Late radiation effects in the dog brain: correlation of MRI and histological changes.
OBJECTIVE
To determine the correlation between sequential changes in the brain of dogs after irradiation, as detected by magnetic resonance imaging (MRI), with the eventual appearance of histological lesions. Histology was performed 77-115 weeks after irradiation.


METHODS
Groups of five beagle dogs were irradiated to the brain with single doses of 10, 12, 14 or 16 Gy of 6 MV photons, at the 100% iso-dose. Sequential MRIs were taken to detect changes in the brain for 77-115 weeks after irradiation. Dose-effect relationships were established for changes in the brain as detected by MRI, computerized tomography (CT), gross morphology and histology. The doses that caused a specified response in 50% of the animals (ED(50)+/-SE) were calculated from these dose-effect relationships for each endpoint.


RESULTS
The ED50 values (+/-SE) for focal and diffuse changes on T2-weighted MR images were 11.0+/-1.1 and 10.8+/-0.9 Gy, respectively. The ED50 values (+/-SE) for contrast enhancement on T1-weighted MR images and on CT were 13.4+/-0.6 and 13.0+/-0.6 Gy, respectively. It was 11.4+/-0.6 Gy for any type of histological lesion (haemorrhage, reactive change or glial scar) 77-115 weeks after irradiation. For a macroscopic lesion the ED50 (+/-SE) value was 13.0+/-1.1 Gy.


CONCLUSIONS
The presence of focal or diffuse changes on T2-weighted MR images was the best indicator for the eventual appearance of any type of histological lesion in the dog brain after irradiation with single doses of photons. The ED50 for any histological lesion did not differ significantly from the ED50 for a focal (P>0.35) or diffuse (P=0.3) change on T2-weighted MR images.
Benczik
Judit
J
Department of Clinical Veterinary Sciences, Faculty of Veterinary Medicine, University of Helsinki, Helsinki, Finland.
Tenhunen
Mikko
M
Snellman
Marjatta
M
Joensuu
Heikki
H
Färkkilä
Markus
M
Joensuu
Raimo
R
Abo Ramadan
Usama
U
Kallio
Merja
M
deGritz
Boris
B
Morris
Gerard M
GM
Hopewell
John W
JW
D000818:Animals; D001921:Brain / Q000473:pathology* / Q000528:radiation effects*; D004285:Dogs; D004307:Dose-Response Relationship, Radiation; D005260:Female; D008279:Magnetic Resonance Imaging; D008297:Male; D017785:Photons; D011829:Radiation Dosage; D013997:Time Factors