Estimated risk level of unified SbRT dose tolerance limits for spinal cord.

Objectives: A variety of guidelines have been reported to estimate spinal cord dose tolerance for patients treated with stereotactic body radiation therapy (SBRT). The purpose of this work is to comprehensively analyze dose-volume data of a cohort of patients treated by SBRT in order to estimate the complication probability and compare these probabilities to dose tolerance limits currently in clinical use. Methods: An extensive literature review of English-language publications identified 59 spinal cord dose tolerance limits suggested for clinical use. These guidelines were partitioned into a unified format of high-risk and low-risk dose tolerance limits. A dataset of spinal cord dose-volume points were interpolated from a previously published cohort of 74 patients with 102 spinal metastases in whom 3 patients developed treatment-related severe myelopathy. Seventy-four percent (50/68) of previously treated patients had prior radiation. Dose-volume data was digitized into the DVH Evaluator software tool where the rest of the DVH was approximated and parameters of the probit dose response model were fitted to the data using the maximum likelihood approach. We estimated risk levels for spinal cord dose-volume parameters including maximum dose, D1cc, and D0.1cc, and compared the corresponding risk level with the published suggested guidelines for spinal cord tolerance. Results: Based on the risk model generated from this limited dataset, the Accuray STARS 20Gy to 1 cc limit and the RTOG 0813 30Gy maximum limit had the highest risk levels, at about 5%, and most of the other unified dose tolerance limits had 1-3% risk. Only one of the unified low-risk limits had higher than 3% risk. Two thirds of patients in this study had prior irradiation so it is likely that in other patient populations the risk level of these limits will be lower. Conclusion: While severe myelopathy is an unacceptable complication, no clinical procedure is without risk. Guidelines, therefore, should aim to minimize the risk of injury to 1-3% or less. No single dose-volume parameter can be relied on to predict this complication, however, it is likely that the use of multiple dose-volume-based guidelines may improve safety. Based on the findings of this study, the current guidelines for D_{0. 1cc} as well as the low-risk guidelines for maximum dose (D_max) may be useful to establish the lower limits of tolerance. Longer follow-up with more patients from more institutions is required to improve the risk estimates. Disclosure: No significant relationships. [ABSTRACT FROM AUTHOR]