Dose-volume effects in rat thoracolumbar spinal cord: the effects of nonuniform dose distribution.
OBJECTIVE
To investigate dose-volume effects in rat spinal cord irradiated with nonuniform dose distributions and to assess regional differences in radiosensitivity.


METHODS
A total of 106 rats divided into three groups were irradiated with (192)Ir gamma-rays at a high dose rate. The groups were irradiated with one, two, or six catheters distributed around the thoracolumbar spinal cord to create different dose distributions. After irradiation, the animals were tested for motor function for 9 months. The response was defined as motor dysfunction and WM or nerve root necrosis. Dose-response data were analyzed with a probit analysis as function of the dose level at a percentage of the volume (D(%)) and with different normal tissue complication probability models. Additionally, the histologic responses of the individual dose voxels were analyzed after registration with the histologic sections.


RESULTS
The probit analysis at D(24) (24% of the volume) gave the best fit results. In addition, the Lyman Kutcher Burman model and the relative seriality model showed acceptable fits, with volume parameters of 0.17 and 0.53, respectively. The histology-based analysis revealed a lower radiosensitivity for the dorsal (50% isoeffective dose [ED(50)] = 32.3) and lateral WM (ED(50) = 33.7 Gy) compared with the dorsal (ED(50) = 25.9 Gy) and ventral nerve roots (ED(50) = 24.1 Gy).


CONCLUSIONS
For this nonuniform irradiation, the spinal cord did not show typical serial behavior. No migration terms were needed for an acceptable fit of the dose-response curves. A higher radiosensitivity for the lumbar nerve roots than for the thoracic WM was found.
Philippens
Mariëlle E P
ME
Department of Radiation Oncology, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands. m.philippens@rther.umcn.nl
Pop
Lucas A M
LA
Visser
Andries G
AG
van der Kogel
Albert J
AJ
D000818:Animals; D004307:Dose-Response Relationship, Radiation; D007496:Iridium Radioisotopes; D008159:Lumbar Vertebrae; D008297:Male; D053580:Muscle Strength / Q000528:radiation effects*; D010243:Paralysis / Q000209:etiology; D010291:Paresis / Q000209:etiology; D011833:Radiation Injuries, Experimental / Q000473:pathology; D011836:Radiation Tolerance; D051381:Rats; D017208:Rats, Wistar; D012018:Reflex / Q000528:radiation effects*; D013116:Spinal Cord / Q000473:pathology / Q000503:physiopathology / Q000528:radiation effects*; D013904:Thoracic Vertebrae
D007496:Iridium Radioisotopes