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Commissioning of a New Wireless Volumetric QAsystem (Delta4 Phantom+) for a Monte Carlo Treatment Planning System

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C Cheung

C Cheung*, F Lee , K Leung , Queen Elizabeth Hospital, Hong Kong SAR


SU-I-GPD-T-353 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Delta⁴ Phantom+ is a new wireless volumetric quality assurance (QA) system. To characterize the system, stability, linearity, angular dependence, doserate dependence and optimization phantom position function were studied. This system was commissioned and evaluated on the QA of VMAT plans generated in the latest Monte-Carlo-based (MC) treatment planning system (TPS), Monaco Version 5.11.

Methods: Readings from selected diodes inside the system were evaluated on stability, linearity, dose rate dependence and angular dependence. VMAT plans of different treatment regions were calculated and measured by pinpoint chamber in solid water and Delta⁴ phantom+. The gamma passing rate is ≥95% with gamma value ≤1 for 3%/3mm and point dose error ≤3%. The optimization position function was evaluated with 4 open field plans and 5 VMAT plans in the same setup. Recommended phantom shifts were recorded for analysis.

Results: The maximum deviations of stability, linearity and doserate dependence were 0.85%, 0.49% and 0.17% respectively. The gamma passing rate of 36 equally spaced gantry angles were above 98% except 0, 90, 180 and 270 degrees (92.3%, 91%, 96.6%, and 86.6%). Monaco VMAT plans showed good gamma passing rate (average 98.6%) and point dose error (average 1.15%). Open field tests showed consistency on phantom position shift (SD <0.15mm), but larger variation (SD <1mm) for VMAT plans.

Conclusion: The system showed good performance on stability, linearity and doserate dependence. All VMAT plans passed the QA. Angular dependence is the worst at the gantry angles parallel to diode detector board. It may be caused by the self-attenuation of the diodes or inaccurate angular correction of the QA software. Optimization phantom position function can be used to correct setup error. The Delta⁴ Phantom+ is a reliable device with phantom position optimization for patient specific QA on Monte-Carlo-based TPS. However, angular response limitation at gracing angles still exists.

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