An iso-effect table for radiation tolerance of the human thoracic spinal cord. A new approach on the basis of the DFT conception.

Forty-four cases of well documented progressive radiation myelopathy of thoracic spinal cord were analyzed in terms of the new DFT conception (Dose-Fraction-Time). The DFT conception is a new radiobiological cell tissue kinetic model comprising explicitly repopulation. The conception has been previously tested against many animal experimental data. Good agreement could be demonstrated. The comparison with clinical data enabled us to establish human thoracic spinal cord tolerance DFT value of DFT = 70. For all cases of thoracic progressive spinal cord radiation myelopathy where the radiation fields were up to 20 cm of the cord length the value of the DFT has been found above the proposed value. Tolerance irradiation doses of various regular irradiation schemes were calculated from the DFT = 70 for practical use. Our predictions are in agreement with tolerance irradiation regimes recommended by various authors on the basis of their clinical experience. Moreover, such an approach enabled to establish safe tolerance doses in cases where currently used NSD or ED conceptions could lead to dangerous consequences--mainly for shorter irradiation schedules. This is the first attempt to utilize the DFT conception in the clinical practice.