Radiation Therapy at the Animal Cancer Center

A new era in Veterinary Radiation Oncology began at the James L. Voss Veterinary Teaching Hospital with the delivery in September 2007 of the Varian Trilogy System. The Trilogy Stereotactic System is the most advanced, sophisticated machine of its type in the world. As the leading image-guided radiotherapy (IGRT) system, Trilogy marks the beginning of a new generation of cancer care. The versatile Trilogy system can be used to deliver the widest range of external beam radiotherapy: 3D conformal radiotherapy, intensity modulated radiation therapy (IMRT), stereotactic radiation therapy (SRT), fractionated stereotactic radiation therapy and intensity-modulated radiosurgery for cancer and neurosurgical treatments. Advanced imaging capabilities built into the system allow therapists to position patients for treatment with sub-millimeter accuracy, making sure the tumor is lined up precisely with the treatment beam before the beam is turned on. A respiratory gating system takes into account the tumor movement that can occur as a patient breathes, turning the beam on and off, so that treatment is delivered only when the tumor is stationary.

This advanced linear accelerator enhances delivery of radiation therapy and reduces adverse side effects in patients thanks to increased accuracy in focusing the beam on the tumor and sparing normal tissue. It also allows us to offer stereotactic radiation therapy, a new form of targeted therapy that can be administered over days, instead of weeks. This exciting new technology is being used for many cancers including osteosarcomas and brain tumors.

Intensity-modulated Radiation Therapy

Intensity-modulated radiation therapy (IMRT) is a is a state-of-the-art cancer treatment method that delivers high doses of radiation directly to cancer cells in a very targeted way, much more precisely than is possible with conventional radiotherapy. IMRT involves varying (or modulating) the intensity of the radiation beam, so that the shape of the resulting dose distribution is tightly matched to the shape of the tumor. IMRT enables an oncology team to direct and narrowly concentrate potent doses of high-energy X rays at a patients tumor while minimizing complications to surrounding healthy tissue.

How it works:
IMRT targets a tumor with intensity-modulated beams delivered from multiple angles. The area where the radiation beams intersect creates a finely sculpted radiation cloud that envelops and has the same shape as the tumor. The IMRT process starts with diagnostic images, for example, computed tomography (CT), magnetic resonance (MR) and/or positron emission tomography (PET) images, of the patients tumor and surrounding anatomy. These images are converted into a custom three-dimensional model of the patients internal anatomy. A powerful computer program creates a treatment plan based on tumor size, shape, and location within the body, along with the doctors dose prescription.

A medical linear accelerator, equipped with a special beam-shaping device called a multileaf collimator (or MLC), delivers the radiation in accordance with the treatment plan. The varian high-resolution MLC has 120 tungsten metal leaves or slats for shaping the radiation beam to deliver unique doses to very small areas, as small as the tip of a pencil.

The linear accelerator rotates around the patient to send beams from multiple angles in order to give the tumor a high dose of radiation while preserving important healthy tissues. A powerful computer program guides the movement of the linear accelerator and the MLC to precisely match the treatment plan, delivering the correct dose directly to the target.

Patient Benefits

Higher doses of radiation can be delivered directly to tumors and cancer cells, while surrounding organs and tissues are protected. Lower doses to healthy normal tissues means fewer complications or side effects. For example, in the case of head and neck tumors, IMRT allows radiation to be delivered in a way that minimizes exposure of the spinal cord, optic nerve, salivary glands or other important structures. In the case of prostate cancer, exposure of the nearby bladder or rectum can be minimized.

The level of normal tissue sparing achieved with IMRT is dramatic. Clinical experience shows a tremendous reduction in the side effects of radiotherapy when IMRT is used.

Clinicians can treat cancers that were previously untreatable with radiation therapy, sparing some patients the invasive techniques of surgery and/or chemotherapy.

Stereotactic Radiation Therapy

Stereotactic radiation therapy is a 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 radiation therapy to tumors in the body, as well.

How it works:
Targeting tumors requires highly accurate patient positioning and immobilization. Patients are positioned using a variety of immobilization devices, depending on the location of the tumor. Multiple beams are used in a similar fashion to IMRT.

Patient benefits

• The use of powerful beams means reduced treatment time.
• The most accurate beam means the most accurate treatment.
• The most advanced patient positioning tools mean the most accurate treatment and a reduction in side effects.
• The ability to deliver treatments that shrink-wrap the radiation dose to the tumor means a reduction in the side effects of the treatment.
• Treatment is generally completed in 1-5 days.
• Acute radiation effects are minimal.

Indications for Stereotactic Radiation Therapy (SRT) in Veterinary Medicine

Pituitary Tumors: SRT 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 SRT as they do to fractionated radiation therapy. Interestingly, the endocrinopathies in humans associated with pituitary tumors respond more rapidly and durably to SRT. We have just completed a study evaluating the endocrine status of pituitary tumors in cats post SRT. Generally 3-4 fractions of SRT are required for pituitary tumors.

Inoperable Menigiomas: SRT is ideal for treating brain tumors. The dose can be focused on the tumor, while sparing the rest of the brain. One to 3 fractions are required, based on the size of the tumor and the relationship between tumor size and brain.

Other Brain Tumors: SRT may prove to be the best treatment alternative for glial tumors and other brain tumors. Although long term cure may not be possible, SRT may provide a fairly durable remission without the morbidity associated with surgery.

Osteosarcomas: Osteosarcomas are often amenable to SRT. SRT 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 lower alpha/beta ratio than other tumor types. SRT has been used in veterinary medicine as a non-surgical approach to limb sparing. In addition, SRT is indicated for axial tumors as well. The Trilogy also has the capability of doing stereotactic intensity modulated radiation therapy Vertebral tumors can be treated while sparing the spinal cord.

Nasal Tumors: Nasal tumors can be safely treated with SRT. Initial results with regard to local tumor control are promising.

Liver Tumors: SRT is ideal for treatment of inoperable liver tumors.

Palliation: SRT 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. Inoperable tumors of the pancreas and adrenals may be amenable to this treatment.

3DCRT: 3-Dimensional Conformal Radiation Therapy

Three Dimensional Conformal Radiation Therapy uses CT-based techniques to generate 3D images of internal structures. The volumes are used to shape radiation beams to spare normal tissue. This remains a useful tool in the radiation armamentarium.

Electron Beam Radiation Therapy

Electrons (4-18 MeV)
Electron therapy is a form of external beam radiation therapy where electrons are directed at the tumor. Electrons don't penetrate as deeply as x-rays, which are normally used for external beam radiation therapy. That makes them well suited for treating tumors close to the skin surface. They are particularly useful for treatment skin tumors over the abdomen, ensuring that excess radiation dose not reach the intestinal track, and for limiting the dose of radiation to the lungs for tumors over the thorax. Electrons are also useful for tumors near the eye. A special ceramic covered tungsten contact lens block can be placed over the eye to greatly diminish dose, while the prescribe dose can be delivered to the area surrounding the eye. At CSU we have electrons ranging in energy from 4-18 MeV. The energy selected for each patient is based on the thickness of the area being treated.

To learn more:
Meet the Radiation Therapy Team

If you are interested in other current cancer treatment options; please refer to chemotherapy and surgery. The ACC offers additional information as Client Handouts that can be obtained on site or via mail if requested.