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Systematic Evaluations of a Flat-Panel Imager for Intensity-Modulated Proton Therapy (IMPT) Plan Patient-Specific QA

Z Su1*, S Rossomme2, W Hsi3, (1) University of Florida, Jacksonville, FL, (2) IBA Dosimetry (3) University of Florida, Jacksonville, FL,


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

Room: AAPM ePoster Library

To evaluate a commercial flat-panel imager fundamental property of proton signal responses, spatial accuracy and quenching effect and its application in IMPT plan patient-specific QA.

The flat-panel signal linearity of 100, 150 and 200MeV proton were obtained for 6 gains settings (0.25, 0.5, 1, 2, 4, 8pF). The flat-panel spatial response was measured by irradiation of 100MeV proton 10x10cm2 beam through 6.3cm solid water then a high-precision perforated aluminum plate with holes in 1cm spacing. The flat-panel measured distances between the centers of adjacent spots were compared to the plate mechanical specifications. PDDs of 100, 150 and 200MeV protons were measured by the flat-panel with different thickness solid water slabs and compared to ones in water. The flat-panel quenching effect was quantified using peak-to-plateau ratios of PDD. A prostate PBS plan was measured at 0, 2, 5, 10 and 15cm depths in solid water using the flat-panel. The obtained images were converted to planar doses through counts-to-dose calibration and compared to corresponding planar doses from the treatment plan using gamma tests with different criteria.

The signal responses were linear in all gain settings. Sixteen of 18 linear fits had coefficient-of-determination greater than 0.9998 and the minimum was 0.999. The flat-panel measured inter-hole distances agreed with the plate specification to 0.1mm. The distal R80 of measured PDDs were within 1mm of those measured in water. However, lower peak-to-plateau ratios (under-response at pristine peak region) of the measured PDD indicated quenching effect of the flat-panel. The gamma test pass rates of prostate IMPT plan at the 5 depths were from 96.6% to 100% (3mm, 3%).

The flat-panel signal response was linear and spatial response was accurate. The detector had quenching effect at pristine peak region of PDD. The IMPT plan patient-specific QAs are acceptable at the measured depths.


Protons, Intensity Modulation, Quality Assurance


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

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