Sequential morphological changes in the dog brain after interstitial iodine- 125 irradiation.

Iodine- 125 seeds (3.55 mCi) were stereotactically implanted into the subcortical white matter of the left coronal gyrus in six beagle dogs. Morphological changes were studied at intervals ranging from 25 to 368 days after implantation. In all of the animals, there was a calcifying necrosis 3 to 6 mm in diameter with an adjacent small zone of demyelination. There were no signs of delayed radiation damage outside the demyelinated perifocal zone. The central tissue necrosis was sharply delineated and did not increase in size after 70 days, i.e., an accumulated dose of 18,000 cGy. In addition, widespread vasogenic edema was present in the homolateral hemisphere. The morphological changes observed differed in many aspects from those found with other radioactive sources, such as gold-198, yttrium-90, or iridium-192. The low energy gamma radiation, the absence of beta radiation, and the half-life of 60.2 days makes iodine-125 a favorable radioactive source to produce a well-defined necrosis without delayed radiation damage in the surrounding brain.