A preliminary safety assessment of vertebral augmentation with 32P brachytherapy bone cement

Comprehensive treatment for vertebral metastatic lesions commonly involves vertebral augmentation (vertebroplasty or kyphoplasty) to relieve pain and stabilize the spine followed by multiple sessions of radiotherapy. We propose to combine vertebral augmentation and radiotherapy into a single treatment by adding 32P, a β-emitting radionuclide, to bone cement, thereby enabling spinal brachytherapy to be performed without irradiating the spinal cord. The goal of this study was to address key dosimetry and safety questions prior to performing extensive animal studies. The 32P was in the form of hydroxyapatite powder activated by neutron bombardment in a nuclear reactor. We performed ex vivo dosimetry experiments to establish criteria for safe placement of the cement within the sheep vertebral body. In an in vivo study, we treated three control ewes and three experimental ewes with brachytherapy cement containing 2.23–3.03 mCi 32P ml−1 to identify the preferred surgical approach, to determine if 32P leaches from the cement and into the blood, urine, or feces, and to identify unexpected adverse effects. Our ex vivo experiments showed that cement with 4 mCi 32P ml−1 could be safely implanted in the vertebral body if the cement surface is at least 4 mm from the spinal cord in sheep and 5 mm from the spinal cord in humans. In vivo, a lateral retroperitoneal surgical approach, ventral to the transverse processes, was identified as easy to perform while allowing a safe distance to the spinal cord. The blood, urine, and feces of the sheep did not contain detectable levels of 32P, and the sheep did not experience any neurologic or other adverse effects from the brachytherapy cement. These results demonstrate, on a preliminary level, the relative safety of this brachytherapy cement and support additional development and testing.