SBRT for Metastatic Disease to the Sacrum

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Abstract
The sacrum is an anatomically complex region composed of five progressively smaller vertebrae, S1 to S5. Unlike the rest of the ‘mobile’ spine (cervical, thoracic, and lumbar regions) where vertebral bodies are separated by intervertebral disc structures, each sacral vertebra is separated by the remnants of previous intervertebral spaces represented as transverse lines of fibrocartilage on complete fusion on the ventral aspect of the sacrum (1–3). In addition to affording the spine flexibility, the intervertebral disc structures are also thought to prevent direct metastatic spread between vertebrae due to factors such as avascularization and high intra-disc pressure. Given the contiguous nature of the sacral vertebrae, it is unlikely the sacrum benefits from similar mechanical or biochemical factors to prevent spread, though there is scant information in the literature on this area (4–7).