Room: Exhibit Hall | Forum 9
Purpose: To evaluate plan quality and treatment delivery efficiency of single-isocenter/two-lesion volumetric-modulated arc therapy (VMAT) lung SBRT treatment.
Methods: Eight patients with two lung lesions underwent single-isocenter highly-conformal non-coplanar VMAT SBRT treatment. Single-isocenter was placed approximately between the two lesions; tumor distance to isocenter was 3.7-9.6 cm. Mean combined planning target volume (PTV) was 44.0Â±23.4cc, (ranged: 20.5-91.8cc) obtained from ITV on 4D-CT co-registered with 3D-CT. Doses were 54Gy (n=6) and 50Gy (n=2) in 3 and 5 fractions. Plans were calculated in Eclipse with AcurosXB and normalized to PTV(D95)=100%. For comparison, two-isocenter plans (isocenter placed centrally in each target) were retrospectively created. Conformity indices (CIs), heterogeneity index (HI), gradient index (GI), gradient distance (GD), R50% and D2cm were calculated. Normal lung V5, V10, V20, MLD and other organs at risk (OAR) doses were evaluated. Number of monitor units (MUs), beam-on time and Octavius QA results were recorded. Metrics were compared using two-tailed student t-tests using an upper bound of p-value<0.05.
Results: No statistically significant difference was obtained in CI, HI, GD, GI, R50%, D2cm and V20 between single-isocenter and two-isocenter plans. In addition, doses to most OAR were statistically insignificant. All values were acceptable per SBRT protocols. However, for single-isocenter plans as the distance between the lesions increased the volume of lung receiving lower dose (V5, V10) and MLD increased. The number of MUs and beam-on time was reduced by a factor of 1.5 for single-isocenter compared to two-isocenter (not accounting for 2nd isocenter setup time). Single-isocenter/two-lesion VMAT lung SBRT plans demonstrated accurate dose delivery of 98.1Â±3.2% for clinical gamma passing rate of 3%/3mm.
Conclusion: Treatment of two lung lesions with centrally located single-isocenter was dosimetrically equivalent to two-isocenter plans. Faster treatment delivery for single-isocenter treatments can improve patient comfort and reduce the amount of intra-fraction motion errors for well-suited patients.