Purpose: Osseous pseudoprogression (OPP) on MRI can mimic true progression in lesions treated with spine stereotactic radiosurgery (SSRS). Our aim is to describe the prevalence and time course of OPP, in order to assist radiologists in assessment of post-SSRS patients.
Materials and Methods: A secondary analysis of two prospective trials was performed. MRIs before and after SSRS were assessed for response. OPP was defined as transient growth in signal abnormality centered at the lesion with a sustained decline on follow-up MRI that was not attributable to chemotherapy.
Results: From the initial set of 223 patients, 37 lesions in 36 patients met inclusion criteria and were selected for secondary analysis. Five of the 37 lesions (14%) demonstrated OPP and 9 demonstrated progressive disease. There was significant association between single-fraction therapy and the development of OPP (p=0.01), and there was a significant difference in OPP-free survival between single- and multi-fraction regimens (p=0.0052). In lesions demonstrating OPP, time to peak size occurred between 9.7 and 24.4 weeks after SSRS (mean: 13.9 weeks, 95% CI: 8.6-19.1 weeks). The peak lesion size was between 4-10 mm larger than baseline. Most lesions returned to baseline size between 23-52.4 weeks following SSRS.
Conclusion: Progression on MRI obtained between 3 and 6 months following SSRS should be treated with caution, as OPP may be seen in > 1/3 of these lesions. Single-fraction SSRS may be associated with OPP. The possibility of OPP should be incorporated into prospective criteria for assessment of local control following SSRS.