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The use of a simultaneous integrated boost in spinal stereotactic body radiotherapy to reduce the risk of vertebral compression fractures: a treatment planning study

In the management of spinal metastases, stereotactic radiotherapy (SBRT) is an emerging technique intended to deliver a high-radiation dose precisely to the target [1]. Concern has been raised about the dose-fractionation schemes used in spinal SBRT as the risk of vertebral compression fractures (VCF) have been reported to be as high as 40% [2]. These rates are much higher compared to conventional radiotherapy, which is typically below 5% [3]. It was hypothesized that radiation effects in the form of bone and tumor necrosis compromise the ability of the vertebrae to withstand the axial loading forces, leading to an increased risk of VCF [4]. Clinicopathologic samples of spinal metastases, obtained after SBRT, showed radiation-induced tumor and osteonecrosis, giving evidence for this hypothesis [5,6]. More support for this hypothesis is provided by studies assessing VCF risk in patients receiving high-dose radiation for primary tumors in the thoracic or abdominal region, showing that vertebral fractures are mostly seen in the high-dose regions [7]. Prevention of VCF is challenging because the metastatic lesions lie within the segment at risk. A simultaneous integrated boost (SIB) dose delivery approach might mitigate the risk of SBRT-induced VCF by boosting the gross tumor volume (GTV) with the non-affected bone included in the clinical target volume (CTV). Currently, a SIB planning design has not been formally compared with a non-SIB approach. In this study, we report on the dosimetric feasibility of a SIB SBRT approach in the treatment of spinal metastases.