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Robustness for Patient Setup Error in Total Body Irradiation Using Volumetric Modulated Arc Therapy (VMAT)

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Y Takahashi

Y Takahashi1,2*, H Tachibana2 , N Nakamura2 , (1) Saitama Medical Center Jichi Medical University, Saitama-city, Saitama, (2) National Cancer Center, Kashiwa, Chiba


SU-F-T-383 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Total body irradiation (TBI) and total marrow irradiation (TMI) using Tomotherapy have been reported. A gantry-based linear accelerator uses one isocenter during one rotational irradiation. Thus, 3-5 isocenter points should be used for a whole plan of TBI-VMAT during smoothing out the junctional dose distribution. IGRT provides accurate and precise patient setup for the multiple junctions, however it is evident that some setup errors should occur and affect accuracy of dose distribution in the area. In this study, we evaluated the robustness for patient’s setup error in VMAT-TBI.
Methods: VMAT-TBI Planning was performed in an adult whole-body human phantom using Eclipse. Eight full arcs with four isocenter points using 6MV-X were used to cover the entire whole body. Dose distribution was optimized using two structures of patient’s body as PTV and lung. The two arcs were shared with one isocenter and the two arcs were 5 cm-overlapped with the other two arcs. Point absolute dose using ionization-chamber and planer relative dose distribution using film in the junctional regions were performed using water-equivalent slab phantom. In the measurements, several setup errors of (+5~-5mm) were added.
Results: The result of the chamber measurement shows the deviations were within ±3% when the setup errors were within ±3 mm. In the planer evaluation, the pass ratio of gamma evaluation (3%/2mm) shows more than 90% if the errors within ±3 mm. However, there were hot/cold areas in the edge of the junction even with acceptable gamma pass ratio. 5 mm setup error caused larger hot and cold areas and the dosimetric acceptable areas were decreased in the overlapped areas.
Conclusion: It can be clinically acceptable for VMAT-TBI when patient setup error is within ±3mm. Averaging effects from patient random error would be helpful to blur the hot/cold area in the junction.

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