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Effect of Beam Geometry and Number of Arcs On VMAT Planning for Large Field Head and Neck Cancer Patients: A Treatment Planning Study

S Srivastava

S Srivastava*, P Metuge , D Pinnaduwage , N Thawani , X Yan , C Ord , S Patel , S Mutyala , S Sorensen , UACC/SJHMC, Phoenix, AZ


SU-I-GPD-T-384 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: : Quality of VMAT plans vary based on number of isocenters, gantry angle, collimator rotation, and field size, specified on a trial-and-error basis. Field size and number of arcs play a significant role on large field plan quality, though this has not been extensively studied. This study investigates the effect of field size and number of arcs on plan quality and delivery efficiency, for large head and neck (HN) targets.
Methods: 70 VMAT plans were generated for 10 patients with large HN targets by changing the field size and no. of arcs for True Beam using Eclipse TPS, 6MV, with same dose and optimization criteria. For RA1 & RA2, X-jaw was opened beyond 15cm to enclose the PTV. In the remaining 5 plans (RA3- RA7) the X jaw size was kept at 15cm while increasing the number of arcs from 2-6. Plan quality was compared based on dosimetric and quality metrics-conformity(CI), homogeneity(HI), gradient index(GI) and delivery efficiency(DE), and based on total MUs and beam-on-time.
Results: Increasing the number of arcs reduced the global max by 5% from RA1-RA6, while HI improved by 40% with PTV mean dose approaching prescription dose (74.22 to 72.80 Gy). CI improved from 2-6arcs, with highest CI for RA7(1.21 ± 0.26) vs. RA1(1.72 ± 0.78). GI showed similar pattern. For OARs, increasing from 2 to 6 arcs decreased mean dose for parotids~1Gy and oral cavity ~3Gy, max dose for cord ~5Gy, mandible ~3Gy and brainstem ~5Gy. No gain in DE was seen beyond 4arcs.
Conclusion: Six arc plans are most conformal, homogeneous with sharpest fall-off and maximum OAR sparing but longest treatment time. Our study suggests using 4arcs may be an optimal strategy for VMAT planning of large volume HN targets, the clinical significance of which needs to be determined in future studies.

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