Stereotaxis (from the Greek: 'spatial arrangement') is a surgical 'mentality' rather than a surgical technique. The technique is based on using the relationships between the 3D space occupied by brain lesions and an external reference system to guide instruments to a pre-established target.
In 1949 the Swedish neurosurgeon Lars Leksell devised a stereotactic 'helmet' for human functional neurosurgery. After the advent of computerized imaging, the procedure was applied to various branches of surgery (biopsies of deep thoracic and abdominal masses, of non-palpable mammary nodules etc.). He later developed the concept of 'radiosurgery', which aimed at destroying discrete anatomical regions within the brain while sparing surrounding healthy tissues. However, it wasn't until the end of the 1960s, with the introduction of the first linear accelerators and their refinements in the 1980s, that Dr. Leksell's idea was put into practice, and research centers in Boston (US) and Vicenza (Italy) developed the technique of stereotactic radiation with accelerators.
Brain stereotactic radiotherapy was first applied to very small, deep lesions, which were not surgically treatable, e.g. vascular lesions. Subsequently, lesions as large as 3 cm or more in diameter, which is often the case of brain tumors and metastases, were treated.
Used also for non-tumoral diseases (e.g. angiomas, aneurisms, malformations), stereotactic radiotherapy has become the treatment of choice for many cerebral neoplasias.

Pre-treatment computerized tomography

Post-treatment computerized tomography

Method
Leksell's stereotactic device consists of a helmet that is fixed to the patient's head with pins. Radio-opaque or paramagnetic samples are inserted into the helmet. Then, a three-dimensional space is defined with computerized tomography and magnetic resonance, and the tumor's coordinates are established on this 3D space. Thus, the stereotactic helmet serves to locate the exact position of the tumor and, subsequently, to administer the radiations exactly on the tumor area.

Fixation device

Invasive

Non-invasive

Today, we use a non-invasive device called the 'stereotactic relocatable frame' instead of Leksell's helmet. This frame provides the stereotactic location, degree of immobilization and precision necessary for radiosurgery. It consists of a helmet that can be adapted to align precisely the laser and the positioning of the isocentric site of the linear accelerator. Using dental impression material, we make a cast of the patient's mouth (mouthpiece). The mouthpiece is attached to two moveable arms on the frame, and fits comfortably into the patient's mouth. In addition, a thermoplastic cast is made of the patient's face and head. When fitted with the thermoplastic mask and with the mouthpiece comfortably in place, the patient is immobilized so as to ensure the millimeter reproducibility of treatment.

Treatment steps
The stereotactic helmet is fixed to the patient's head under local anesthesia. The patient feels no pain.

Then examinations such as computerized tomography, magnetic resonance and angiography are performed to locate, diagnose and measure the neoplastic mass.

Subsequently, the treatment strategy is planned at the computer. During therapy the linear accelerator moves in synchrony with the treatment bed.

Using the computerized volumetric reconstructions, we simulate various surgical approaches. We also evaluate the best way to reach a target in case of obstruction by nerve or vascular structures. Besides simulating surgery, it is possible to plan and perform the surgery.

Supine on a bed, with the helmet well-centered, the patient is irradiated for 15-20 minutes, with the radiation source that circles according to a prefixed scheme, so that the rays reach the tumoral target from various directions, following different pathways each time, so as to spare healthy tissue as much as possible.

Conclusions
Radiosurgery is used to treat:

- malformations, small benign neoplasias that are difficult to reach and remove (auditory neurinomas, meningiomas of the base of the skull, pinealomas);
- single and multiple encephalic metastases;
- such diseases as thalamotectomy for Parkinson's disease, refractory pain etc.

The mortality and morbidity of stereotactic procedures are very low, in the order of 1% and 2%, respectively, whereas the mortality of free-hand biopsy for malignant neoplasias reaches 13%.
Described simply, stereotactic radiotherapy may seem banal. But it requires a high level of expertise, organization and an interdisciplinary approach: radiotherapists, health physicists, neuroradiologists and neurosurgeons closely interact during this procedure.

Dose released with conformational beams

Dose released with arc therapy

Result of the 3D distribution

A single stereotactic radiotherapy session may be sufficient, or the dose can be divided into various sessions. Stereotactic radiotherapy can be used to treat deep-sited neoplastic masses, which cannot be treated surgically because of the risk of damaging important cerebral areas. In addition, the technique can be applied to slow-growing tumors so as the do not reach a size that is difficult to treat surgically. With the advent of stereotactic radiotherapy it has become possible to cure the 'very small' and the 'very deep'.
Stereotactic radiotherapy can be applied also to larger brain tumors. After the neurosurgeon has removed as much of the tumor as possible, stereotactic radiotherapy can be used where surgery might cause damage. Radiosurgery is now being applied to other types of tumors, such as chest and abdomen tumors.