Stereotactic Radiosurgery (SRS) And Stereotactic Radiotherapy (SRT)

Radiosurgery(SRS) is a term originally referring to a procedure that delivers three dimensional Stereo tactic External Beam Irradiation(SEBI). This technique delivers a relatively large single dose of radiation to a small intracranial target with great accuracy. In contrast the term stereo tactic radiotherapy (SRT) refers to similarly accurate therapy given over multiple treatments.

Encompassing both single and fractioned treatments, stereo tactic irradiation has developed quite rapidly, particularly over the past 10 years in terms of manpower, cost, utilization, and applied technology. Several thousand patients with arteriovenous malformations (AVMs) and several hundred patients with benign and malignant lesions have been treated.

A stereo tactic frame is fixed to the patient's skull, providing highly accurate fiducial landmarks that allow for stereo tactic localization of intracranial targets after cross-registration with neuro imaging studies, such as magnetic resonance imaging (MRI), computed tomography (CT), or angiography. The frame provides the basis by which a target can be identified in the image study set with respect to the stereo tactic frame and specified in an X,Y,Z coordinate system after cross-registration with neuroimaging studies. This coordinate system is used during target localization to define the shape and extent of the lesion to be treated.

A target can be selected on the radiographic image and its localization with respect to the stereo tactic frame determined in an X,Y,Z coordinate system. It is used during target localization studies. (MRI, CT, angiography) A known relationship between this stereo tactic coordinate system and the radiation source allows cross registration and accurate delivery of radiation to the target.

SEBI (SRS, SRT) differs from conventional external - beam radiation therapy in several important respects:

Small volumes in the range 1 to 30cm³ are treated. A single fraction of radiation is typically delivered. Some institutions deliver more than one fraction, but this approach is still being evaluated. Extra precision with target localization and treatment geometries is required. High-dose gradients at field edges minimize dose deposition outside the target volume. The volume of tissue beyond the target that receives significant dose is strongly dependent on target size and the conformity of the isodose to the target. Beams intersect at a common point within the skull after entering through points distributed over the surface of the skull. Three-dimensional distribution of beams reduces the volume tissue receiving moderate or high doses of irradiation. The sequence of planning and treatment for radio surgery is labor intensive and requires ,multiple personnel. A typical team with a linac system involves a neurosurgeon, radiation oncologist, neuro radiologist, radiation physicist, dosimetrist, treatment planning technologist, therapist, nurse and electronic maintenance person.

A radiosurgery system consists of a stereo tactic frame, radiation delivery system, and computer hardware and treatment planning software. Combined with the use of a conventional MRI or CT scanner, the system allows accurate determination of target size and location, treatment planning, and delivery of radiation.

Indications for radiosurgery include the presence of a suitably sized, radiographically distinct lesion (generally = 4cm) that has the potential to respond to a single, large dose of irradiation. Depending on the clinical situation, this response may involve cessation of neo plastic growth, obliteration of a vascular malformation, or selective destruction of a neural pathway. Both benign and malignant lesions, as well as certain functional disorders, have been treated. The largest worldwide experience has been in the treatment of AVMs. Both primary and metastatic brain tumors have been treated. Functional radiosurgery has been attempted in efforts to relieve pain, reduce tremor, or reduce seizure frequency. The more common treatment indications are discussed below. Radiosurgery is used to treat a variety of benign and malignant lesions and some functional disorders.

Arteriovenous malformations Meningiomas, primary and recurrent Acoustic neuromas, primary and recurrent Pituitary adenomas, primary and recurrent Nasopharynx carcinomas, primary and recurrent Solitary brain metastasis, initial and recurrent High-grade gliomas, primary and recurrent Low-grade gliomas, primary and recurrent Medulloblastomas, recurrent

In some categories, only a small number of cases have been treated, with results unreported and indications not established. A survey indicated that vascular lesions (AVMs) constituted approximately 44% of the patients treated; meningeal lesions, 11% ; metastases, 12%; glial tumours, 8%; and acoustic schwannomas, 14%. The role of radiosurgery in vascular lesions has been well established at the gamma-knife institutions and confirmed with linac systems.

Evidence suggests that the natural history of inoperable AVMs may be favorably influenced after radiosurgery. For many other lesions the indications for treatment have not been clearly established; for some, radiosurgery must be regarded as investigational. Ideal target volumes for radiosurgery are nearly spherical and small, upto about 3 cms in maximum dimension. Irregular volumes may require treatment to multiple isocenters to shape a selected isodose surface to conform to the target volume.