Room: Exhibit Hall | Forum 9
Purpose: To evaluate the efficacy of utilizing multiple isocenters to achieve non-coplanar beam geometries in SBRT.
Methods: Ten clinically treated lung SBRT plans that contained non-coplanar geometry via couch rotations (NC-CR) were chosen for evaluation. All cases were planned for 6MV-FFF Varian Truebeam photon RapidArcÂ® delivery using the Eclipse v13.6 PO optimization tools and a final dose calculation with the Acuros 13.6.23 algorithm. Prescriptions varied from 50-55 Gy in 3-5 fractions. Modified plans were obtained by replacing the NC-CR non-coplanar arcs with non-coplanar arcs achieved by superior and inferior shifts of the isocenter (NC-IS). If axial plane arcs were used in the clinical plans (i.e. no couch rotation), they were unchanged in the modified plans. New optimizations were obtained using identical constraints and the final NC-IS plans were normalized to the minimum PTV dose as achieved in the clinic plans. The plan metrics PTV maximum, conformity index (CI), R50, D2cm, MU, Lung V20, Lung V5, and Body V2Gy were evaluated for each plan.
Results: With the exception of MU, other evaluated plan metrics showed similar behavior between the NC-CR and NC-IS approaches. For the NC-IS plans R50, Lung V20, Lung V5 and V2Gy showed small average increases while CI and D2cm showed small average decreases. All non-MU metrics evaluated showed improvement in some cases while showing degradation in others. MU increased in all NC-IS cases and averaged 49% larger values.
Conclusion: Results suggests use of NC-IS techniques to obtain non-coplanar geometry can produce plans with similar dosimetric characteristics to those obtained using the NC-CR approach. This has potential implications for new ring gantry LINAC designs, such as the Varian Halcyon, without couch rotation functionality. Further investigation is required to evaluate advantage of this technique in cases requiring complex beam geometries, such as targets adhered to the rib or chest wall.