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Portal Dosimetry Based Patient-Specific QA of a New Ring-Gantry Linac Yielded Similar Passing Rate Compared to a Portable Ion Chamber Array Based QA

P Teo*, F Li , K Fallon , P Kosterin , Z Shen , S Jang , R Lalonde , G Bednarz , M Huq , University of Pittsburgh Cancer Institute, Pittsburgh, PA


(Sunday, 7/29/2018) 4:00 PM - 4:30 PM

Room: Exhibit Hall | Forum 7

Purpose: To compare patient-specific QA results obtained from the portal imager of a new ring-gantry linac with those obtained from a 2D ionization chamber array (MatriXX, IBA Dosimetry) for VMAT and IMRT plans. In addition, the portal dosimetry (PD) QA of a large-field IMRT treatment planned using the new multi-isocenter feature of a research version of Eclipse (v15.5) is compared with the QA results obtained from MatriXX.

Methods: Ten prostate and one large-field pelvis patients were selected for this retrospective study and replanned for the new linac. The prostate plans were replanned with IMRT (7- and 9-fields) and VMAT (2- and 3-arcs) and the pelvis plan (field size of 34cm superior-inferior) was replanned with 2 isocenters (7.02cm apart) assigned automatically using the new multi-isocenter feature of Eclipse. Their portal dose was calculated using the AAA-15151 portal dose module. While PD of the large-field pelvis plan was delivered in a single plan, two separate QA plans were needed for the MatriXX. The region-of-interest for the PD analysis was selected to encompass 1cm beyond the MLC irradiated region. The gamma passing criteria was set to 3%/3mm. Gamma and point dose results obtained from PD were compared with those from MatriXX.

Results: For the prostate plans, average gamma of 97.3% was obtained from the MatriXX. This compares with 98.8% (average) for the IMRT/VMAT plans obtained from the PD of the new linac. For the pelvis case, gamma of 99% was obtained from MatriXX and PD. Average ratios (measured vs predicted point dose) of 1.02 and 0.96 were obtained from MatriXX and PD respectively.

Conclusion: Portal dosimetry QA of IMRT/VMAT plans on the new linac passes QA (>95%) with the same criteria as MatriXX QA. Without the need of a separate setup, PD offers both an efficient and reliable tool for performing patient-specific QA.


Portal Imaging, Calibration, Quality Assurance


TH- External beam- photons: portal dosimetry

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