Linac radiosurgery as a tool in neurosurgery

Abstract

Deinsberger and Tidstrand review the current treatment of different malignant and benign diseases by radiosurgery. The authors focus on an adjunct role of radiosurgery, although this method represents an appropriate treatment modality in its own right. Some radiobiological considerations should be added concerning linac radiosurgery as well as gamma knife radiosurgery. In particular, the mature neuron cells are considered the most radioresistant cells in the central nervous system, due to their postmitotic status. The proliferating glial and endothelial cells are more radiosensitive. They act like the connective tissue, to feed and stabilise the neurons. Damage to these cells will consecutively damage the nerve tissue. Already 1 day after high-dose irradiation, dose-dependent extravasation of serum proteins occurs as a sign of the disrupted blood–brain barrier. This damage increases for a few more days and in the course of the next few weeks it fades away to the normal condition. Two to 3 months after irradiation, reversible demyelinisation can occur as a subacute reaction. Dose-dependently, these demyelinated plaques can become confluent. In addition, capillary damage occurs. Later on, the neuron and glial cells and the vessels become further alterated. Increasing gliosis and demyelination are associated with vascular changes. Consecutively, a progressive vasogenic oedema results. After all, the oedema causes a further exacerbation of cell nutrition, thus closing a vicious circle. To understand the clinical implications for the efficacy and toxicity of radiosurgery, one must take into account the difference between earlyand late-responding tissues. According to the linear-quadratic model, a single dose of 20 Gy corresponds to about 50 Gy for early-responding and about 90–100 Gy for late-responding tissues. The target may be earlyor late-responding, whereas the surrounding nerval structures (brain, cranial nerves) always are late-responding. Arteriovenous malformations (AVM) appear also to be late-responding. This means that reactions following irradiation depend very strongly on the applied single dose. For these lesions, no therapeutic gain for fractionation is expected. However, by analogy with surgical treatment, it is assumed that no angiographically visible AVM after radiosurgery means no risk of bleeding. This analogy may not be appropriate as the mechanism of radiation induced blood vessel obliteration is an occlusion of vessels remaining in situ. Radiosurgery is not without complications, as the aim of high dose irradiation is to cause late normal tissue damage to the blood vessels. AVM are often embedded in normal functioning brain, which is also responsible for treatment toxicity as it receives the full radiosurgery dose. Circumscribed radionecroses or leukoencephalopathies following a conventional fractionated irradiation are a typical late effect which can be seen after 9–36 months. After radiosurgery, a manifestation of a necrosis is possible within only a few weeks. The risk of development of this adverse reaction strongly correlates with the volume of irradiated brain and the received dose. In particular, the brain volume irradiated with more than 10 Gy is a predictive parameter. The sensitivity of the optic apparatus seems to be relatively high. Applying 10–22 Gy leads to a complication rate of 20%. The authors supposed the radiosensitivity of the oligodendroglial tissue building the myelin sheath of the optic nerves to be the cause. Existing injury of the visual system increases the sensitivity to radiosurgery with reduced tolerance. The tolerance dose for a single fraction for the optic system is considered 8 Gy. The main acute reactions after radiosurgery with a frequency of 0–7% are headache, nausea and vomiting, and seizures. Usually, these symptoms decline within a few hours. Probability and extent of these effects depend on lesion size and localisation as well as duration of treatment. Headache can develop due to the fixation of the stereotactic headring on the skull. Symptoms will increase This commentary refers to the article http://dx.doi.org/10.1007/ s10143-005-0376-7