The clinical value of different treatment objectives and degrees of freedom in radiation therapy optimization.
With inverse radiation therapy planning methods, both biological and physical objective functions can be used to perform the optimization. A biological objective function, namely the probability of achieving tumor control without causing severe complications in normal tissues, P+, has been used to evaluate six different optimization methods. All six methods have been tested on two different clinically relevant treatment geometries. The results show that optimization with a physical objective function which gives the best possible agreement with the desired dose distribution in the least squares sense, may result in severe loss of complication-free tumor control due to insufficient consideration of the organs at risk. It is generally better to use a physical objective function which minimizes the over-dosage when the desired dose distribution can not be exactly reproduced. In all cases the use of physical objective functions results in a lower probability of controlling the tumor without causing severe normal tissue reactions than if the biological objective function, P+, is used. However, the results also show the importance of accurately accounting for beam divergence, dose build-up, beam attenuation, and lateral scatter during the optimization procedure, particularly when the biological objective function is used. The loss in P+ by assuming that all energy deposition kernels are identical and that all the constituent beams have fixed relative weights can be 15% or more. When lateral scatter is not accounted for during the optimization, serious injury to organs at risk may result. This problem is specially severe for organs that are partly or totally encapsulated by the target volume. For superficial target volumes accurate consideration of the dose build-up of the incident pencil beams is fundamental.