Inhibition of peridural fibrosis after laminectomy using low-dose externalbeam radiation in a dog model

OBJECTIVE: Clinical studies have demonstrated a significant association bet ween the presence of extensive postlumbar discectomy peridural scar formati on and the recurrence of low back and radicular pain. Low-dose perioperativ e:radiation therapy has previously been demonstrated to inhibit peridural f ibrosis after laminectomy in a rat model.:The current study was designed to measure the effect of low-dose radiation on postlaminectomy peridural fibr osis development in a larger animal model. METHODS: Three dogs underwent a total of 12 lumbar hemilaminectomies. For e ach animal, two levels received 1) external beam radiation 24 hours before surgery, 2) surgery alone, or 3) radiation alone. Radiation was administere d in a single fraction of 700 cGy using computed tomographic guidance for d osimetry planning. The isodose distribution was such that the dose conforme d to the posterior epidural space with minimal exit dose to normal tissue. Port films were used to confirm the correct levels. Gadolinium-enhanced mag netic resonance imaging (MRI) of the lumbar spines was obtained before the animals were killed 12 weeks after surgery. The spines were harvested, and axial sections through the laminectomy defect were stained with hematoxylin and eosin and Masson's trichrome. All specimens were evaluated for extent of fibrosis along the dura, density of fibrosis, nerve root entrapment, and sublaminar fibrosis. RESULTS: There were no complications from the surgery, and no new neurologi cal deficits were noted. There was a:statistically significant difference b etween the irradiated and nonirradiated groups regarding the extent of fibr osis (P = 0.001) and the density of fibroblasts (P = 0.001). There was also a marked difference in nerve root entrapment (P = 0.182) and the presence of sublaminar fibrosis (P = 0.061) between the treatment and control groups . MRI revealed less gadolinium enhancement at the irradiated levels compare d with the nonirradiated levels, confirming the usefulness of MRI in predic ting the degree of epidural fibrosis. CONCLUSION: Low-dose external beam radiation therapy administered 24 hours before laminectomy in a dog model significantly decreased the extent and de nsity of peridural fibrosis as well as nerve root entrapment and sublaminar fibrosis. This treatment strategy may be efficacious in patients with recu rrent radicular pain after lumbar discectomy that is thought to be secondar y to peridural fibrosis on the basis of gadolinium-enhanced MRI studies, an d who might benefit from reoperation for nerve root decompression.