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Spinal cord dose constraints are a critical feature for stereotactic body radiation therapy (SBRT). Spinal cord maximum point dose (Dmax) by Monte Carlo (MC) calculations is used as a critical cord tolerance limit for SBRT, but information is lacking about its reproducibility. This study examines uncertainty of MC dose calculations for small volumes in spine SBRT. Seven consecutive spine radiosurgery cases were randomly selected to measure precision of the Dmax calculation in comparison to other volumes. Each plan was calculated five times using MC with a 2% uncertainty objective, and variabilities in dose-volume histogram (DVH) parameters across recalculations were evaluated with coefficient of variation (standard deviation divided by mean). The average ratio of D0.03 cc/Dmax was calculated across a larger series of 130 cases. The variability of Dmax was twice as high for D0.03 cc and five times as high for D1 cc across recalculations for the seven cases. For larger volumes, the variability was lower. The standard deviation of Dmax was 0.1959 Gy, compared to 0.0931 Gy, 0.0569 Gy, and 0.0364 Gy for D0.03 cc, D0.1 cc, and D1 cc, respectively. The average D0.03 cc/Dmax among 130 cases was 0.93. Dmax has greater variability compared to D0.03 cc, D0.1 cc, and D1 cc, potentially creating risks when used for guidance for spinal cord. D0.03 cc may be an attractive alternative with higher reliability while its limits could be obtained by scaling the reported Dmax limit by a factor of 0.93. This may help guide treatment planning and aid in discovering true dose constraints for spine SBRT.

Stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), and stereotactic ablative body radiotherapy (SABR) are commonly used in the treatment of central nervous system (CNS) disease. This study has refined the radiation toxicity estimates for some normal tissues of the CNS based on review and analysis of the clinical evidence for single fraction radiosurgery, hypofractionated SBRT, and conventionally fractionated radiation therapy. Published guidelines and protocols are reviewed. In the past, many normal tissue tolerances were compiled based on the experience of the investigators and publications in the literature. Some tolerances were determined by modeling or calculation using the existing biological formulas, in particular the linear quadratic (LQ) model. In the present study, the estimate of risk for each dose tolerance limit in some CNS tissues is provided exclusively with normal tissue complication probability (NTCP). The clinical outcomes are compared to understand the difference in biological effect between radiosurgery and radiotherapy. Normal tissue dose tolerances and the corresponding complication rates are provided for brainstem, optic nerves, cochlea, and spinal cord, including single fraction SRS, five-fraction SBRT, and conventional radiation therapy. Calculation of biologically effective dose (BED) or single fraction equivalent dose (SFED) alone using the LQ model conveys no consensus on the biological effect across different fractionations. Comparison of conventional radiation therapy to brain and spinal cord with single fraction equivalent dose leads to even conflicting clinical outcomes. Effective differences between single fraction SRS and conventional radiotherapy need to be better understood. The existing biological model might not be valid to predict the radiosurgical outcomes based on conventionally fractionated radiotherapy. However, application of the statistical dose response models of clinical SRS and SBRT outcomes data to selected current dose tolerance guidelines into simple tables can be a clinically useful resource.