Treatment Planning and Delivery Evaluation of Volumetric Modulated Arc Therapy for Stereotactic Body Radiotherapy of Spinal Tumours: Impact of Arc Discretization in Planning System

The purpose of this study was to compare single arc volumetric modulated arc therapy (VMAT) to intensity modulated radiotherapy (IMRT) for spine SBRT in terms of target coverage, organ at risk (OAR) sparing and delivery performance. VMAT plans with 91 control points (VMAT-91CP) were generated for 15 spine metastases patients previously treated with a nine-field IMRT technique. VMAT and IMRT plans were compared based on target coverage, maximum spinal cord dose, maximum plan dose and volume of normal tissue receiving 20% to 80% of the prescribed dose. Treatment delivery time and monitor units (MU) were measured to determine delivery efficiency. To assess the impact of arc discretization in the treatment planning system (TPS), the VMAT-91CP plans were modified by almost doubling the number of CPs (VMAT-181CP). Planned-to-delivered dose agreement for both techniques was assessed using two types of 3D detector arrays. VMAT-91CP target coverage was equivalent to IMRT while maintaining or improving spinal cord sparing. This was achieved without increasing the volume of normal tissue receiving low or intermediate dose levels. Planned-to-delivered dose agreement equivalent to IMRT was achieved with VMAT, but required decreasing the CP angular spacing from 4 degrees to 2 degrees (VMAT-181CP plans). On average, VMAT-181CP plans reduced delivery time by 53% compared to IMRT. Single-arc VMAT for spine SBRT improved delivery efficiency while maintaining target coverage and OAR sparing compared to IMRT. VMAT plans generated with a CP gantry angular spacing of 2 degrees is recommended to avoid a discretization effect in the TPS and ensure acceptable planned-to-delivered dose agreement.