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Determination of Machine-Specific Tolerances Based On Long-Term Monthly QA Dosimetry Results in Proton Therapy

S Rana1*, H Grewal1, M Chacko1, C Eckert1, M Storey1, J Chang1, A Rosenfeld2, (1) The Oklahoma Proton Center, Oklahoma City, OK, (2) University of Wollongong, Wollongong, Australia

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
The purpose of this study is twofold: (1) report the long-term monthly quality assurance (QA) dosimetry results of our uniform scanning proton therapy (USPT) system, and (2) derive the machine-specific tolerances and compare them against the AAPM TG224 recommendations.

Methods:
Our proton center has four treatment rooms (TR1, TR2, TR3, and TR4) with a cyclotron and a universal nozzle from the IBA. Monthly QA dosimetry results of four treatment rooms over a period of 6 years (Feb 2014—Jan 2020) were retrieved from the QA database. The dosimetry parameters included the dose output, range, flatness, symmetry, and beam coincidence (proton vs. x-ray). The difference (?) was calculated by comparing the results of each parameter against its baseline value of the respective treatment room. In order to derive the machine-specific tolerances, statistical process control (SPC) analysis was performed such that upper-control-limit (UCL) and lower-control-limit (LCL) are defined by +2s and -2s, respectively, from the average value.

Results:
The mean ? in dose output (TG224:±2%) of our standard beam (R16M10) was -0.3±0.8% (2s=±1.7%). The mean ?s in range (TG224:±1mm) were 0.8±0.5 mm (2s=±1.0 mm) for R10, 0.2±0.5 mm (2s=±1.0 mm) for R16, 0.2±0.5 mm (2s=±1.0 mm) for R22, and 0.0±0.8 mm (2s=±1.7 mm) for R28. The mean flatness (TG224:±2%) and symmetry (TG224:±2%) of four beams (R10M6, R16M10, R22M8, and R28M10) were 1.8±0.6% (2s=±1.2%) and 0.2±0.4% (2s=±0.9%), respectively. The mean beam coincidence (TG224:±2mm) was 0.1±0.5 mm (2s=±1.0 mm).

Conclusion:
The SPC analysis was performed to assess whether various dosimetry parameters of our USPT system were in statistical control. The 2s of dose output, flatness, symmetry, and beam coincidence were within TG224 recommended values. For range, we observed the 2s results within ±1 mm (TG224 recommendation) for beams R10, R16, and R22, whereas the 2s was slightly higher (±1.7 mm) for higher-energy beam (R28).

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Keywords

Protons, Quality Assurance, Statistical Analysis

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – quality assurance

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