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Initial Experience of Patient-Specific QA Using a Pencil Beam Scanning Proton Therapy System

F Piskulich

F Piskulich*, Y Zhang , L Perles , A Mascia , R Lepage , A Giebeler , L Dong , Scripps Proton Therapy Center, San Diego, CA


SU-E-CAMPUS-T-6 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To illustrate patient QA results for the first 10 patients treated at Scripps Proton Center by comparing point dose measurement using an ion chamber and in-house developed secondary MU program, and the measurement of 2D dose distribution using an ion chamber array.

Methods: At the time of writing, 10 patient plans were approved for treatment using Varian ProBeam pencil beam scanning system and Eclipse treatment planning software. We used the IBA CC04 0.04 cm3 ion chamber and PTW Unidos E electrometer for point dose measurement in a small water tank (Sun Nuclear 1D scanner). We developed independent MU check software based on measured pencil beam dose profiles for various energies. We used PTW Octavius 729 XDR array to evaluate 2D planar dose distribution. The 3D gamma at 3%/3 mm local dose was used to compare a 3D calculated dose plan with a 2D measured dose distribution using PTW Verisoft software. All fields were exported to a verification phantom plan and delivered at 0 degrees for simplicity.

Results: Comparisons between the CC04 ion chamber measurement and calculated dose agree well within 1%. The PTW Octavius 729 XDR array exhibited some dose rate dependence in high dose rate pencil beam delivery. Nevertheless, the results, used as a relative measurement, passed the gamma criteria of 3%/3mm for greater than 90% of area in all patient fields. Visual inspection showed good agreement between ion chamber dose profile and the calculated plan. The in-house secondary check for MU agreed very well with the plan dose and measurement. The results will be updated with more patients treated.

Conclusion: The initial patient specific QA results are encouraging for a new pencil beam scanning only proton therapy system.

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