Osseous Pseudoprogression in Vertebral Bodies Treated with Stereotactic Radiosurgery: A Secondary Analysis of Prospective Phase I/II Clinical Trials

BACKGROUND AND PURPOSE: Osseous pseudoprogression on MR imaging can mimic true progression in lesions treated with spine stereotactic radiosurgery. Our aim was to describe the prevalence and time course of osseous pseudoprogression to assist radiologists in the assessment of patients after spine stereotactic radiosurgery. MATERIALS AND METHODS: A secondary analysis of 2 prospective trials was performed. MRIs before and after spine stereotactic radiosurgery were assessed for response. Osseous pseudoprogression was defined as transient growth in signal abnormality centered at the lesion with a sustained decline on follow-up MR imaging that was not attributable to chemotherapy. RESULTS: From the initial set of 223 patients, 37 lesions in 36 patients met the inclusion criteria and were selected for secondary analysis. Five of the 37 lesions (14%) demonstrated osseous pseudoprogression, and 9 demonstrated progressive disease. There was a significant association between single-fraction therapy and the development of osseous pseudoprogression (P = .01), and there was a significant difference in osseous pseudoprogression-free survival between single- and multifraction regimens (P = .005). In lesions demonstrating osseous pseudoprogression, time-to-peak size occurred between 9.7 and 24.4 weeks after spine stereotactic radiosurgery (mean, 13.9 weeks; 95% CI, 8.6-19.1 weeks). The peak lesion size was between 4 and 10 mm larger than baseline. Most lesions returned to baseline size between 23 and 52.4 weeks following spine stereotactic radiosurgery. CONCLUSIONS: Progression on MR imaging performed between 3 and 6 months following spine stereotactic radiosurgery should be treated with caution because osseous pseudoprogression may be seen in more than one-third of these lesions. Single-fraction spine stereotactic radiosurgery may be associated with osseous pseudoprogression. The possibility of osseous pseudoprogression should be incorporated into the prospective criteria for assessment of local control following spine stereotactic radiosurgery.