Bone Abstracts

Radiosurgery of the spine is used to treat patients with spinal metastases. Recent studies have shown increased fracture risk in patients treated with high doses of irradiation. This study tested if there is a dose response relationship between prescribed radiation and induced bone loss and metabolism.

Methods: Vertebral bodies C5–C7 of ~50 week old female Yucatan minipigs were irradiated at 16, 20, and 24Gy (n=5/dose). The cervical spine regions were scanned on a clinical PET-CT scanner prior to irradiation, 1 month and 3 months after irradiation. Following CT scanning, dynamic 1h PET scans were acquired after injection with 18Fluorodeoxyglucose (18FDG). CT scans were analyzed using the Mindways’ Bone Investigational Toolkit. Changes in volumetric integral and cancellous BMD, mineral mass, and cortical thickness were calculated for the central 75% of the distance between endplates. Pharmacokinetic modeling was done of the 18FDG uptake using Siemens IRW pharmacokinetic software. Statistical analysis of CT and pharmacokinetic data was performed by ANOVA or 2-tailed t-test when appropriate.

Results: Cortical thickness and integral BMD increased in a dose dependent fashion. The increase in vertebral mineral mass was greater in the high dose group. Cancellous BMD decreased in a dose dependent fashion. 18FDG retention decreased in the irradiated vertebrae. The overall vertebral metabolic response (ki) showed significant (P<0.05) dose response at 3 months between high, medium, and low dose groups. The magnitude of decrease was similar for the 16 and 20Gy doses which was less than the decrease seen in the 24Gy treated group. At 3 months, the flux between the plasma and vertebral bodies (k1/k2) showed statistically significant difference (P<0.05) between irradiated and non-irradiated vertebral bodies in the high dose group. Conclusion: In conclusion, radiation decreased bone growth, increased cortical thickness and integral vBMD, and decreased glucose utilization in a dose dependent fashion but with a threshold effect.