The radiation dose-response of the human spinal cord.
OBJECTIVE
To characterize the radiation dose-response of the human spinal cord.


METHODS
Because no single institution has sufficient data to establish a dose-response function for the human spinal cord, published reports were combined. Requisite data were dose and fractionation, number of patients at risk, number of myelopathy cases, and survival experience of the population. Eight data points for cervical myelopathy were obtained from five reports. Using maximum likelihood estimation correcting for the survival experience of the population, estimates were obtained for the median tolerance dose, slope parameter, and alpha/beta ratio in a logistic dose-response function. An adequate fit to thoracic data was not possible. Hyperbaric oxygen treatments involving the cervical cord were also analyzed.


RESULTS
The estimate of the median tolerance dose (cervical cord) was 69.4 Gy (95% confidence interval, 66.4-72.6). The alpha/beta = 0.87 Gy. At 45 Gy, the (extrapolated) probability of myelopathy is 0.03%; and at 50 Gy, 0.2%. The dose for a 5% myelopathy rate is 59.3 Gy. Graphical analysis indicates that the sensitivity of the thoracic cord is less than that of the cervical cord. There appears to be a sensitizing effect from hyperbaric oxygen treatment.


CONCLUSIONS
The estimate of alpha/beta is smaller than usually quoted, but values this small were found in some studies. Using alpha/beta = 0.87 Gy, one would expect a considerable advantage by decreasing the dose/fraction to less than 2 Gy. These results were obtained from only single fractions/day and should not be applied uncritically to hyperfractionation.
Schultheiss
Timothy E
TE
Department of Radiation Oncology, City of Hope Cancer Center, 1500 Duarte Road, Duarte, CA 91010, USA. schultheiss@coh.org
D000465:Algorithms; D002574:Cervical Vertebrae; D016009:Chi-Square Distribution; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D006931:Hyperbaric Oxygenation; D011832:Radiation Injuries / Q000209:etiology*; D011836:Radiation Tolerance; D011879:Radiotherapy Dosage; D013116:Spinal Cord / Q000528:radiation effects*; D013904:Thoracic Vertebrae