THE CLINICAL-VALUE OF DIFFERENT TREATMENT OBJECTIVES AND DEGREES OF FREEDOM IN RADIATION-THERAPY OPTIMIZATION

With inverse radiation therapy planning methods, both biological and p hysical objective functions can be used to perform the optimization. A biological objective function, namely the probability of achieving tu mor control without causing severe complications in normal tissues, P- +, has been used to evaluate six different optimization methods. All s ix methods have been tested on two different clinically relevant treat ment geometries. The results show that optimization with a physical ob jective function which gives the best possible agreement with the desi red dose distribution in the least squares sense, may result in severe loss of complication-free tumor control due to insufficient considera tion of the organs at risk. It is generally better to use a physical o bjective function which minimizes the over-dosage when the desired dos e distribution can not be exactly reproduced. In all cases the use of physical objective functions results in a lower probability of control ling 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 optim ization procedure, particularly when the biological objective function is used. The loss in P-+ by assuming that all energy deposition kerne ls are identical and that all the constituent beams have fixed relativ e weights can be 15% or more. When lateral scatter is not accounted fo r during the optimization, serious injury to organs at risk may result . This problem is specially severe for organs that are partly or total ly encapsulated by the target volume. For superficial target volumes a ccurate consideration of the dose build-up of the incident pencil beam s is fundamental.