Improving tumour target coverage in spine stereotactic radiotherapy

Abstract

Spine stereotactic body radiotherapy (SBRT) has high rates of local control. Local control failure rates are higher in cases with epidural disease and radioresistant histologies. A limiting factor is the proximity of the spinal cord and the competing risk of radiation myelitis. The overlap of the target and spinal cord planning margins is often a deciding factor in target coverage and epidural dose. The aim of this thesis was to develop solutions to improve target coverage in spine SBRT. Chapter 2-4 were designed as a step-wise approach to developing spine SBRT on the MR-linac platform for the purposes of utilising adaptive radiotherapy to improve target coverage. Chapter 2 developed an imaging protocol with observer analysis of image structure clarity relevant to spine SBRT. Chapter 3 explored the capabilities of the platform and software to generate spine SBRT radiotherapy plans of equal quality and matching clinical goals compared to CyberKnife (Accuray, Sunnyvale), the current radiotherapy platform used at RMH. Chapter 4 simulated an online adaptive MR-linac workflow to show that standard spine SBRT workflows exceed the reported dose to the spinal cord by up to 27% in up to 80% of cases. Adaptive MR-linac workflows achieved small gains in PTV coverage we seen in a 3/5 of cases. Chapter 5 and 6 looked at positioning patients for CyberKnife based treatments in the prone setup to improve target coverage in posteriorly based vertebral tumours. 15 cases were retrospectively replanned in Chapter 5 to identify patient anatomical factors to guide a prospective study in chapter 6. The incomplete study was unable to show a benefit from prone treatment to improve PTV coverage to allow recommendations for clinical adoption. Chapter 7 utilised data from prior chapters to calculate spine planning risk volume (PRV) margins around the spinal cord. Reduced margins are expected to reduce the overlap of target and spinal cord planning volumes and improve coverage. A 1.75mm MR-linac spine PRV was recommended which is a reduction on current RMH margins and importantly was a markedly smaller margin than other standard radiotherapy platforms. A smaller CyberKnife supine PRV margin was achieved but in the prone position a smaller margin could not be recommended.