SBRT for Spine Lesions: A Novel Optimized Delivery Technique

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Spine SBRT has increased in use due to advances in image guidance, dose calculations, and patient immobilization techniques. It remains challenging to deliver a sufficiently high dose for local control while sparing sufficiently the spinal cord. Advanced optimization in treatment planning systems can result in conformal plans which sufficiently spare critical structures. The ability to plan SBRT with dose escalation is examined in this study using a new segmentation and optimization treatment planning technique in which regions of the PTV are considered separately in order to produce concave dose distributions. Ten patients treated to the thoracic and lumbar spine were selected for this planning study. In all cases, a single vertebral body was selected for treatment, regardless of the presence of other spinal metastases. The vertebral body and pedicles were included in the target delineation. Two VMAT arcs were used with a prescription of 16 Gy with a dose constraint to the spinal cord of a maximum point dose (0.035 cc) of 14 Gy. Three patients (representing cases where cord sparing difficulty was the highest, moderate, and lowest) were then selected for dose escalation to 20 Gy and to 24 Gy. Simultaneously, the use of one or two VMAT arcs was examined as well as the differences between Monte Carlo and Pencil Beam. In the 16 Gy plans, the spinal cord maximum dose was kept to 13.3 Gy on average. For 20 Gy (three cases), the maximum dose increased to 13.9 Gy and to 14.2 Gy at 24 Gy. Modulation increased with dose escalation as expected (for the most difficult case, total monitor units per Gy increased from 251 at 16 Gy to 396.1 at 24 Gy). Targets were covered at 95% in all cases except the 24 Gy plan where coverage ranged from 93% to 93.6%. The treatment plans resulted in dosimetrically optimized results for spinal cord sparing while maximizing target coverage. This is achieved by an optimization which is specific to spine SBRT anatomy. Dose escalation between 20 and 24 Gy is possible depending on constraints.