Background: This study evaluated the impact of the distance from planning target volume (PTV) to spinal cord on planning dosimetry of stereotactic body radiation therapy (SBRT) in patients with isolated spine metastasis and established planning criteria and parameters to restricting low dose spillage.Methods: Six modified PTVs from each of 10 patients with isolated spinal metastasis were created by artificial uniform extension from clinical target volume (CTV) to ensure a minimum PTV-to-cord distance of 0, 1, 2, 3, 4 and 5 mm;respectively.The prescription dose (PD) was 22 Gy in a single fraction.PTV dosimetric parameters including V100, Dmin, D98D95D1conformity index (CI), R50% (ratio of the 50% prescription isodose volume to the PTV), D2 cm (maximum dose in percentage of PD at 2 cm from PTV in any direction), and cord dose were measured and compared.Results: PTV V100, Dmin, D98, D95were enhanced markedly along with the increase of the PTV-to-cord distance, with a significant difference.PTV CI,R50%, D2cm, and cord dose were not statistically significant in different PTV-to-cord distances.A PTV-to-cord distance of 2 to 3 mm meet PTV Dmin, D98, D95.R50%, and D2cm could restrict low dose spillage in SRS panning.Conclusions:PTV V100, Dmin, D98, D95 are directly correlated with PTV-to-cord distance for spine SBRT in single fraction.A distance of 2 to 3 mm from PTV to spinal cord meets planning dose requirements.R50% and D2cm can quantitatively restrict low dose spillage and improve plan quality.