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Radiation Modalities

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Strontium-90 therapy (Plesiotherapy)

Strontium-90 (Sr-90) therapy does not penetrate very deep into the body, only treating a couple of millimeters into the skin. This therapy involves placing a radioactive isotope over the tumor area and delivering a large dose of radiation once or twice. SR-90 therapy is very good at treating and curing a number of superficial tumors such as squamous cell carcinoma of the ear pinnae and nasal planum in cats, melanoma of the eye in dogs and cats, and squamous cell carcinoma of the eye in horses.

Intensity-modulated radiation therapy (IMRT)

Intensity-modulated radiation therapy (IMRT) allows us to deliver high doses of radiation directly to cancer cells in a much more precise way than is possible with conventional radiotherapy. IMRT involves varying 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 a radiation oncology team to direct and narrowly concentrate potent doses of high energy X-rays at a patient’s tumor while minimizing complications to surrounding healthy tissues.

IMRT targets a tumor with intensity-modulated beams (X-rays) delivered from multiple angles. The area where the radiation beams intersect creates a finely sculpted area of radiation that surrounds, and has the same shape as, the tumor.

The IMRT process starts with computed tomography (CT), magnetic resonance (MR), and/or positron emission tomography (PET) images, of the patient’s tumor and surrounding anatomy. These images are transferred to the radiation treatment planning computer, where a custom three-dimensional model of the patient’s internal anatomy is created. A specially trained radiation oncologist uses a powerful computer program to design a treatment plan based on the tumor’s size, shape, and location within the body.

The multi-leaf collimator is used to shape 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 angels 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 of IMRT

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 nasal tumors, IMRT allows radiation to be delivered in a way that minimizes exposure of the brain and eyes. 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.