MRI-Guided SABR of Spinal Metastases: A Safety and Quality Comparison of Co-60 and Linac Treatments

Abstract

MRI-guided radiotherapy systems visualize the spinal cord and cauda equina for spine SABR (stereotactic ablative radiotherapy) setup and tracking of motion during treatment. However, concerns have been raised regarding the dosimetric limitations of the first generation of Co-60 MRI-guided systems. In prior work, we demonstrated excellent visualization of spinal structures and tolerance for Co-60 MRI-guided spine treatments. In this work we demonstrate dosimetric improvements for Linac based MRI guided SABR based on recalculation of the original Co-60 plans. Nine spinal metastases were treated with Co-60 MRI-guided radiotherapy at our institution from May 2016 to January 2017. Seven received a single 16 Gy dose and two received 30 and 24Gy in three fractions. Coverage was at least 80% (acceptable deviation per RTOG 0631, prefer >90%) in all cases. Contours were based on the International Spine Radiosurgery Consortium Consensus. Objectives are based on RTOG 0631 protocol. OAR objectives for single fraction are: spinal cord V10Gy<0.35 cc, V14Gy<0.03 cc, partial cord V10Gy<10% and cauda equina V16Gy<0.03cc. Dose spillage objectives for single fraction are volume of 105% isodose out of PTV <3 cc (V105% out of PTV <3cc), V115% out of PTV 0.0 cc, and conformity index < 1.5. OAR objectives for three fractions are based on NRG BR001 protocol: spinal cord V22.5Gy<0.03cc, V13Gy<1.2cc, cauda equina V25.5Gy<0.03cc and V21.9Gy<5cc. Dose spillage objectives were the same as the single fraction cases. Linac-based MRI guided SABR plans were generated for the Co-60 MRI guided SABR cases using vendor supplied software. Co-60. Organ at risk (OAR) objectives were met in all cases. However, dose spillage for Co-60 plans would often be considered unacceptable by RTOG 0631 protocol: median V105% out of PTV: 22.8 cc (11.4 cc – 26.9 cc) and median V115% out of PTV: 0.3 cc (0.0 cc – 8.2 cc). Treatment was delivered considering that this dose spillage was mostly into para-spinal soft tissues and not into OARs. No adverse effects have been observed, and all cases had excellent visualization of spinal anatomy with real-time patient motion tracking and gating. Linac. All OAR objectives were met in all cases. Dose spillage for Linac plans were either per RTOG 0631 protocol or acceptable deviation in all cases. Median V105% out of PTV: 1.4cc (0.18 cc – 3.0 cc) with V115% out of PTV 0.0cc. Co-60 MRI-guided SABR achieves an adequate target coverage and OAR sparing for spinal osseous metastases. Treatments are safe, well tolerated, and excellent setup and real-time guidance of treatment are observed. However, dose spillage guidelines were not achieved in many Co-60 delivered cases due to MLC width and cobalt penumbra. However, dose spillage guidelines can be met with newer generation Linac-MRI systems. MRI guidance can improve safety and dose escalation for spine SABR given clear visualization of the cord and other structures during treatment.