SU-E-T-556: Verification and Evaluation of Acuros XB Dose Calculations for Stereotactic Ablative Radiotherapy of the Thoracic Spine

Abstract

Purpose: To evaluate the Eclipse Acuros XB (AXB) dose calculation engine for Stereotactic Ablative Radiotherapy (SAbR) of the thoracic spine using step-and-shoot IMRT and RapidArc. Methods: The accuracy of the AXB dose calculation engine was first verified by comparing with film and ion chamber measurements for open beams with field sizes from 2×2cm2-10×10cm2, delivered to a heterogeneous phantom of a slab-geometry. Acquired images of an anthropomorphic thoracic phantom, composed of tissue equivalent lungs, vertebrae, and soft tissue, were then used to create step-and-shoot IMRT and RapidArc treatment plans. Point dose measurements in vertebral bone and spinal cord were compared to Acuros calculations. Finally, step-and-shoot IMRT and dual-arc RapidArc plans were created using the Analytical Anisotropic Algorithm (AAA) for five patients with thoracic spinal metastases following our institutional protocol. All treatment plans were then recalculated with AXB while keeping beam parameters the same. Clinically significant dosimetric parameters, such as GTV coverage and spinal cord dose were compared. Results: Excellent agreement was achieved between point dose and planar dose measurements and the AXB calculations in the heterogeneous slab and anthropomorphic phantoms. Patient IMRT and RapidArc plans had GTV D90 and mean dose values 0.2%–2.6% and 0.2%–2.1% lower, respectively, for AXB calculations compared with AAA. The maximum cord dose was also lower for AXB compared with AAA (−2.7%±2.4% for IMRT; −2.6%±1.2% for RapidArc). AXB dose calculations were 4−6 times faster than AAA for RapidArc plans, but 7−9 times slower for IMRT plans. Conclusion: AXB calculations were shown to be dosimetrically accurate for heterogeneous media. This work demonstrates that AXB calculates a reduced dose compared with AAA in both the target and spinal cord for thoracic spine SAbR. The dosimetric differences should be considered when using AAA for thoracic spine treatment planning.