SRS is a process of delivery of radiation that was primarily pioneered by Dr. Lars Leksell in Sweeden in the 1950’s and 1960’s. Dr. Leksell invented the term radiosurgery since he was creating effects in the brain using radiation that were similar to effects that he previously created with open surgery. SRS utilizes many or even hundreds of small beams of radiation (from 4 to 60mm in diameter). The beams of radiation may each have a unique entrance point or the beams may consist of multiple arcs of radiation. Each beam of radiation has a single or a few common focus points within the tumor or target. Each of these focus points is described in terms of x, y and z coordinates in stereotactic or three dimensional space.
SRS can shape the radiation to conform to the three dimensional shape of the tumor or target. For example, if the tumor or target is in the shape of a donut then the radiation can be made to be quite similar in form. SRS also has the advantage that the dose of radiation dissipates very rapidly at increasing distances from the tumor. SRS incorporates enhanced immobilization of the body and/or enhanced image guidance. Image guidance is the process whereby the accurate position of the beams of radiation is confirmed by x-ray images before and/or during the delivery of radiation. The enhanced accuracy of SRS enables the physician to greatly increase the dose of radiation.
When a single dose of radiation is given (over minutes or hours) this is termed SRS. When 2-5 doses of radiation are given over 2-5 separate days, this is termed SBRT (stereotactic body radiation therapy). Delivery of larger doses of radiation over one to five days results in much more effective radiation. For example, SBRT of stage I lung cancer appears to have an effectiveness in local tumor control that is similar to surgical removal of the tumor. For another example, SRS of small tumors that spread to the brain can often control the tumors as effectively as surgical removal.