Strategies for treating possible tumor extension: some theoretical considerations.

When there is a small possibility of cancer having extended to a region some distance from the main bulk of disease, it may be unclear whether to include that region in the target volume and, if so, what dose should be delivered to it. We have constructed a theoretical model that includes dose and volume relationships for both diseased and normal tissue. With this model one can calculate the change in tumor control probability (TCP) when varying doses are delivered to the regions of known and suspected disease. Values of TCP as a function of dose to the region of suspected disease have been calculated for a wide range of the variables on which the model depends. We conclude that the strategy of treating the region of suspected disease to about 70% of the dose delivered to the region of known disease is almost always better than not treating it at all, or treating both regions to a uniform but reduced dose designed to keep the probability of complication the same. The gain in TCP could be from 5 to 15% for situations of clinical interest.