Effects of photodynamic therapy on the structural integrity of vertebral bone.

Study Design. This study investigates the effects of photodynamic therapy (PDT) on the structural integrity of vertebral bone in healthy rats. Objective. To determine the short-term (1 week) and intermediate term (6 weeks) effects of a single PDT treatment on the mechanical and structural properties of vertebral bone. Summary of Background Data. Spinal metastasis develops in up to one-third of all cancer patients, compromising the mechanical integrity of the spine and thereby increasing the risk of pathologic fractures and spinal cord damage. PDT has recently been adapted to ablate metastatic tumors in the spine in preclinical animal models. However, little is known about the effects of PDT on the structural integrity of vertebral bone. Methods. A single PDT treatment was administered to healthy Wistar rats at photosensitizer and light doses known to be effective in athymic rats bearing human breast cancer metastases. At both 1 and 6 weeks post-treatment, changes in trabecular architecture, global stiffness and strength of vertebrae were quantified using micro-CT stereological analysis and axial compression testing. Results. At 6 weeks, there was a significant increase in bone volume fraction ( to 55.7 +/- 11.1% vs. 38.5 +/- 6.4%, P < 0.001) and decrease in bone surface area-to-volume ratio (16.9 +/- 5.0/mm vs. 22.8 +/- 4.5/mm, P = 0.001), attributed to trabecular thickening (130 +/- 40 mu m vs. 90 +/- 20 mu m, P < 0.001). Similar trends were found at 1 week after PDT. There was a significant increase in stiffness from control (306 +/- 123 N/mm) at 1 week (399 +/- 150 N/mm, P = 0.04) and 6 weeks (410 +/- 113 N/mm, P = 0.05) post PDT. There was a positive trend toward increased ultimate stress at 1 week, which became statistically significant at 6 weeks compared with control (39.3 +/- 11.3 MPa vs. 27.5 +/- 9.5 MPa control, P = 0.002). Conclusion. Not only may PDT be successful in ablating metastatic tumor tissue in the spine, but the positive effects of PDT on bone found in this study suggest that PDT may also improve vertebral mechanical stability.