The
Radiation Dose–Response of the Human Spinal Cord
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 α/β 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 α/β = 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. The estimate of α/β is smaller than usually
quoted, but values this small were found in some studies.
Using α/β = 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.
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