Feasibility of spinal stereotactic body radiotherapy in Elekta Unity® MR-Linac

The Elekta Unity MR-Linac (MRL) is expected to benefit spine stereotactic body radiotherapy (SBRT) due to the improved soft tissue contrast available with onboard MR imaging. However, the irradiation geometry and beam configuration of the MRL deviates from the conventional linear accelerator (Linac). The purpose of the study was to investigate the feasibility of spine SBRT on the MRL. Treatment plans were generated for lumbar and thoracic spines. Target and spinal cord doses were measured with two cylindrical ion chambers inserted into an anthropomorphic spine phantom. Our study indicated that the Monaco treatment planning system (TPS) could generate clinical treatment plans for the MRL that were of comparable quality to the RayStation TPS with a conventional Linac. For both Linacs the planned dose within the gross tumor volume agreed with measurements within +/- 3%. For the spinal cord, while the measured doses from the TrueBeam were 1.8% higher for the lumbar spine plan and 6.9% higher for thoracic spine plan, the measured doses from MRL were 0.6% lower for the lumbar spine plan and 3.9% higher for the thoracic spine plan. In conclusion, the feasibility of spine SBRT in Elekta Unity MRL has been demonstrated, however, more effort is needed for such as optimizing the online plan adaptation method.