A Phase Ii Isotoxicity Study Of Spinal Radiosurgery/Sbrt

To establish the toxicity of single fraction spinal radiosurgery (SBRT) utilizing standardized spinal dosimetry in patients with and without previous RT to the treated area. In the phase I portion of this trial, QA procedures and intrafraction motion was studied in a five fraction regimen. We now present results of the phase II portion of the trial where patients received a single fraction of radiation therapy delivering 9-24 Gy to the planning target volume depending upon proximity of the spinal cord. Small portions of the PTV were planned to be relatively underdosed so that 0.5 cc of spinal cord would receive 10 Gy (no prior RT) or 8 Gy (prior RT greater than six months). The PTV was defined as the gross disease on CT/MRI plus the entire vertebral body if it was involved. If the tumor only involved the posterior elements, the vertebral body was not targeted. All treatment was administered with a Tomotherapy device utilizing multiple intrafraction MVCT image guidance. Patients had clinical and radiographic follow-up at 1 month and 3 months, then q3 months for two years. Forty-two patients were enrolled including 11 in the phase I portion and thirty-one in the phase II portion. All patients had secondary metastases either to bone or paravertebral soft tissue. Twenty-nine of the thirty-one phase II patients received treatment according to the protocol (one refused treatment after enrollment and another received a higher than protocol dose to the spinal cord). Fifteen patients had clinical follow-up greater than six months. Two patients developed acute chest pain (grade 2 and grade 3) within 24 hours of treatment. Both responded to medical management with steroids. Two patients developed vertebral body destabilization requiring surgical management. No patient experienced a spinal cord injury. Although follow-up continues for some patients, this initial experience suggests that 10 Gy to 0.5 cc of spinal cord in patients without prior RT and 8 Gy to 0.5 cc with prior RT are associated with a low risk of spinal cord toxicity in this group of patients with modest life expectancies. This finding supports the absolute dose limit of 10 Gy to 0.35 cc utilized in ongoing study RTOG 0631.