Stereotactic Radiosurgery (SRS)
DESCRIPTION AND OVERVIEW
Stereotactic radiosurgery is a radiation therapy modality in which high doses of radiation are delivered using a very precise beam during a single treatment session. It is generally used for smaller lesions and metastases. Historically, this approach was used primarily in neurosurgery, for treating brain lesions and abnormalities. However, the Trilogy system is optimized to deliver stereotactic radiosurgery to tumors in the body, as well. Because the tumors treated with stereotactic radiosurgery are generally quite small, this approach requires precise patient positioning and exact targeting of the beam.
HOW IT WORKS
Targeting such small volumes requires highly accurate patient positioning and immobilization. Varians optical guidance system uses infrared light and optical tracking cameras to provide a precise readout of patient displacement relative to the isocenter, with 0.3mm accuracy. Varians Trilogy system comes with a number of technologies for positioning patients accurately, including a headring, bite block, optical tracking cameras and fiducial markers that go onto fixation devices.
- Small lesions can be treated with radiotherapy, reducing the need for more invasive treatments such as surgery or chemotherapy.
- Trilogy can then deliver the most powerful radiation beams with unparalleled accuracy.
Indications for Stereotactic Radiosurgery (SRS) in Veterinary Medicine:
Pituitary Tumors: Radiosurgery is an ideal tool for the treatment of pituitary tumors. Dose can be focused on the tumor and extraneous dose to the brain can be minimized. Neurological signs from these tumors, in both dogs and cats, respond extremely well to fractionated radiation therapy. Although this technique has not been done often in veterinary medicine, in human patients, neurological signs respond as well to SRS as they do to fractionated radiation therapy. Interestingly, the endocrinopathies in humans associated with pituitary tumors respond more rapidly and durably to SRS. We have an ongoing study evaluating the endocrine status of pituitary tumors in cats post SRS (link to acc website). Generally 2 fractions of SRS are required for pituitary tumors.
Inoperable menigiomas: SRS is ideal for treating brain tumors. The dose can be focused on the tumor, while sparing the rest of the brain. Two-3 fractions are required, based on the size of the tumor and the relationship between tumor size and brain.
Other brain tumors: SRS may prove to be the best treatment alternative for glial tumors and other brain tumors. Although long term cure may not be possible, SRS may provide a fairly durable remission without the morbidity associated with surgery.
Osteosarcomas: Osteosarcomas are often amenable to SRS. SRS can deliver an intense dose to the tumor while at the same time deliver a dose appropriate to the soft tissue extension of the disease. From a biological standpoint, treatment in large doses/fraction makes sense for osteosarcoma, because there is data to suggest that these tumors have a higher alpha/beta ratio than other tumor types. SRS has been used in veterinary medicine as a non-surgical approach to limb sparing. In addition, SRS is indicated for axial tumors as well. The Trilogy also has the capability of doing stereotactic intensity modulated radiosurgery (SIMRS). This means that the beam can be sculpted using the sliding collimator leaves. Vertebral tumors can be treated while sparing the spinal cord.
Liver tumors: SRS is ideal for treatment of liver tumors.
Palliation: SRS is commonly used in humans to achieve durable palliation of tumors in the abdominal cavity. A higher dose can be delivered that using regular external beam radiation therapy because dose to the intestines can be minimized.