THE POTENTIAL AND LIMITATIONS OF THE INVERSE RADIOTHERAPY TECHNIQUE

The objective of the work presented in this paper is to explore the sc ope of the applicability of the inverse radiotherapy technique for des igning optimized intensity distributions to achieve a desired dose dis tribution. A specified desired uniform dose to the target volume is in verted, subject to constraints on the surrounding normal tissue dose, to produce optimum intensity distributions in a set of beams arranged around the target volume. We employed the inverse technique and softwa re developed by Bortfeld and evaluated results both qualitatively and quantitatively using dose distribution displays, dose-volume histogram s and biological indices including tumor control probability and norma l tissue complication probabilities. So far we have applied this metho dology to prostate and lung treatment plans. For prostate the inverse technique produces satisfactory approximations of the desired dose dis tributions. However, for lung its performance is considerably inferior . Our investigations point to a number of factors for this difference, the primary ones being differences in the tolerance doses of neighbor ing normal tissues, magnitudes of volume effect, tissue architectures, and the achievability of the specified desired dose distributions. We conclude that, for certain clinical situations, it is not sufficient to specify the objectives of optimization purely in terms of the desir ed pattern of the dose. The objectives must also include dose-volume e ffects and biological indices. Furthermore, the mathematics of optimiz ation must be able to incorporate these factors into the process. We f ind that the inverse technique is not suitable for situations where do se-volume considerations and biological indices are important and that other methods of optimization of intensity distributions should be ex plored.