RadiosurgeryThe future of radiosurgery: radiobiology, technology, and applications
Section snippets
The future: blood vessels
Radiosurgery causes a proliferative vasculopathy within the blood vessels of an arteriovenous malformation (AVM). The process begins with endothelial cell injury secondary to exposure to ionizing radiation in high doses 62, 76. Blood vessels become hyalinized and thickened, which leads eventually to luminal closure. Gliotic tissue may surround the AVM. When this response becomes chronic, gliotic scar or fibroblasts replace much of the mass of the AVM. Szeifert showed that myofibroblasts could
The future: functional disorders
The use of radiosurgery as a lesion generator for functional neurosurgery already has a 49-year history. Nevertheless, its use is controversial because physiologic information is not obtained during the procedure. Experiments in the 1960s showed that high radiosurgical doses (above 150 Gy) delivered to small volumes (3 mm × 5 mm diameter) caused consistent but limited focal tissue necrosis within one month. The lesion did not change significantly over time 1, 29, 30. Similar findings were
The future: benign tumors
Over the last two decades, radiobiologists have agreed on the use of radiosurgery for benign brain lesions such as arteriovenous malformations and benign tumors, but questioned its role in the management of malignant tumors [24]. They derived data to argue that the treatment of malignant tumors with a single radiation fraction would result in a suboptimal therapeutic ratio between tumor control and the late response. Improved results might be expected from fractionation [24]. Their argument was
The future: malignant tumors
With the introduction of radiosurgery, the management of malignant primary and metastatic brain tumors has changed considerably over the last 10 years and is continuing to evolve [18]. The evaluation of different glioma cell lines and the response to varying radiation regimens has provided several experimental models for study of the response to radiosurgery 5, 13, 25, 68, 75, 77. Clinically, another development was better definition of the toxicity of radiosurgery. The Radiation Therapy
The future: other organ systems
The first extracerebral applications of radiosurgery began with the irradiation of retropharyngeal cancers that could be imaged and targeted within the same stereotactic space as the brain [33]. Results were similar to those obtained for brain metastases whose primary site of origin was also outside the brain. New radiosurgery systems may advance radiosurgical applications for the spine and other body targets. The requirements for radiosurgery remain the same. These include fixation, image
Modification of the dose rate
Reducing the dose rate of irradiation reduces biologic effectiveness. Over time, cells can repair sublethal radiation-induced damage [24]. The rate of repair (expressed as the half time for disappearance of repairable injury) is approximately 1.5 hours in the central nervous system. An obvious effect of dose rate that pertains to radiosurgery systems is the decay in cobalt activity found with the Gamma Knife. Because the half life of cobalt is approximately 5 years, significant dose rate
Acknowledgements
The authors thank the staff of the Center for Image-guided Neurosurgery at the University of Pittsburgh, and Drs. Salvador Somaza, Yoshimasa Mori, Satoshi Maesawa, and Ajay Niranjan who worked on many of our research studies. Dr. Kondziolka was supported by National Institutes of Health grant K08-NS01723.
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