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Model Comparison for Commissioning of a Real-Time Dose Delivery Detector for a 6MV Photons Beam

N Maffei

N Maffei1,2*, G Mistretta1 , A Chendi3 , M Gutierrez4 , P Ceroni1 , L Morini1 , L Boni1 , A Bernabei1 , B Meduri1 , T Costi1 , F Lohr1 , G Guidi1,3 , (1) Azienda Ospedaliero Universitaria di Modena, Modena, Modena, (2) University of Turin, Torino, Torino, (3) University of Bologna, Bologna, Bologna, (4) International Center of Theoretical Physics, Trieste, Trieste


SU-I-GPD-T-480 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study provides an analysis of the influence of the Dolphin-IBA® (D) transmission detector on treatment beam characteristics comparing the LINAC model with/without the detector in place. Beam parameter modifications introduced by D in photon beams were evaluated regarding PDD, beam profiles, flatness, symmetry, field size, penumbra and transmission.

Methods: The detector size is 24x24cm2 with 1513 ionization chambers. It was interfaced with an ElektaSynergy®. LINAC models were obtained from RayStation® ATPcurves without D (NoD) and from the auto-modeling function after experimental measurements with D and output factor (OF) optimization. Measurements were carried out using PinPoint/Semiflex ion chambers within PTWwaterTank. Analysis were referred to 6MV photons with a field size [3x3;5x5;10x10;15x15;20x20;40x40]cm2 at depth of [1.7;5;10;20]cm. Statistical analysis was performed using SPSS tStudent.

Results: A SDD of 60.3cm (58.5cm default) was obtained comparing the measured field size of 10x10cm2 at 90cm SSD with D and NoD. Comparing profiles, measured at different depths and field sizes, a best flatness of 0.7% was obtained using D model with a difference between in-plane/cross-plane of 1.0%; the 40x40cm2 shows best flatness>4% using D model. A mean increment of 1.0% was obtained in symmetry using D model. Between the two models no statistical differences (sign=18%) were observed comparing field sizes, also penumbra difference of 0.5mm was obtained on all field sizes with a sign=47%. An OF discrepancy of 0.18% was detected between D and NoD up to 20x20cm2; it reaches 1.7% in the 40x40cm2. A transmission factor of 91.9±1.2% was measured.

Conclusion: The D model seems to smooth PDD curves resulting in more homogenous profiles. A finer field size step between 20x20cm2 and 40x40cm2 is under investigation. No relevant statistical difference was detected comparing the models. Changing the LINAC model after mounting D may not be mandatory for clinical use; attenuation must, however, be considered.

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