Extra-cranial Stereotactic Radiosurgery
Dear OncoLink "Ask the Experts,"
Is stereotactic radiosurgery used for inoperable tumors in non-brain areas? If so, where can I find further information about it, and where is such work taking place?
Thanks for your consideration,
Kenneth Blank, MD and John Han-Chih Chang, MD, OncoLink Editorial Assistants, respond:
Thank you for your interest and question.
Stereotactic radiosurgery (SRS) is a modality that utilizes external beam radiation therapy. Its proposed benefit over conventional externalbeam radiation therapy is that higher doses of radiation can be given to tumors and some benign lesions without damage to the surrounding normal tissues.
With any external beam radiation therapy, the highest dose of radiation develops at the intersection of multiple beams. Thus, the less beams there are, the more dosereaches the other areas the beam traverses. For example, a treatment that only includes two beams, such as an anterior and posterior field, would distribute the dose tothe front and back of the patient. The highest dose would be in the middle, but dose would be distributed from front to back. If a four-field technique was utilized, the dose again would be highest at the middle. The "extra" dose would be distributed from front to back and laterallymore area would receive radiation, but the dose wouldbe diminished in each of those areas respectively.
SRS uses the above principle to tightly conform a maximum dose around a small target and minimize dose to surrounding structures by employing many fields simultaneously. The key to SRS is immobilization of the patient so that targeting can be accurate and precise, since conforming the radiation dose to the target leaves little margin for error. This has been successfully performed in brain lesions, both benign and malignant, with the assistance of the head frame (immobilization devicethat "screws" into the skull). The head frame allows reproducibility of patient positioning between the planning/targeting and treatment sessions.
SRS has been attempted in extra-cranial areas. It continues to be investigational. As we alluded to earlier, SRS relies on reproducibility of positioning. With the extra-cranial sites, reproducibility and immobilization are difficult to come by, since just normal breathing can move a pulmonary or abdominal tumor more than 1 cm. A body frame has been utilized to immobilize patients for such treatment. Reports are few and far between on the success of such treatment. One articlefrom Stockholm, Sweden reports on 31 patients treated for solitary lesions in the liver, lungs and retroperitoneum. They stated that there was a 50% response rate. In the United States, Dr. Gil Lederman at the Staten Island University in New York and possibly the University of Florida in Gainesville have the capability to perform suchSRS on extra-cranial sites. No data has yet been reported from these two institutions on SRS on the other areas of the body, but trials are underway.