ABSTRACT
BACKGROUND: The ability of a geometry-based method to expeditiously adapt a "2-Step" step and shoot IMRT plan was explored. Both changes of the geometry of target and organ at risk have to be balanced. A retrospective prostate planning study was performed to investigate the relative benefits of beam segment adaptation to the changes in target and organ at risk coverage. METHODS: Four patients with six planning cases with extraordinarily large deformations of rectum and prostate were chosen for the study. A 9-field IMRT plan (A) using 2-Step IMRT segments was planned on an initial CT study. The plan had to fulfil all the requirements of a conventional high-quality step and shoot IMRT plan. To adapt to changes of the anatomy in a further CT data set, three approaches were considered: the original plan with optimized isocentre position (B), a newly optimized plan (C) and the original plan, adapted using the 2-Step IMRT optimization rules (D). DVH parameters were utilized for quantification of plan quality: D(99) for the CTV and the central planning target volume (PTV), D(95) for an outer PTV, V(95), V(80) and V(50) for rectum and bladder. RESULTS: The adapted plan (D) achieved almost the same target coverage as the newly optimized plan (C). Target coverage for plan B was poor and for the organs at risk, the rectum V(80) was slightly increased. The volume with more than 95% of the target dose (V(95)) was 1.5+/-1.5 cm(3) for the newly optimized plan (C), compared to 2.2+/-1.3 cm(3) for the original plan (A) and 7.2+/-4.8 cm(3) (B) on the first and the second CT, respectively. The adapted plan resulted in 4.3+/-2.1 cm(3) (D), an intermediate dose load to the rectum. All other parameters were comparable for the newly optimized and the adapted plan. CONCLUSIONS: The first results for adaptation of interfractional changes using the 2-Step IMRT algorithm are encouraging. The plans were superior to plans with optimized isocentre position and only marginally inferior to a newly optimized plan.
