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Statistic Characteristics of Dose-Volume-Histogram Deviations by Patient Setup Error in Spot-Scanning Carbon-Ion Radiotherapy

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J Sun*, W Hsi , W Wang , G Jiang , Shanghai Proton and Heavy Ion Center, Shanghai, China


SU-I-GPD-T-163 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the statistic characteristics of dose-volume-histogram (DVH) deviations induced by setup error in carbon-ion radiotherapy.

Methods: 2 approaches to statically calculate deviation for setup error were investigated for a liver case with prescribed 55Gy (RBE) over 10 fractions. One approach, ‘Truncated’, is to sum with Gaussian-like weights of dose deviations with discrete setup displacements. Another approach, ‘Metrics’, utilize the ‘Plan Robustness Analysis’ tools in the Computational Environment for Radiotherapy Research (CERR) planning system. Mean DVHs of ‘Truncated’ approach is manually obtained in three directions separately with a maximum 10 mm displacement which is equivalent to twice of 5 mm standard setup error in ‘Metrics’ approach. The mean dose deviation with statistic confidence of ‘Metrics’ approach was obtained by moving the planned dose distribution with random Gaussian probability of a defined setup displacement. Calculated mean DVHs between 2 approaches are compared.

Results: Mean DVHs of the GTV and CTV by ‘Truncated’ approach for max 10mm displacement show 3-5% lower volumes at dose of 98-99% (D-A) , and also 5-15% lower volumes at dose of 100-101% (D-B) to planned DVHs. With a sigma of 5mm displacement in lateral direction, the mean DVHs of GTV/CTV by ‘Metrics’ approach results little 2-3% larger volumes at both D-A & D-B to ‘Truncated’ approach. However, mean DVH of normal liver reduces constantly 10-15% for doses of 30-90% by ‘Metric’ approach, but increases <3% by ‘Truncated’ approach to planned DVHs. Incorrect range uncertainty in ‘Metric’ approach can result large different DVHs to‘Truncated' approach.

Conclusion: ‘Metric’approach utilizes a robustness evaluation for plans with 7-9 photon fields with dose deviations that mainly introduce by shifting the doses perpendicular to the field edge. Combined effects of setup error and range uncertainty for 2-3 field of carbon-ion radiotherapy in ‘Metric’ approach need to be investigated for its full utilization.

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