Purpose: To demonstrate the plan quality and delivery efficiency of volumetric modulated arc therapy (VMAT) with Halcyon ring delivery system (RDS) in the treatment of single-isocenter/two lesion lung stereotactic body radiation therapy (SBRT).
Methods: Fourteen patients with two lung lesions previously treated with 6MV-FFF VMAT lung SBRT using a single-isocenter on C-arm TrueBeam Linac were selected. The mean isocenter to lesion distance was 6.0 ± 2.4 cm and combined target volume was 56.8 ± 37.4 cc. Prescribed dose was 50 Gy to each lesion in five fractions, treated every other day with 2-4 non-coplanar partial/full arcs using 6MV-FFF beam and AcurosXB dose calculation engine for tissue heterogeneity corrections. For comparison, TrueBeam single-isocenter plans were re-optimized for Halcyon RDS coplanar geometry. TrueBeam and Halcyon plans were normalized for identical combined target coverage. High and intermediate-dose spillage were compared. Normal lung metrics and dose to organs at risk (OARs) were evaluated. Plan complexity was assessed via total monitor units, beam-on time and modulation factor.
Results: Halcyon plans were statistically similar to clinically delivered TrueBeam plans. No statistical differences in target conformity, dose heterogeneity and intermediate-dose spillage were observed (all, p>0.05). Halcyon plans on average demonstrated statistically insignificant reduced maximum dose to most adjacent OAR. However, Halcyon yielded statistically significant lower maximal dose to the ribs (p=0.02) and normal lung V10Gy (p=0.04). Plan complexity slightly increased as seen in the average increase of monitor units, modulation factor and beam-on time by 533, 0.5 and 2.8 min, respectively. However, the estimated overall treatment time was reduced by 2.4 min.
Conclusion: SBRT treatment of synchronous lung lesions via single-isocenter VMAT on Halcyon RDS is feasible and dosimetrically equivalent to clinically delivered TrueBeam plans. Halcyon provides excellent plan quality and shorter overall treatment time that may improve patient compliance, reduce intra-fraction movement and improve clinic efficiency.
Funding Support, Disclosures, and Conflict of Interest: This work is partially supported by Varian Medical Systems.