Stereotactic radiosurgery (SRS) is a specialized application of 3-D conformal treatment planning for treatment of an assortment of brain tumors and arteriovenous malformations. The technique delivers a high dose with millimeter tolerances, with a sharp dose gradient. Dr. Lars Leksell, from the Karolinska Institute in Stockholm, Sweden, first described SRS in 1949. However, practice of the technique did not experience explosive growth until the 1980s. SRS can be accomplished using one of three radiation modalities—gamma knife, linear accelerator (LINAC), or heavy particles such as protons.
With the advent of IMRT-based treatment planning, intensity modulated radiosurgery (IMRS) has yielded conformal plans that are easily reproducible. The gamma knife is a large device that uses a fixed hemispherical assembly of 201 cobalt-60 sources focused at a defined point. Plugging the apertures of selected cobalt sources results in the shaping of the desired radiation field.
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The linear accelerator generates megavoltage x-rays and electrons that are used to provide routine external beam radiotherapy. An SRS treatment may typically involve rotating the radiation beam in arcs around the patient. Since linear accelerators are readily available in radiation oncology centers, LINAC-based radiosurgery is most commonly used.
The use of heavy particles or protons is sometimes referred to as Bragg-peak therapy. This requires bulky and expensive particle generators called cyclotrons or synchrotrons. Their singular advantage is a rapid decrease in radiation dose beyond the focus (Bragg peak) depth. Currently, there are a limited number of such facilities in the United States capable of treating patients.
Because of small target volumes, SRS demands high precision in all aspects of treatment planning and delivery. A special headframe (often screwed into the patient’s cranium) is used in conjunction with CT and MRI imaging to provide precise localization information, and is utilized to position the patient during treatment. Modern planning systems allow for fusion of an MRI image with the corresponding CT slice in order to maximize the strengths of each imaging modality.
Most published reports of SRS have focused on its use to treat primary malignant brain tumors, metastases from other solid tumor sites, schwannomas, and arteriovenous malformations.
For glioblastoma multiforme (GBM), SRS has been utilized as a form of boost therapy and has yielded two-year survivals approaching 30%, without significant acute toxicity. However, more than 10% of patients may develop clinically detectable radionecrosis. Compared to historical controls, the use of SRS as a boost to external beam radiotherapy is superior to external beam radiotherapy alone. Patients with lower pathologic grade, younger age, better performance status, smaller tumor volume, and unifocal disease benefit most from SRS. This subgroup is being further investigated in the RTOG 9305 trial.
In the treatment of brain metastases, most studies have suggested patients with controlled extracranial disease, higher performance status, greater disease-free interval, and age <70 are more likely to benefit from SRS. Median survivals of 7 to 18 months are reported. Emerging data also suggest that SRS is more cost-effective than surgical resection for isolated brain metastases. The RTOG 9508 trial may help clarify if SRS is beneficial following whole brain radiation.
Vestibular schwannomas (or acoustic neuromas) are controlled locally (95%) following SRS, although most lesions may take up to two years to show an objective response. The most serious toxicity is trigeminal neuralgia, which can occur in up to one-half of patients.
Arteriovenous malformations have also been managed effectively with SRS. A complete radiographic response may take several years, but most patients experience symptomatic relief much sooner.
Movement disorders and trigeminal neuralgia have been treated successfully in patients who have failed more conventional medical and surgical approaches. In most cases, resolution of tremor or neuropathic pain has been seen within one to two months of treatment. This remains a fertile area of ongoing research and efforts are being made to evaluate this approach in the first-line setting.