Radiation therapy for stereotactic body radiation therapy in spine tumors: linac or robotic?

Purpose: Stereotactic body radiotherapy (SBRT) is used for spine treatments as it precisely delivers high radiation dose to tumors in close proximity to organs-at-risk (OARs). The goal of this work is to evaluate dosimetric properties of SBRT for spinal treatments with linear accelerators and CyberKnife (CK). Materials and methods: Plans of 27 patients, treated with CK for spine tumors, were also retrospectively optimized for linac-based (LB) intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). One nine-field IMRT plan and five VMAT plans were generated for each patient. The LB target volumes were uniformly expanded by 0.1 cm to accommodate for the uncertainty in patient positioning. All plans were optimized to cover 90% of the target volumes with a prescription dose of 27 Gy in three fractions. If dose constraints to OARs were not met, the prescription dose was decreased to 24 Gy. Target dose conformity and falloff were evaluated with Paddick's conformity (CI) and gradient (GI) indices. Results: PTV expansion resulted in a 31.5% volume increase in the LB plans. The three full-arcs VMAT (VMAT_3full) plans resulted in the best average CI(0.820) compared to CK(0.758) with worst average from one half-arcs VMAT (VMAT_1half) plans (0.747). Dose falloff was also superior with the VMAT_3full plans with an average GI value of 3.596, in comparison to CK(3.786) and IMRT(4.447). In 6 cases CK plans were unable to meet OAR constraints and the prescription dose was decreased to 24 Gy, compared to only 2 for VMAT_3full. Conclusion: Regardless of the larger target volumes, LB plans were comparable to CK plans. Conformity of target doses of the VMAT_3full plans were better than CK in all cases and dose fall-off was better 23 of 27 plans. Dose to OARs were lower for CK, but constraints met for all plans. The use of VMAT would reduce the treatment time.