photon fluence profiles for each beam. The method has been used to investigate for a given target volume a l arge number of combinations of beam entry directions to find the best beam orientations with respect to the probability of achieving complic ation-free tumor control. It is demonstrated that when nonuniform dose delivery is available it is unsuitable to combine parallel opposed be ams in two-beam techniques and to a lesser extent also to use perpendi cular beams. In two-beam techniques the best angle between the beams i s generally in the 100-degrees-120-degrees range. The major symmetry c haracteristics of the P+ phase space for two-beam techniques are also identified. The method can easily be extended from two to three dimens ions and noncoplanar geometry, but it is presented here in its two-dim ensional form for clarity and speed of calculation.