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Variable Dose-Rate Conformal Arc Planning for Stereotactic Body Radiotherapy

B Zhang

b zhang*, M Lee , S Chen , W D'Souza , K Prado , B Yi , Univ. of Maryland School Of Medicine, Baltimore, MD


SU-F-FS2-7 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: Four Seasons 2

Purpose: Stereotactic body radiotherapy (SBRT) is often delivered with 9~12 static beams or 2~3 VMAT beams. Static plans require longer treatment time and VMAT plans use modulated beams which may not be desired for small fields and moving targets. There have been suggestions on the benefit of using conformal arc with variable dose rate. This study developed and tested a script for a planning system (RayStation, Raysearch, Sweden) to convert a weight-optimized plan with static beams into a single variable dose-rate conformal arc (VDCA) beam.

Methods: Beam weights of a plan with 22 coplanar static beams of 16 degree gantry spacing are optimized based on clinical goals and critical organ dose constraints. Then the plan is converted to a single-arc-beam plan of 4 degree gantry spacing. Ten lung SBRT plans (5 with 9~12 static fields, 5 with VMAT) were retrospectively used for comparisons. Plan quality, planning time and treatment delivery time were compared between the clinical static/VMAT and the VDCA plans.

Results: Planning time for a VMAT and a VDCA plan are 95 min (60~120 min) and 35 min (25~45 min) on average, respectively. Plan qualities for VDCA plans were comparable or better than the clinical ones. For VMAT, it is not unusual to have 2~3 arc beams. On the other hand, one arc beam is enough for VDCA plans unless it reaches the maximum MU limit of the machine. Variation of conformity index was minimal. Delivery times for the static plans were 5~8 minutes, while those for VMAT and VDCA are 3~5 and less than 3 minutes, respectively.

Conclusion: A method to generate VDCA plans for SBRT has been developed. VDCA plans are efficient in planning time and beam delivery, while plan quality is not compromised.

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