Stereotactic radiosurgery (SRS) is a treatment delivery technique that utilizes a single high dose of radiation to ablate a tumor, hence the term “surgery”, although this is a non-invasive treatment modality. The word stereotactic refers to a three-dimensional coordinate system that enables accurate correlation of a tumor visualized in the patient’s diagnostic images with the actual tumor position in the patient’s body. This technique is typically used for brain and spinal tumors; however, it may be used for other types of tumors as well. Stereotactic body radiation therapy (SBRT) refers to the same delivery method, but a given dose is delivered in multiple treatments over several days. Both of these treatment methods rely on accurate patient positioning, strict quality assurance, on-board imaging and highly conformal delivery methods, which result in a steep dose gradient between the edge of the tumor and surrounding normal tissue. This results in a high dose of radiation being received by the tumor and a relatively low dose being received by surrounding tissue. As such, there is an exceedingly low probability of normal tissue toxicity (manifesting as side effects). This is compared to conventional radiation therapy (RT) techniques and protocols, in which a total dose of radiation is delivered in multiple small daily fractions that may span a 4-6 week period. Using conventional methods, breaking up the total dose into many smaller doses is necessary because of the relatively large amount of normal tissue surrounding the tumor that needs to be included in a given field. With conventional techniques, there is a very high probability of acute radiation side effects in the normal tissue included in a given field. Acute radiation toxicity and associated side effects are self-limiting and will resolve in 1-3 weeks. Radiation therapy for veterinary patients requires anesthesia, as accuracy of dose delivery depends on patient immobilization. Conventional radiotherapy requires daily anesthesia over 4 weeks. Stereotactic radiotherapy treatments may only require 1-3 anesthetic episodes, which are typically performed in one week or less.
Stereotactic radiosurgery may be delivered using a conventional linear accelerator that is equipped with special cones and collimators that allow precise radiation delivery to a tumor. Rigid immobilization may or may not be required for a patient, depending on the type of linear accelerator being used. Planning software utilizes a pre-treatment CT or MRI scan which allows the radiation oncologist to sharply delineate the tumor and define sensitive surrounding tissues that will be avoided. Immediately prior to a treatment, a cone beam CT scan is typically performed, which allows visualization of the tumor and surrounding anatomy and ensures proper dose delivery. The success of radiosurgery depends on rigid quality assurance and delivery of radiation with sub millimeter accuracy.
Technological advancements have also allowed the development of a compact linear accelerator mounted on a robotic arm. Accuray in Sunnydale, CA manufactures the CyberKnife® system. This method of delivering radiation is accomplished by directing the source of radiation from up to 1200 different angles around a tumor. A robotic arm accomplishes rapid repositioning of the linear accelerator during a patient’s treatment. In addition, reimaging of the patient and robot corrections for tumor movement is done continuously throughout a treatment. The CyberKnife® system does not rely on rigid immobilization, but instead tracks tumor position via bony anatomy or by utilizing the placement of fiducial markers, which are tiny gold seeds that are implanted in or around a tumor. These fiducial markers are tracked by the use of orthogonal kilovoltage x-ray sources and floor panel detectors (see Figure 1). These allow tracking by the robot in real-time and can account for changes in patient position during a treatment. Advances in the CyberKnife® system have allowed tumors that are constantly moving, such as lung or liver tumors, to be treated with sub-millimeter accuracy, as the robotic arm moves the beam of radiation with the tumor as it changes position with the varying phases of respiration. As with the previous SRS/SRT delivery method, the CyberKnife® relies on computerized software planning, strict adherence to quality assurance and a team of radiation oncologists and medical physicists.
There are two CyberKnife® facilities on the east coast that are dedicated to veterinary use. One is at the VCA Animal Specialty Center in Yonkers, NY and the second is the privately owned Veterinary CyberKnife Cancer Center in Malvern, PA. CyberKnife® radiosurgery has allowed treatment of tumors that were rarely treated with radiation therapy in veterinary medicine, such as lung and liver tumors. This advancement has also allowed successful treatment of tumors such as bone bladder/ urethral tumors and prostate tumors (see Figure 2), as the concern for normal tissue toxicity has previously kept us from attempting radiotherapy using conventional methods.
These novel therapies are on par with many human facilities and offer our patients options never before thought possible in veterinary medicine. These options will enable treatment for cancers never thought possible with previous therapy units. Although the term stereotactic radiotherapy has become more commonplace in veterinary oncology, many facilities use the term loosely but are not truly “sterotactic”. For the primary care clinician, it is important to recognize the difference and to also ensure that a radiation oncologist is “on site” to evaluate the patient and play an active role in the therapy protocol. A search for radiation oncologist can be found via http://www.acvr.org/public/search
Submittted by Dr. Siobhan Haney; Radiation Oncologist Hope Veterinary Specialists
Figure 1. The CyberKnife® Radiosurgery system
Figure 2. This is an axis CT image at the level of the pelvis. The orange circle at the center of the image is the urethra, which is effaced with transitional cell carcinoma. The colored lines around this area represent the different isodose lines, or the amount of radiation that is being received by the surrounding tissue. The colon is directly above the urethra and is receiving a very small portion of the dose of radiation. This decreased the chance of normal tissue side toxicity to an almost negligible level.