Objective: Spinal cord injury is arguably the most feared complication in radiotherapy and has historically limited the aggressiveness of spinal tumor treatment. We report a case series of 6 patients treated with radiosurgery who developed delayed myelopathy.
Methods: Between 1996 and 2005, 1075 patients with benign or malignant spinal tumors were treated by CyberKnife (Accuray, Inc., Sunnyvale, CA) robotic radiosurgery at Stanford University Medical Center and the University of Pittsburgh Medical Center. Patients were followed prospectively with clinical and radiographic assessments at 1- to 6-month intervals. A retrospective review identified patients who developed delayed radiation-induced myelopathy. Six patients (5 women, 1 man) with a mean age of 48 years (range, 25-61 years) developed delayed myelopathy at a mean of 6.3 months (range, 2-9 months) after spinal radiosurgery. Three tumors were metastatic; 3 were benign. The metastases were in the upper to midthoracic spine, whereas the benign tumors were partially in the cervical region. Three cases involved previous radiation therapy.
Results: Dose volume histograms were generated for target and critical structures. Clinical and dosimetric factors were analyzed for factors predictive of spinal cord injury. Specific dosimetric factors contributing to this complication could not be identified, but one-half of the patients with myelopathy received spinal cord biological equivalent doses exceeding 8 Gy.
Conclusion: Delayed myelopathy after radiosurgery is uncommon with the dose schedules used in this case series. Radiation injury to the spinal cord occurred over a spectrum of dose parameters that prevented identification of specific dosimetric factors contributing to this complication. Primarily, biological equivalent dose estimates were not usable for defining spinal cord tolerance to hypofractionated dose schedules. We recommend limiting the volume of spinal cord treated above an 8-Gy equivalent dose, because half of the complications occurred beyond this level.