Pre- And Post Sbrt Metabolic Response For Osseous Spine Metastases

SBRT, predominantly, causes indirect cell kill through (1) ceramide induced vascular apoptosis which starts few hours after treatment & peaks around 3rd day (2) immunological alteration in the host tumor microenvironment, a process which starts around 3rd day post-treatment & peaks at 10-12 days. Routine Post SBRT follow-up imaging is advocated after 3 months. Present study evaluated the clinicoradiobiologic response of single or multi-fraction spine SBRT post-treatment for spinal oligometastases with serial FDG PET-CT scans at specified time points. 10 patients with localized spinal metastases with primaries from lung-3, breast - 2, kidney - 3, Melanoma - 2, Thyroid - 1 were included in proof-of-concept study. Patients were having lesions anywhere from C1 to L5, less than 5 cms paraspinal mass, >3mm gap between edge of the lesion, 2 contiguous spine levels with <50% body involvement & No spinal instability. All patients underwent S board rigid fixation / body fix immobilization, advanced image guidance using 6 DOF corrections on C-arm Linac. Fractionation used for treatment was either 16 / 1 Frc. or 24gy/3 Frc based on the clinical scenario, goals of treatment & projected life expectancy. All patients underwent limited metabolic imaging Pre- treatment and Post-treatment PET-CT scans up to 3mo. Post SBRT serial PET-CTs scans were timed serially at 72hrs, 10 days, 30 days, 60 days, 90 days & 180 days post treatment. PET-CT images were reviewed in order to determine the pre- and post- treatment maximum standardized uptake value (m-SUV) of the lesion, including “complete resolution” of FDG- avidity. Corresponding Morphologic changes in the target lesions and surrounding normal bony were studied on the PET-CT images during the 3mo. evaluation period. All the 10 patients showed nearly consistent serial regression patterns in metabolic activity posttreatment. The observed metabolic regression was between 55%-60% (median 53%) 48hrs after treatment, 70% metabolic regression (median – 68%) was observed 10 days post-treatment. A median SUV increase of 2.7 was observed in all pts in the first- and second-month scan when compared with 10th day post Treatment scan. At 3rd month 7 out of 10 pts had FDG non-Avid disease. During the evaluation period at 3rd month there was some change in bony architecture (mineralization starting) & regression of paraspinal mass lesions. The mineralization process is nearly completed by 6 months in 8/10 pts. Early metabolic response post Spine SBRT was seen at 48hrs post treatment & maximum metabolic response seen at 3mo. post SBRT. The bony mineralization starts in 3mon. and nearly completed by 6mo.