Stereotactic radiosurgery (SRS) is a highly precise form of radiation therapy initially developed to treat small brain tumors and functional abnormalities of the brain. The principles of cranial SRS, namely high precision radiation where delivery is accurate to within one to two millimeters, are now being applied to the treatment of body tumors with a procedure known as stereotactic body radiotherapy (SBRT).
Despite its name, SRS is a non-surgical procedure that delivers precisely-targeted radiation at much higher doses, in only a single or few treatments, as compared to traditional radiation therapy. This treatment is only possible due to the development of highly advanced radiation technologies that permit maximum dose delivery within the target while minimizing dose to the surrounding healthy tissue. The goal is to deliver doses that will destroy the tumor and achieve permanent local control.

SRS and SBRT rely on several technologies:

• three-dimensional imaging and localization techniques that determine the exact coordinates of the target within the body

• systems to immobilize and carefully position the patient and maintain the patient position during therapy

• highly focused gamma-ray or x-ray beams that converge on a tumor or abnormality

• image-guided radiation therapy (IGRT) which uses medical imaging to confirm the location of a tumor immediately before, and in some cases, during the delivery of radiation. IGRT improves the precision and accuracy of the treatment

Three-dimensional imaging, such as CT, MRI, and PET/CT is used to locate the tumor or abnormality within the body and define its exact size and shape. These images also guide the treatment planning—in which beams of radiation are designed to converge on the target area from different angles and planes—as well as the careful positioning of the patient for therapy sessions.

Although SRS commonly refers to a one-day treatment, physicians sometimes recommend multiple stereotactic delivered treatments. This is important for tumors larger than one inch in diameter as the surrounding normal tissue exposed to the single high dose of radiation must be respected and limited, and the volume of normal tissue treated increases proportionally to the tumor size. Delivering the radiation in a few sessions as opposed to one, can improve safety and allow the normal tissue to heal in between treatments. Therefore, fractionating the treatment allows for high doses to still be delivered within the target, while maintaining an acceptable safety profile. This procedure is usually referred to as fractionated stereotactic radiotherapy (SRT), and typically refers to the delivery of two to five treatments of focused radiation.

SRS and SBRT are important alternatives to invasive surgery, especially for patients who are unable to undergo surgery and for tumors and abnormalities that are:

• hard to reach

• located close to vital organs/anatomic regions

• subject to movement within the body

SRS is used to treat:

• many types of brain tumors including:

  • benign and malignant

  • primary and metastatic

  • single and multiple

  • residual tumor cells following surgery

  • intracranial, orbital and base-of-skull tumors

• arteriovenous malformations (AVMs), a tangle of expanded blood vessels that disrupts normal blood flow in the brain and sometimes bleeds.

• other neurological conditions like trigeminal neuralgia (a nerve disorder in the face), tremor, etc.

SBRT is currently used and/or being investigated for use in treating malignant or benign small-to-medium size tumors in the body and common disease sites, including the:

• lung

• liver

• abdomen

• spine

• prostate

• head and neck

SRS fundamentally works in the same way as other forms of radiation treatment. It does not actually remove the tumor; rather, it damages the DNA of tumor cells. As a result, these cells lose their ability to reproduce. Following treatment, benign tumors usually shrink over a period of 18 months to two years. Malignant and metastatic tumors may shrink more rapidly, even within a couple of months. When treated with SRS, arteriovenous malformations (AVMs) may begin to thicken and close off slowly over a period of several years following treatment. Many tumors will remain stable and inactive without any change. Since the aim is to prevent tumor growth, this is considered a success. In some tumors, like acoustic neuromas, a temporary enlargement may be observed following SRS due to an inflammatory response within the tumor tissue that overtime either stabilizes, or a subsequent tumor regression is observed called pseudoprogression.