（a）Stereotactic Radio Surgery, SRS
Stereotactic Radiosurgery（SRS) is a radiation therapy technique that precisely concentrates large doses of radiation into a target area, resulting in tissue necrosis. The peripheral tissue adjacent to the target area is not irradiated due to rapid dose fall off. The treatment forms a knife-like interface, the x-rays acting as a scalpel. Therefore, it is called radiation surgery or radiation knife.
Equipment that uses 60-Cobalt to produce gamma rays and SRS treatment is called gamma-knife. Equipment that uses an X-ray linear accelerator as the energy source for SRS treatment is called X-knife. Because X-rays are photons, X-knife may be referred to as photon knife.
（b）stereotactic radiation therapy, SRT
Stereotactic radiation therapy (SRT) also refers to a radiation therapy technique that precisely concentrates large doses of radiation into a target area, but generally it is delivered over a few treatment sessions. This technology is also known as fractionated stereotactic radiation therapy (FRST). When it is delivered to the body (instead of the brain) it is referred to as SBRT (stereotactic body radiation therapy). Fractionated irradiation (multiple treatments instead of just one), may have some radiobiology advantages for some tumors, including the protection of adjacent normal tissues, increasing the total dose to the tumor, and can killing of hypoxic tumor cells.
2．Working principle and characteristics of γ knife and X knife
The working principle of the knife is to arrange 201 60Co sources withina helmet-shaped shell so that the rays of each directionally focus into a single point to form a spherical irradiation field (diameter ≤ 18mm). Although the dose emitted by each 60Co source is very small, after being directionally focused to a point, the dose concentration at that point will be significantly increased to achieve the therapeutic purpose of treatment. At the same time, treatment will not damage the normal structures surrounding the treatment sphere.
The working principle of X-knife is similar. Multiple non-coplanar rotating treatment arcs are delivered from the linear accelerator, controlled by rotation of the gantry and couch. The X-ray beams of each arc can vary in energy (6 ~ 15MV) and the size of each beam is kept small through collimation. The target is fixed at the center of these treatment arcs, at the isocenter of rotation. This has a similar affect as gamma knife, with limited dose to surrounding structures.
3． Indications for γ（X）knife
（a）Those who are not candidates for surgery, such as the elderly or frail, children, unable to undergo anesthesia, deeply located lesions, high surgical risk, etc.
（b）Recurrent, residual and uncontrolled intracranial lesions after previous surgery.
（c）Patient refuses craniotomy.
（d）Gamma knife used as a comprehensive treatment plan which combines with surgery, radiotherapy and chemotherapy.
In terms of size, gamma knife is only suitable for tumors smaller than 3cm, while X-knife can be used up to 4cm. For tumor that is 4 ~ 5cm, the X knife system can be used at times.
It is mainly used to treat intracranial tumors, such as pituitary adenoma, meningioma, brain metastasis, acoustic neuroma, pineal gland tumor, solid craniopharyngioma, etc. It also has good therapeutic effect on cerebrovascular diseases such as cerebral arteriovenous malformation and cavernous hemangioma.
SRS can be an alternative to brain surgery in the treatment of intracranial benign or malignant tumors. In addition to using the disposable fixed head ring for stereotactic radiotherapy, it can also provide repeated (fractionated) treatments, which provides a safe and effective supplementary treatment for inoperable brain tumors and other tumors of the head and face, including skull base tumors. It can also be used to treat residual tumors after brain surgery or combined with other radiotherapy for comprehensive treatment.
GCCC introduces SupeRay™ transcendental knife, a product of OUR UNITED Co., which is the first head gamma knife with integrated mobile image-guided radiotherapy to function in China, supporting non-invasive radiosurgery or fractionated radiotherapy.