Room: AAPM ePoster Library
The objective of this work was to compare dosimetric differences between plans with lateralized targets and isocenter in patient’s midline using RapidArc with jaw tracking technique (JTT) and static jaw technique (SJT).
Twenty-seven SBRT patients were planned with JTT and SJT using Eclipse v15.1 and TrueBeam STx (Varian) with 6MV and 10MV. Comparison were done using average PTV's D95%, D2%, Paddick's CI and V5Gy, V10Gy and Dmean for Body-GTV. The average area defined by the primary jaws was calculated for all JTT and SJT plans. Its ratio was related to Dmean variation to establish a comparison criterion. Fields portal images were taken for verification.
No differences were found for D95%. JTT showed an increase of D2% about 3% (p=0.0006). The PCI for JTT and SJT were 0.67±0.20 and 0.57±0.25 respectively (p=0.006). For Body-GTV the dosimetric parameters for JTT and SJT were: V5Gy, 2798±1385 cc versus 3047±1439 (p=2.7E-6); V10Gy, 1116±934 versus 1266±1004 (p= 1.6E-5) and Dmean, 1.76±1 Gy versus 1.98±1 Gy (p=6.3E-8). The relationship between area ratio and Dmean variation resulted in Spearman and Pearson coefficients of 0.765 and 0.757. Increased dose ring shaped areas were observed in some SJT plans (wider primary collimation openings). The corresponding portal images of those SJT plans showed radiation leakage between closed opposed leaves with a peak about 30% of the maximum fluence.
JTT plans were capable of delivering equivalent or better PTV coverage with better healthy tissue protection. The decrease of the primary collimator opening areas with JTT showed a significative relationship with the healthy tissue dose sparing.