Improvement of tomographic intensity modulated radiotherapy dose distributions using periodic shifting of arc abutment regions

Based on the study of treatment are positioning versus target length, a met hod that allowed periodic shift of are abutment regions through the course of intensity modulated radiotherapy (IMRT) was developed. In this method, t wo treatment plans were developed for the same tumor. The first plan contai ned the original target (Planning Target Volume as defined by radiation onc ologist) and the second one contained a modified target. The modification o f the original target consisted of simply increasing its length, adding a s mall extension to it, or creating a distant pseudo target. These modificati ons cause are abutment regions in the second plan to be shifted relative to their positions in the first plan. Different methods of target modificatio n were investigated because in some cases (for instance, when a critical st ructure might overlap with the target extension) a simple extension of the target would cause an unacceptable irradiation of the sensitive structures. The dose prescribed to the modified portion of the target varied from 10% to 100% of the original target dose. It was found that a clinically signifi cant shift(greater than or equal to 5 mm) in abutment region locations occu rred when the dose prescribed to the extended portion of the target was gre ater than or equal to 95% of the original target dose. On the other hand, t he pseudo target required only similar to 10% to 20% of the original target dose to produce the same shift in are positions. Results of the film dosim etry showed that when a single plan was used for the treatment delivery, th e dose nonuniformity was 17% and 25% of the prescribed dose with 0.5 and 1 mm errors in couch indexing, respectively. The dose nonuniformity was reduc ed by at least half when two plans were used for IMRT delivery. (C) 2000 Am erican Association of Physicists in Medicine. [S0094-2405(00)01507-8].