Optimized 3-D planning by simple means. An example

Abstract

A treatment technique favorable for linacs with asymmetric jaws, which combines cranio-caudal matching fields with fields enclosing the whole target volume, is investigated with respect to field matching and sparing of normal tissue and organs at risk. For a pelvic target volume rapidly varying in cranio-caudal direction a 5-field technique was planned with individually weighted and blocked fields. Three fields adjoining in cranio-caudal direction were completed by 2 fields enclosing the whole target volume. The matching line was measured and calculated with Helax TMS. Furthermore a 4-field box and opposing fields were planned. The dose-volume histograms for target, bladder, intestine and soft tissue were exported. Normal tissue complication probability and tumor control probability, respectively, were calculated for all techniques. In the region of the matching line the summation of the measured normalized curves resulted in relative dose maxima of 6.0% (caudal) and 4.5% (cranial), respectively. For fields enclosing the whole target volume the dose maxima in the region of the matching line decreased to 2.0% (caudal) and 1.8% (cranial), respectively. For the dose profiles calculated with Helax TMS no overdose was found. The 5-field technique with adjoining fields results in a better sparing of the organs at risk compared to the other techniques, whereas the tumor control remains the same. In specific cases a technique with cranio-caudal adjoining fields can be recommended with respect to sparing of normal tissue and organs at risk.