Brachytherapy--interstitial implant radiosurgery

With both surgery and radiosurgery a selective destruction and removal of a lesion is undertaken. Using interstitial irradiation a calculated volume of necrosis is induced, which is subsequently removed by macrophage activity. Interstitial irradiation is therefore considered a radiosurgical method. To better understand the side effects of radiosurgical lesions in the brain, we studied the effects of three physically distinct gamma emitters iridium-192, gold-198 and iodine-125 with respect to the morphological development of radionecroses and the consecutive vasogenic oedema using different dose rates and application times. Experimental dose effects on normal brain (beagle dogs) revealed that depending on the energy range, application time and dose rate, calcified or liquified necroses develop from the center around the seed into the periphery. These well-defined radionecroses are surrounded by reactive glia and, temporarily, by fibrinoid necrosis of endothelial cells. In contrast to permanent implants, effects of temporary implants on tissue are characterised in the early stages of oedema, in later stages by progressive resorption of necrotic tissue and less permanent damage. Dose rate and energy range determine the volume of the necroses and the magnitude of vasogenic oedema. It is the vasogenic oedema which primarily determines the extent of demyelination and reactive glioses in the vicinity of the radionecrosis. Toxic side effects of interstitial implant radiosurgery can be minimised by using temporary implants with moderate dose rates, in which case repair processes are free to take place after removal of the radioactive source from the brain. Based on experimental and clinical findings a biological dosimetry has evolved which favours temporary implants of iodine-125 with dose rates of 10 cGy/h.