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Stereotactic Radiosurgery

What is stereotactic radiosurgery?
Stereotactic radiosurgery is a highly precise form of radiation therapy used primarily to treat tumors and other abnormalities of the brain and pituitary gland. It is a non-surgical procedure that delivers a single high-dose of precisely-targeted radiation using highly focused gamma-ray or x-ray beams that converge on the specific area or areas of the brain or pituitary gland where the tumor resides, minimizing the amount of radiation to healthy surrounding tissue. Although stereotactic radiosurgery is often completed in a one-day session, doctors sometimes recommend multiple treatments, especially for tumors larger than one inch in diameter. The procedure is usually referred to as fractionated stereotactic radiotherapy and is completed in three to five treatments.

Stereotactic radiosurgery is an important alternative to invasive surgery, especially for tumors located deep within or close to vital areas of the brain. Radiosurgery is used to treat many types of brain tumors, either benign or malignant primary or metastatic lesions and either single or multiple tumors.

Stereotactic radiosurgery 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 time. Malignant and metastatic tumors may shrink more rapidly, even within a couple of months.

What equipment is used?
Linear accelerator (LINAC) radiosurgery consist of four phases: head frame placement and imaging, computerized dose planning and radiation delivery. The LINAC's gantry rotates around the patient, delivering radiation beams through multiple arcs. The LINAC is able to use a larger x-ray beam, which enables it to treat larger tumors more uniformly and it can be used for fractionated radiosurgery or stereotactic radiotherapy using a relocatable frame, which is an advantage for larger tumors or particularly critical locations.

Who will be involved in this procedure and who operates the equipment?
The treatment team is comprised of a number of specialized medical professionals, typically including a radiation oncologist, a radiologist, a medical radiation physicist, and several radiation therapists.

  • The radiation oncologist leads the treatment team and oversees the treatment; they outline the targeted area(s) to be treated, decide on the appropriate radiation dose, approve the treatment plan, and interpret the results of radiosurgical procedures.
  • A radiologist interprets brain imaging that identifies the targeted area(s) to be treated.
  • The medical radiation physicist ensures the delivery of the precise dose or radiation.
  • The radiation oncologist along with the physicist uses a specialized computer software program to devise a treatment plan, calculates the exposures and beam configuration to conformally treat the targeted area(s) to the prescribed dose.
  • A highly trained radiation therapist positions the patient on the treatment table and operates the machine from an adjacent protected area. The radiation therapist can observe the patient on a closed circuit monitor.
  • A neurologist or neurosurgeon may participate with the radiation oncologist in the multidisciplinary team that considers various treatment options for individual cases and helps decide who may benefit from radiosurgery.

Is there any special preparation needed for the procedure?
Stereotactic radiosurgery is performed on an outpatient basis. The patient should not eat or drink anything after midnight on the night before imaging or the treatment. The owner should ask the oncologist if it is ok to take medications on the day of the treatment.

What will the patient feel during and after the procedure?
Radiosurgery treatments are similar to having an x-ray. There is no pain or discomfort from the actual treatment. Patients are anesthetized for the procedure as even slight movements can cause the oncologist to miss the tumor. In most cases, the patient can resume all normal activities within one or two days.       

Canine Appendicular Osteosarcoma

Osteosarcoma is the most common bone tumor in dogs. Although it is mostly a disease of older large or giant breed dogs, it can affect dogs of any size or age. Osteosarcoma may be found in many areas, but it most commonly appears in areas bordering the shoulder, wrist and knee.
The first step in diagnosing osteosarcoma and other primary bone tumors is to take radiographs ("x-rays") of the affected limb. A careful physical exam is also important to assess the overall health of your dog.

Next, radiographs ("x-rays") will be taken of your dog's chest to see if the tumor has spread to the lungs. It is important to find out whether the cancer has spread (metastasized) to the lungs, as treatment options and prognosis are very different if lung metastasis has occurred. In the initial diagnostic stages, we may also aspirate local lymph nodes and any skin masses, or perform an abdominal ultrasound. Again, these steps are necessary in order to assess the spread of the cancer.

Another diagnostic test that may be performed is a bone scan. This test, which requires an overnight stay in the hospital, will help determine whether the tumor has spread to other bones and how much of the bone where the primary tumor is located is affected. This test is most often done on patients who will undergo a treatment that spares the affected limb, and is not required for every patient.

