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Correlation Between the Gamma Pass Rate and the Clinically Relevant Index Based On Measurement-Guided Reconstructed Dose in Two Commercial Treatment Planning Systems for Breast Tangential IMRT

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A KANEKO*, I SUMIDA , K TAMARI , K OHTANI , O SUZUKI , Y SEO , F ISOHASHI , Y YOSHIOKA , K OGAWA , Department of Radiation Oncology, Osaka University Graduate School of Medicine, Suita, Osaka


SU-I-GPD-T-303 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the relationship between the gamma pass rate and clinically relevant indices (organ specific gamma pass rates, dose-volume metrics and TCP/NTCP values) derived from the measurement-guided reconstructed dose in two commercial treatment planning systems (TPS) with different concepts of beam source modeling.

Methods: Twenty one left-sided early breast cancer patients were selected. Two opposed tangential 6 MV x-ray beams were used in fixed gantry step-and-shoot IMRT delivery. The DICOM RT-plan file was exported from the XiO, and was imported to another TPS (RayStation) to calculate dose distribution. Per-beam planar dose verification was performed to make a relative local error distribution. Following analyses were conducted on calculated doses by each TPS. 1) the 2D gamma passing rate, 2) the measurement-guided reconstruction of the predicted dose incorporated the local error distribution, 3) the organ specific 3D gamma passing rate and 4) the clinically relevant index.

Results: Mean 2D gamma pass rates (3% global/3 mm: lower threshold 10%) for XiO and RayStation were 98.1% and 100.0%, respectively. There were no correlations between the 2D gamma pass rate and the organ specific 3D gamma pass rate in the planning target volume (PTV) and the ipsilateral lung. Differences of V5Gy and NTCP value in the ipsilateral lung between calculated and predicted doses on RayStation were significantly smaller than those on XiO (p<0.001). Differences of D95% and TCP value in the PTV on XiO were significantly smaller than RayStation (p<0.001).

Conclusion: There were weak correlations between 2D gamma passing rate and organ specific gamma passing rate under the gamma criteria of 3% global/3 mm. The difference between the predicted and the planned doses derived from two different TPSs showed significant difference of dose-volume metrics and TCP/NTCP values in the PTV and the ipsilateral lung due to the difference of beam source modeling.

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