Spinal cord tolerance for stereotactic body radiotherapy.
OBJECTIVE
Dosimetric data are reported for five cases of radiation-induced myelopathy after stereotactic body radiotherapy (SBRT) to spinal tumors. Analysis per the biologically effective dose (BED) model was performed.


METHODS
Five patients with radiation myelopathy were compared to a subset of 19 patients with no radiation myelopathy post-SBRT. In all patients, the thecal sac was contoured to represent the spinal cord, and doses to the maximum point, 0.1-, 1-, 2-, and 5-cc volumes, were analyzed. The mean normalized 2-Gy-equivalent BEDs (nBEDs), calculated using an alpha/beta value of 2 for late toxicity with units Gy 2/2, were compared using the t test and analysis of variance test.


RESULTS
Radiation myelopathy was observed at the maximum point with doses of 25.6 Gy in two fractions, 30.9 Gy in three fractions, and 14.8, 13.1, and 10.6 Gy in one fraction. Overall, there was a significant interaction between patient subsets and volume based on the nBED (p = 0.0003). Given individual volumes, a significant difference was observed for the mean maximum point nBED (p = 0.01).


CONCLUSIONS
The maximum point dose should be respected for spine SBRT. For single-fraction SBRT 10 Gy to a maximum point is safe, and up to five fractions an nBED of 30 to 35 Gy 2/2 to the thecal sac also poses a low risk of radiation myelopathy.
Sahgal
Arjun
A
Department of Radiation Oncology, Sunnybrook Health Sciences Centre, Princess Margaret Hospital, University of Toronto, Toronto, Ontario, Canada. arjun.sahgal@sunnybrook.ca
Ma
Lijun
L
Gibbs
Iris
I
Gerszten
Peter C
PC
Ryu
Sam
S
Soltys
Scott
S
Weinberg
Vivian
V
Wong
Shun
S
Chang
Eric
E
Fowler
Jack
J
Larson
David A
DA
D000328:Adult; D000704:Analysis of Variance; D005260:Female; D006801:Humans; D020714:Maximum Tolerated Dose; D008875:Middle Aged; D011832:Radiation Injuries / Q000209:etiology*; D016634:Radiosurgery / Q000009:adverse effects* / Q000379:methods; D012062:Relative Biological Effectiveness; D012189:Retrospective Studies; D013116:Spinal Cord / Q000528:radiation effects*; D013118:Spinal Cord Diseases / Q000209:etiology*; D013120:Spinal Cord Neoplasms / Q000601:surgery