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Achieving Both High Plan Quality and High Delivery Efficiency with Hybrid Radiation Therapy Combining Fixed-Field IMRT and VMAT

G Kirlik

G Kirlik*, W D'Souza , H Zhang , University of Maryland School of Medicine, Baltimore, MD


SU-I-GPD-T-321 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To develop a radiation therapy approach that combines intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for hybrid delivery.

Methods: Our method starts with an empty set of apertures for all control points in a single arc. In each iteration, a rigorous mathematical optimization problem is solved using column generation to obtain an aperture to a control point. With the user input of number of IMRT stopping points and the maximum number of apertures for each stopping point, the algorithm terminates when either no more apertures can be obtained to improve the treatment plan quality or all VMAT and IMRT control points are filled with apertures. Besides satisfying all delivery constraints when generating apertures, the optimal IMRT stopping points are also decided by the algorithm. We demonstrate our method with generating hybrid single-arc VMAT plans with 3, 5 and 8 stopping points and at most 5 apertures at each stopping points (s3-n5, s4-n5 and s8-n5) for 10 locally advanced head-and-neck cancer cases retrospectively. We compared our method with 2-arc clinical VMAT plans.

Results: At least 95% target coverage was achieved by all plans. The average of conformity indices of primary target for s3-n5, s4-n5 and s8-n5 plans were 1.52, 1.49, 1.49 respectively comparing to 2.44 for clinical VMAT (p<0.01). For organ-at-risk sparing, the hybrid plans achieved reduction in right parotid mean dose of 32%, oral cavity mean dose of 38%, spinal cord maximum dose of 34%, brain stem maximum dose of 56%, and other normal tissue maximum dose of 6%, comparing to clinical 2-arc VMAT plans (p<0.01). The average delivery time of the three plans were 143s, 157s and 226s respectively.

Conclusion: We demonstrated that the proposed hybrid method was able to achieve high dosimetric plan quality with efficient delivery of single-arc plus 3-4 IMRT stopping points.

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