Virtual simulation of targets with concave surfaces using a static-segmentation technique

In many clinical situations, local control rates are restricted because of radiation-dose limitations imposed by radiation-sensitive normal tissues invaginating into the target. In order to avoid these dose limitations, we developed a method of virtual simulation that had to meet the following requirements: 1. The use of static beams with a single isocenter; 2. Beam intensity modulation by field segmentation executable with a standard Philips MLC multileaf collimator; 3. Virtual simulation and dose computation executable on Sherouse's GRATIS 3D-planning system; 4. A sharp dose gradient between the target and the invaginating normal tissue at the predictable location; 5. Heuristic computation of segment intensities leading to 85-90% dose homogeneity at the target and a predictable dose distribution at the invaginating normal tissue. Results of virtual simulations of clinical cases (n = 10) showed that requirements 1-4 could be accomplished using 5-8 beam incidences and 25-50 segments; requirement 5 could be fulfilled in targets with a single concavity (n = 8) in their surfaces