Spine SRS, also known as stereotactic body radiotherapy, is increasingly being applied to treat primary and metastatic spinal tumors.12,13 Spine SRS is a paradigm shift from conventional radiotherapy practice, where the aim is to deliver low dose-per-fraction treatments (1.8-4.0 Gy per day) daily to a total dose limited by the tolerance of the spinal cord. Spine SRS overcomes this convention by using modern radiation technologies that allow for concentrating the dose within the tumor and away from the spinal cord, to permit tumor dose escalation while maintaining safe spinal cord dose constraints. 5 Delivery precision has been reported to be within 1.5 mm and 1° with a 95% confidence interval. Recently, the Canadian Association of Radiation Oncology defined stereotactic body radiotherapy (including as it pertains to the spine) as "The precise delivery of highly conformal and image-guided hypofractionated external beam radiotherapy, delivered in a single or few fraction(s), to an extracranial body target with doses at least biologically equivalent to a radical course when given over a protracted conventionally fractionated schedule."17 Philosophically, this translates to delivering a "locally curative" dose as opposed to a "locally palliative" dose for spinal metastatic disease. As experience with high dose-per-fraction spine SRS is gained (such that the most common practices range from 30 to 40 Gy in 5 frac- Spine stereotactic radiosurgery (SRS) is increasingly being used to treat metastatic spinal tumors. As the experience matures, high rates of vertebral compression fracture (VCF) are being observed. What is unknown is the mechanism of action; it has been postulated but not confirmed that radiation itself is a contributing factor. This case report describes 2 patients who were treated with spine SRS who subsequently developed signal changes on MRI consistent with tumor progression and VCF; however, biopsy confirmed a diagnosis of radiation-induced necrosis in 1 patient and fibrosis in the other. Radionecrosis is a rare and serious side effect of high-dose radiation therapy and represents a diagnostic challenge, as the authors have learned from years of experience with brain SRS. These cases highlight the issues in the new era of spine SRS with respect to relying on imaging alone as a means of determining true tumor progression. In those scenarios in which it is unclear based on imaging if true tumor progression has occurred, the authors recommend biopsy to rule out radiation-induced effects within the bone prior to initiating salvage therapies.

Image-guided radiotherapy is widely used in NZ; however, there is a wide variation in its application between centres. Detailed tumour site-specific, imaging modality-specific national guidelines will allow standardization of IGRT practices.

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