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Small Field Spine Radiosurgery IMRT QA with Octavius 4D: Chamber Comparison and Beam Model Assessment

Z Wen

Z Wen*, X Wang , D Luo , E Han , T Briere , J Yang , U. of Texas MD Anderson Cancer Ctr., Houston, TX


SU-I-GPD-T-527 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: IMRT plans for stereotactic spine radiosurgery (SSRS) are often highly modulated and comprised of small segmented fields that can cause discrepancy between QA measurement and calculation. A PTW Octavius 4D system, including a rotational phantom and a liquid-filled chamber array (1000 SRS, 2.5 mm detector spacing), was commissioned and tested with 7 clinical SSRS IMRT plans. To evaluate system QA performance, measured data was compared with ionization chamber measurement and dose calculation using Pinnacle treatment planning system (TPS).

Methods: The QA plans were calculated with the Octavius phantom with two beam models, one general model for all field sizes (BM1), and the other model optimized for small SSRS fields (BM2). Plans were delivered with a Varian TrueBeam Linac with high-definition MLC (2.5 mm leaf width). For analysis, a 3D gamma index was used with passing criteria of 2.5 mm/4% local dose and dose threshold at 30% of max dose. These plans were previously measured with a PTW CC04 chamber in a Scanditronix IMRT body phantom. The point dose differences between the measurement by Octavius and Pinnacle calculation were evaluated at the same chamber locations (10 measurement points from the 7 plans) for comparison.

Results: The average gamma passing rate for the 7 plans were 92.7% (BM1) and 96.4% (BM2); the average point dose difference was -3.6% and -2.1%, comparing to average chamber measurement -3.5% and -1.9%.

Conclusion: The point dose measurement with the Octavius 4D system is consistent with chamber measurement, and its 3D volumetric dose analysis is adequately sensitive to assess different SSRS beam models. The Octavius 4D system can be a suitable QA device for spine SSRS IMRT QA purposes.

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