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Impact of Additional Intensity Modulation On Arc-Based Stereotactic Body Radiotherapy (SBRT) for Lung Cancer

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H Liu

H Liu*, J Ye, J Kim, J Deng, Z Chen, Yale New Haven Hospital, New Haven, CT

MO-F-108-7 Monday 4:30PM - 6:00PM Room: 108

Purpose:
A single-isocenter multi-segment dynamic conformal arc technique (SiMs-arc) has been used in our department in the past three years for efficient planning and delivery of over 200 lung SBRT treatments. The purpose of this study is to investigate the impact of additional intensity-modulation made available by Varian RapidArc on the dosimetric quality and delivery efficiency of arc-based lung SBRT.

Methods:
Four patients previously treated with lung SBRT using SiMs-arc were selected. SiMs-arc plans were generated with the isocenter located in the geometric center of patient axial plane (to allow for collision-free gantry rotation around the patient) and six 60°-arc segments spanning from 1° to 359°. Two RapidArc plans, one using the same arc geometry as SiMs-arc and the other using typical partial arcs (210°) with the isocenter inside PTV, were generated for each patient. All plans, generated using Eclipse V10.0, were normalized with PTV V100 to 95%. PTV coverage, dose to organs-at-risk and total MUs were compared to assess the impact of additional intensity-modulation provided in RapidArc.

Results:
RapidArc plans produced higher PTV D99 (by 1.0%-3.1%) and minimum dose (by 2.4% to 9.8%); better PTV conformality index (by 1%-8%); and less volume of 50% dose outside 2cm from PTV (by 0-20.8cc) than the corresponding SiMs-arc plans. No significant dose differences were observed for lungs, trachea, chest wall and heart. RapidArc using partial-arcs had lowest maximum dose to spinal cord. RapidArc plans required 1.5 to 1.91 times more MUs than SiMs-arc plans to deliver the same dose.

Conclusion:
Additional intensity-modulation enlisted by RapidArc produces modest dosimetric improvements over conformal arcs for lung SBRT, but requires more MUs (by a factor > 1.5) to deliver. The dosimetric improvements, most notably in PTV minimum dose and in dose conformality for irregularly shaped PTVs, may outweigh the increased MUs in using RapidArc.

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