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Effect of Beam Geometry and Number of Arcs On Integral Dose in VMAT Planning for Large Field Head and Neck and Pelvic Cancer Patients

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-439 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Complex, large field cancers are treated with VMAT to deliver more conformal radiation with better organ sparing, possibly at the risk of higher ID. Increased number of arcs are utilized to improve plan quality. We investigate the impact of beam geometry and number of arcs on ID.
Methods: Five each of head & neck and pelvic cases were selected for this IRB approved retrospective study. 70 VMAT plans were generated (7/ patient) by changing the field size and no. of arcs for Varian TrueBeam with Eclipse TPS. All plans used full arcs, same isocenter, collimator angles and dose optimization criteria. Dosimetry data for PTV, OARs and Body-PTV was extracted and analyzed. The ID (defined as an integration of the absorbed dose over the normal tissue volume excluding PTV) was calculated for threshold doses of 2, 5, 10, 15, 20, 40, 50 & 60 Gy, along with mean dose. Delivery efficiency was evaluated with total MUs and beam-on-time.
Results: All plans achieved ~ 95% PTV coverage. Better OAR sparing was achieved by increasing the number of arcs. Going from RA 1 to RA 7, the ID decreased with increasing arcs, with lowest ID seen with RA3 in H&N, and RA4 in pelvis. The mean ID decreased by 7% (H&N) and 16% (pelvis) from RA1 to RA7. Total MUs increased up to RA4 (2x compared to RA1), but did not change significantly beyond that. The beam on-time and total delivery time increased from RA1-RA7; beam-on-time by 6x and delivery time by 7x for H&N, and 9x for Pelvic cases.
Conclusion: Mean ID decreases with increasing number of arcs contrary to expectation, even at lower threshold doses. Improved ID with 6 arcs may be of benefit in certain clinical situations. Clinical significance of this study needs further investigation.

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