"Optimized" 3-D planning by simple means. An example

Aim: A treatment technique favorable for linacs with asymmetric jaws, which combines cranio-caudal matching fields with fields enclosing the whole tar get volume, is investigated with respect to field matching and sparing of n ormal tissue and organs at risk. Patient and Methods: For a pelvic target volume rapidly varying in cranio-c audal 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(C). Furthermore a 4-field box an d opposing fields were planned. The dose-volume histograms for target, blad der, intestine and soft tissue were exported. Normal tissue complication pr obability and tumor control probability, respectively, were calculated for all techniques. Results: In the region of the matching line the summation of the measured n ormalized 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) a nd 1.8% (cranial), respectively. For the dose profiles calculated with Hela x TMSO no overdose was found. The 5-field technique with adjoining fields r esults in a better sparing of the organs at risk compared to the other tech niques, whereas the tumor control remains the same. Conclusions: In specific cases a technique with cranio-caudal adjoining fie lds can be recommended with respect to sparing of normal tissue and organs at risk.