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Evaluation of Flattening-Filter-Free Arcs for Lung SBRT

L Ouyang

L Ouyang*, H Lee , A Pompos , Y Yan , S Jiang , R Foster , UT Southwestern Medical Center, Dallas, TX


SU-E-T-389 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To evaluate dynamic conformal arc therapy (DCAT) and volumetric-modulated arc therapy (VMAT) using flattening-filter-free (FFF) beams for treating non-small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT).

Methods:Five clinical patients, previously treated with SBRT using non-coplanar 3D conformal radiation therapy (3DCRT), were selected and re-planned with DCAT and VMAT with both FFF beam (6xFFF with a dose rate of 1200MU/min) and flattened beams (6x with 600 MU/min). All the arc plans were planned with one 360° arc and normalized to the same PTV coverage (100% prescription cover 95% PTV volume) for comparison. Treatment planning metrics such as R100%, R50%, D2cm and lung V20 were compared to the original plans. To evaluate the treatment efficiency differences, all the arc plans were delivered and plan delivery time was compared to that of the clinical treatment as recorded in Mosaiq.

Results:All plans meet RTOG conformality constraints and normal tissue tolerances. Average R100% was similar for FFFDCAT (1.07±0.05), FFFVMAT (1.03±0.08) and flattened VMAT (fVMAT) (1.03±0.09) while flattened DCAT (fDCAT) (1.09±0.07) and 3DCRT (1.11±0.06) were significantly inferior (p<0.05, t test). FFFDCAT produced the best average intermediate dose conformality as indicated by R50% (3.86±0.44) and D2cm (43.7±5.3%) when compared to all the other techniques. Significant improvement (p<0.05) in lung V20 was also found with FFFDCAT (2.33±2.06%) when compared to FFFVMAT (2.48±2.03%), fVMAT (2.52±2.07%) and fDCAT (2.64±2.11%) and was slightly better than 3DCRT (2.43±2.04%), though not significant. The FFFDCAT delivery significantly improves the treatment efficiency with an average plan delivery time of 2.70±1.57 min (p<0.05), as compared to fDCAT (5.98±3.45min), fVMAT (6.51±2.94) and 3DCRT (25.14±5.67), but is not significantly better than the FFFVMAT (3.18±1.04).

Conclusion:Combining FFF beams and DCAT provide promising improvements in NSCLC SBRT treatment in both plan quality and treatment efficiency and unlike VMAT, tumor coverage is not affected by MLC interplay effects.

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