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Multiple Isocenter IMAT: Impact of Technique On Junction Error Robustness

E Ehler

E Ehler*, P Higgins, K Dusenbery, Univ Minnesota, Minneapolis, MN

TH-A-116-11 Thursday 8:00AM - 9:55AM Room: 116

Purpose: Many techniques of delivering multiple isocentric IMRT or IMAT have been described. However, these studies have considered different treatment sites, reported different dosimetric parameters, and considered a limited degree of junction error, typically a 1D beam placement error of 3 mm. This work provides a direct comparison of these techniques by utilizing a consistent set of dosimetric parameters and a common treatment, craniospinal irradiation.

Methods: Seven IMAT treatment plans were created, with three main features: (Method1) target subvolumes used to explicitly control the transition of dose from one set of beams to another in the junction region, (Method2) staggering the jaw settings for a set of arcs that overlap with another set of arc fields but not directly controlling the dose transition, (Method3) arranging beams to overlap 5-10 cm and allowing the inverse planning system to control the junction dose. Method3 was subdivided into three and four isocenter plans; four isocenter plans had greatest beam overlap. Then, the beam isocenters were off set 0.25 and 0.5 cm in each dimension (3D); therefore junction errors of 1.0 cm or greater were considered. For all plans, a total of 6 junction errors were considered.

Results: Dosimetric impact of junction error was limited mostly to target volumes. The PTV V95 degraded by 6% to 9% and 1% to 3% for the 0.5 and 0.25 cm junction errors, respectively. For the CTV the V95 was impacted by 0.5% to 2.6% and 0.1% to 0.7%, respectively. The four isocenter plans of Method 3 consistently showed the largest degradation in V95 and V105 for both the PTV and CTV. Method1 and three field plans of Method 3 showed the least change in V95 and V105.

Conclusion: Robust multiple isocenter IMAT plans, in terms of junction error, can be achieved with proper inverse planning techniques.

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