To obtain a definitive diagnosis, it is necessary to aspirate the tumor with an ultrasound-guided needle or perform a bone biopsy in order to attempt to identify the type of tumor. These procedures require sedation or anesthesia. There is often a large amount of bony reaction associated with this type of tumor, so it can be difficult to obtain an adequate sample of the actual tumor cells. Additionally, small samples are taken in order to minimize the spread of the tumor cells along the biopsy pathway and to avoid fracturing the bone. For these reasons, the biopsy procedure does not always yield a diagnostic sample. Even though only very small samples are taken, and bone biopsies are performed with the utmost care, this procedure always carries a small risk of fracturing the bone.

The tumor must be removed from your dog's leg. Aside from the possibility that the cancer will spread, the tumor is painful, and once it has destroyed enough of the bone, even normal activities such as walking or running can cause the bone to break. The standard treatment for an osteosarcoma is the amputation of the affected limb. Most dogs recover quite well from this procedure and are running and playing in a very short time.

Dogs who have other orthopedic problems or who are obese may not be good candidates for amputation. If this is the case, we may be able to offer your dog a limb sparing procedure. In this procedure, the tumor is removed and the bone is replaced either with another bone from your dog or with a bone from a bone bank. This operation cannot be performed in all locations and the tumor must be of a relatively small size at the time of diagnosis. Because of the high complication rate, amputation may eventually be necessary.
If neither amputation nor a limb sparing procedure is possible or desirable, another option is to attempt to relieve pain with palliative radiation therapy. Although palliative radiation does not generally increase a dog's survival time, in about 75% of dogs treated it does alleviate the pain caused by the tumor. Radiation therapy requires one treatment per week for four weeks, and these are administered while the patient is under general anesthesia.

Chemotherapy is an important follow-up step in the treatment of osteosarcoma. Though we may not have found gross evidence of the spread of tumor in your dog, it is estimated that over 90% of dogs with this type of tumor have microscopic spread before amputation or limb sparing procedures are performed. Although chemotherapy has not been shown to be very effective in treating osteosarcoma when there are visible signs of spread, it is very effective in treating microscopic disease. The protocol we currently use requires six treatments given three weeks apart. Chemotherapy as a follow-up treatment increases the median life expectancy to about one year. The median survival time with amputation alone is about three months. By two years 10-20% of the dogs who have received chemotherapy appear to be free of cancer. Most dogs tolerate the therapy very well and experience few side effects. The possible risks of treatment will be discussed with you, and you can also refer to the hospital's handout on chemotherapy.

This handout is designed to answer some of the basic questions relating to canine appendicular osteosarcoma. Your dog's particular situation may vary. If you have any questions please ask us; we will be happy to answer them to the best of our ability.

Feline Vaccine Associated Fibrosarcoma

Vaccine associated fibrosarcomas are tumors that arise at sites where cats have been vaccinated. They are most commonly associated with the rabies vaccine and the vaccine for feline leukemia virus. We continue to give these vaccines because of the relatively high risk of contracting these diseases and the relatively low risk of developing a fibrosarcoma. The incidence of these tumors is not known, but is reported to be about 1 in 10,000. Fibrosarcomas tend to be very aggressive locally, and can also metastasize (spread to other areas of the body).

The first attempt at treatment is thought to have the best chance of curing or slowing the disease, although it is possible to treat disease recurrence. Treatment generally involves combinations of surgery, radiation therapy and chemotherapy. It is necessary to determine the extent of the cancer in order to determine which treatment option is best for your cat.

A thorough physical exam, chest radiographs ("x-rays") and blood work are all part of the initial work-up. Although pulmonary metastasis, or the spread of the tumor to the lungs, occurs only about 10-25% of the time, it changes the prognosis and treatment options available for your cat. A CT scan, which allows careful evaluation of the tumor and the tissues around it, is usually required as part of the diagnostic plan. If a biopsy has not been done one will be done at the same time as the CT scan.

With the information obtained from the CT scan we will be able to formulate a treatment plan. If the tumor is very small or non-invasive, surgery alone may be recommended. In other cases we may recommend radiation therapy followed by surgery, or we may recommend chemotherapy alone, which tends to be very well tolerated by cats. If surgery or radiation therapy followed by surgery is recommended we will offer follow-up chemotherapy to try to decrease the likelihood that lung metastasis will occur.

If your cat has a recurrence of a fibrosarcoma, the diagnostic procedures will be very similar. Again, imaging the tumor with radiographs and a CT scan will help us determine the extent of the tumor and help us to formulate a treatment plan for your cat.

This handout is designed to answer some of the basic questions relating to feline vaccine associated fibrosarcoma. Your cat's particular situation may vary. If you have any questions please ask us; we will be happy to answer them to the best of our ability.