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Dosimetric Comparison Between Conventional 2.5mm MLC-Based and Jawless Flattening-Filter-Free 5mm/10mm Double-MLC Based Plans to Determine the Feasibility of Re-Irradiation SBRT for Locoregionally Recurrent Or Second Primary Head and Neck Carcinoma

S Jang*, P Teo , D Clump , D Heron , M Huq , University of Pittsburgh Cancer Institute, Pittsburgh, PA


(Wednesday, 8/1/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 9

Purpose: We compared the dosimetric-metrics of jawless 6MV-FFF and 5mm/10mm double-MLC linac with those of conventional FFF and 2.5mm-MLC linac. The impact of 5mm versus 10mm double-MLC layer on the plan quality of re-irradiation head-neck SBRT was also investigated to find the impact of MLC-resolution. Sensitivity-analysis with varying number of arcs was performed.

Methods: Nine challenging clinical cases planned with 2.5mm-MLC and 6MV-FFF linac were selected, and treatment plans for jawless 6MV-FFF and 5mm/10mm-double-MLC machine were generated using the Eclipse 15.6 Test-Version. Prescription was 44Gy/40Gy in 5fxs depending on the size of targets. To minimize any subjective bias, VMAT-techniques with the same arcs or gantry angles, which were identical to the clinical plans, were used. We also used the same optimization objectives to achieve comparable modulation-factor (RX-dose/total-MU), After achieving comparable target coverage, dosimetric-metrics(CI/R50%/D2cm(Gy)/IDL/OAR-dose) were compared between two different plans. Regarding sensitivity-analysis with arcs, the number of arcs varied from the same arcs in clinical plans to two times of the arcs with the same gantry angles but different collimator angles.

Results: With the same number of arcs, the dosimetric-metrics of jawless 5mm-double-MLC plans were slightly better than those of jawless 10mm-MLC plans;mean CI=1.20±0.13(10mm-MLC) versus 1.16±,0.12(5mm-MLC), R50=5.0±1.4 versus 4.7±1.3, D2cm(Gy)=48.6±1.7 versus 48.1±1.6. The dosimetric-metrics of jawless MLC plans were improved by increasing the number of arcs, however clinical plans showed better dosimetric plan qualities with comparable modulation-factor;mean CI=1.08±0.08 (clinical plans) versus 1.14±0.11 (jawless 10mmMLC/double arcs), R50=4.1±1.0 versus 4.6±1.2, D2cm(Gy)=46.5±2.5 versus 47.9±1.6. With the same arcs and comparable modulation-factor, the mean IDLs of clinical plans, jawless 10mm/5mm MLC plans were 84.4±2.6%/78.8±6.1%/81.3±3.7%, respectively, showing that higher modulation-factor is needed for jawless linac to improve the plan quality.

Conclusion: With the same arcs and comparable modulation-factor, conventional 2.5mm-MLC 6MV-FFF linac showed better dosimetric-metrics compared to jawless double-MLC linac for re-irradiation head-neck SBRT.:


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