(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: analyze the portal dosimetry (PD) patient-specific quality assurance (PSQA) results of our center's newly installed Varian Halcyon v1.0 linear accelerator from the first case to now, a total of 7 months, 1203 patients.
Methods: Varian Halcyon v1.0 linear accelerator with a dual-layer MLC capable of delivering only 6 MV-FFF beams and a standard aSi EPID panel were used.1.For all patients, calculate the composite field PD?As and the average value of each fields PD?As.2.Compare the PD?As between VMAT and IMRT techniques 3.The QA results of Portal dosimetry (PD), ArcCheck (AC) and point dose at a point inside the radiation field (PDinF) method from randomly selected 40 patients before and after EPID&AD (absolute dose) calibration based on criteria of: For PD method, the 2D ?As were calculated using 2mm/2% criteria. For AC method, the 2D?As were calculated using 2mm/2% and 3mm/3% criteria.For PDinF method, CC13(0.13cc) IC and Mobius Verification Phantom were used to measure one point in radiation field and TPS/measured differences were calculated.4.Analyze EPID uniformity value in Machine Performance Check, suggestions are given for the EPID/AD calibration period.
Results: Spearman's correlation coefficient of the ?As between the composite field and separated fields was 0.529, the significance index was less than 0.01.2.The PD?As (2 mm/2%) of all VMAT plans were 96.45%±4.37% and FF-IMRT were 93.24%±6.79%. There was statistical difference in the results of PD patients before and after EPID&AD calibration (P<0.05), but no statistical difference (P>0.05) in AC, PDinF method.3.Averaging 31.33±2.31 days is needed for EPID uniformity changes from 0 to 1%.
Conclusion: average PD?As of VMAT is higher than FF-IMRT, so the intervention level needs to be determined separately.2.There is a moderate correlation between the composite field and separated field.3.PD uniformity deviation will affect the PSQA result.The EPID&AD calibration needs to be performed on a regular basis, preferably one month.
Keywords
Quality Assurance, Commissioning, Portal Imaging
Taxonomy
TH- External Beam- Photons: portal dosimetry, in-vivo dosimetry and dose reconstruction
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