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Dosimetric Differences in Non-Coplanar Vs Coplanar Treatment of SBRT of Lung Lesion with VMAT Do Not Yield Differences in Clinically Observable Outcomes

L Sanford*, J Molloy , R McGarry , M Randall , D Pokhrel , University of Kentucky, Lexington, KY


(Tuesday, 7/31/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 7

Purpose: To quantify and compare the dosimetry, including estimated local tumor control rates, for non-coplanar vs coplanar VMAT (n/c-VMAT) for SBRT of lung lesion treated with Acuros-based dose calculation.

Methods: Clinical SBRT plans for 10 early-stage NSCLC patients receiving 30 Gy in 1 fraction using n-VMAT were replanned using 3-4 coplanar partial arcs, c-VMAT. These patients were treated at Truebeam using 6MV-FFF beam and Acuros-based dose calculation. Average planning target volume (PTV) from 4D-CT (MIP) was 13.4cc ± 13.8cc. Plans were normalized to receive same target coverage. Conformity (CI), heterogeneity indices (HI), intermediate dose spillage (R50%, D2cm, Gradient Index (GI), Gradient Distance (GD)), normal lung V20, V10, V5, dose to organs at risks (OARs) including ribs and skin, and predicted local control rates were compared for both plans. Two-year actuarial local tumor control rates were estimated by utilizing tumor size-adjusted biophysical model (s-BED, accounts for tumor size) and dose received by 99% of the PTV (PTVD99) for both plans. Metrics were compared using two-tailed paired t-tests using upper bound of p-value <0.05.

Results: No statistically significant difference was obtained in CI, HI, GI, GD, R50%, D2cm, normal lung dose, or dose to most OARs between the n-VMAT and c-VMAT plans. Only statistically significant variable was dose to 10 cc of skin, being about 9.0% higher with c-VMAT (p-value=0.003) compared to n-VMAT. Predicted 2-year local control rates utilizing s-BED model was statistically insignificant (p-value=0.824) between two plans.

Conclusion: For these cases, n-VMAT of a single fraction lung SBRT treatment was dosimetrically equivalent to c-VMAT. However, c-VMAT provides faster treatment due to faster treatment time between arcs (reducing couch kick time for therapists), potentially resulting in less intra-fraction motion error and improving patient comfort. Clinical follow up results are warranted to validate our predictions.


Lung, Bioeffect Dose, Stereotactic Radiosurgery


TH- External beam- photons: extracranial stereotactic/SBRT

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