(Tuesday, 7/31/2018) 1:15 PM - 1:45 PM
Room: Exhibit Hall | Forum 7
Purpose: The Varian Halcyonâ„¢ fast-rotating O-ring gantry linac system is delivered with a fully pre-configured photon beam model in the treatment planning system. This work reports on the commissioning and achieved treatment delivery quality of IMRT/VMAT.
Methods: The dosimetric calibration was performed according to IAEA TRS-398, delivering 600cGy/min at 10cm depth. Commissioning of IMRT/VMAT was conducted following AAPM codes-of-practice (MPPG5, TG-119) and was supplemented with external mailed dosimetry audits for calibration and IMRT/VMAT delivery. Next, end-to-end (E2E) testing was performed for VMAT (cranial, rectum, spine and head&neck) and IMRT (lung) using anthropomorphic phantoms. E2E dose measurements were performed using 2D radiochromic film and using ionization chambers (IC) point measurements. Additionally, extensive patient-specific QA was performed for the first patients for every indication (cranial=5, rectum=2, spine=5, head&neck=7, lung=5, thorax=2) using portal image dosimetry, ArcCHECK™ diode array, radiochromic film dosimetry in a slab phantom and IC point measurements. Planar dose measurements were analyzed using 3%(local)/3mm γ-criteria and a low dose exclusion threshold set to 10% of the maximum value in the plane.
Results: The AAPM MPPG5 measurements were fully compliant with the guideline criteria. More stringent criteria were used to assess the limitation of the system: a 2%(local)/2mm γ-criteria revealed deviations for large fields (28x28cm²) and at large depths (>20cm). TG-119’s confidence levels were obtained for both IMRT and VMAT. The IROC beam calibration audit showed agreement of <1%. The IROC lung IMRT audit had γ-agreement scores (γAS) above 93%. All E2E tests had γAS above 90.6% and point differences below 2.3%. All patient-specific QA had γAS above 95.7% and point differences below 4.1%.
Conclusion: The fully pre-configured Halcyonâ„¢ fast-rotating O-ring gantry linac system fulfills the international commissioning codes-of-practice. The good agreement between the measured and calculated dose of IMRT/VMAT for a wide range of treatment indications confirmed the treatment delivery quality.
Keywords
Commissioning, Linear Accelerator, Quality Assurance
Taxonomy
TH- External beam- photons: Quality Assurance - VMAT
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