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Dosimetric Comparison of RapidArc Photon and Pencil-Beam Scanning Proton Plans for Lung Stereotactic Body Radiation Therapy

M Parham1*, S Ahmad2 , H Jin3 , (1) University of Oklahoma | Health & Science Center, Midwest City, OK, (2) University of Oklahoma Health Sciences Center, Oklahoma City, OK, (3) University of Oklahoma Health Science Center, Oklahoma City, OK


(Tuesday, 7/16/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 1

Purpose: To compare Varian RapidArc with Mevion proton pencil-beam scanning (PBS) for lung stereotactic body radiotherapy (SBRT).

Methods: Ten lung SBRT RapidArc plans treated with a Truebeam linac were retrospectively selected. The prescriptions ranged from 50.0 to 55.5 Gy in 3 or 5 fractions. The RapidArc plans were generated with 6 MV-FFF and 2-4 half arcs and calculated using AcurosXB (version 15.6). The plans were normalized to cover at least 95% of planning target volume (PTV) by 100% prescription. The patient data were transferred to a RaySearch RayStation (version 8A) that was commissioned for Mevion S250i with Hyperscan. Proton plans were generated by robust planning (position uncertainty: 5 mm; range uncertainty: 3.5%) with 3-4 beams and single-field optimization and calculated by Monte Carlo. The gross tumor volume was overridden by muscle density as instructed by a published guideline. The PBS plans were normalized to cover at least 99% of clinical target volume (CTV) by 100% prescription. The plans were compared in terms of target coverage (PTV for RapidArc and CTV for PBS; homogeneity index (HI=(D₂-D₉₈)/Rx), conformity index (CI=V[95%PTV or 99%CTV]/TV), Dmax, and Dmean), V₂₀ and V₅ of ipsilateral-lung, and Dmax and Dmean of 5-mm expanded spinal cord.

Results: The PBS plans delivered higher Dmax and Dmean to the targets with better CI (RapidArc=1.3±0.1 vs PBS=1.0±0.0) and worse HI (RapidArc=0.08±0.03 vs PBS=0.12±0.03). V₂₀ and V₅ of ipsilateral-lung (RapidArc/PBS) were 10.0±7.3%/13.5±10.1% and 23.9±14.7%/25.9±16.3%, respectively. Dmax and Dmean of the expanded cord (RapidArc/PBS) were 19.7±7.1/7.1±9.3% and 3.1±1.8/0.5±0.6%, respectively.

Conclusion: While PBS provided more conformal dose to the targets, RapidArc delivered better homogenous dose with less maximum and mean dose. RapidArc spared lung slightly better due partly to PBS robust planning; however, the difference was marginal. PBS has high potential to save surrounding critical organs with limited beam angles and Bragg peaks.


Protons, Lung


TH- External Beam- Particle therapy: Proton therapy - treatment planning/virtual clinical studies

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