Functional Neurosurgery, Stereotaxy, Stereotactic Radiotherapy, Gamma Knife and X-knife or Cyberknife

Lot of people are not aware about the stereotaxy and gamma knife and to a certain extent some medical professionals find it difficult understand these terms. There is ample of literature and academic material on these subjects but I would like to explain in brief and in a very simple manner.
Gamma knife or Gamma surgery or cyber knife is a misnomer and it is nothing to do with surgery or scalpel. These are the non invasive method of intervention. Gamma knife and cyberknife are the procedures in which radiation is used to treat a lesion in brain. 

Now there are variety of lesions which can be treated with Gamma knife or Cyber Knife including vascular malformations like arterivenous malformations ( AVM), small tumors  and even malignant metastasis to the brain.

Brain Metastasis is the commonest intracranial malignancy and constitutes 20% to 40% of patients with neoplastic lesion. The most frequent primary cancers which metastasize to brain are lung, breast , renal cancers and melanoma.

Surgical Resection with adjuvant WBRT (whole brain radiation therapy) is considered the standard of treatment. However, in metastatic lesion localized in deep structures or in eloquent brain areas not amenable to surgery, patients may benefit  from stereotactic radiotherapy ( SRS) treatment.

SRS is based on focusing multiple , high dose, ionizing radiation beams using stereotactic guidance on an intracranial target.

Although both these procedures involve Photon Radiation Technology, there is some difference between Gamma knife and Cyber knife. Gamma Knife uses Cobalt-60 sources and a stereotactic system but Cyberknife uses LINAC ( Linear Accelerator) without need for head fixation.

Stereotactic Radiotherapy( SRT) is a non invasive external beam Radiation therapy for the malignat lesions of the brain where the targeting of the lesion is done using a stereotactic apparatus. One of the important indication for SRT is the brain metastasis and upto three metstatic lesions of about 3.5 cm size or less in the brain can be treated without invasive surgery.
Stereotactic procedures became famous with the pioneering work of Leksell.
At present various types of Stereotactic apparatus available which are compatible with CT scan or MRI. The aim of this apparatus is to to localize a lesion very precisely ( sub millimeter) and three dimentionally. Therefore to localize sub  thalamus or a small AVM of a very small deep seated lesion in brain a stereotactic frame is fixed to the head of the patient without giving general anesthesia and patient remains conscious.
This stereotactic apparatus can be used to localize a lesion for stereotactic biopsy or putting an electrode in Deep brain stimulation in DBS surgery for Parkinson disease.
The procedures like vagal stimulation for epilepsy,  Deep brain stimulation (DBS )for Parkinson disease and dystonia come under a broad term called Functional Neurosurgery.

References-

Dr. Daniele Rigamonti, Schmidek & Sweet operative Neurosurgical techniques Indication, Methods, and Results, Sixth Edition, section V.