Radiation oncology for equine tumors - DVM
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Radiation oncology for equine tumors
The current state of this therapeutic technique in horses


The future

"I think the advances in the field are going to be limited, in that there are very few of us who can do radiation therapy with linear accelerators," says Green. "If people wanted to, they could purposely build a radiation therapy vault in their practices to accommodate horses. But radiation therapy is not going to be a lucrative venture, so unless someone wanted to subsidize that, it probably wouldn't be the best idea to deliberately build a vault for horses, unless you could also use it to treat small animals. Ninety-five percent of what I do is small animals. This is due simply to the fact that horses don't get a lot of tumors."

However, there are advances in human radiotherapy, including intensity-modulated radiation therapy (IMRT), radiosurgery and TomoTherapy.

IMRT is a procedure that uses multiple radiation beams from multiple angles. The beam shape changes during treatment to allow more precise delivery of radiation dose to the tumor with better avoidance of normal tissues. Like conventional radiation protocols, it requires multiple anesthetized treatment sessions. IMRT can be done with a conventional linear accelerator equipped with a multileaf collimator. It also requires cross-sectional imaging of the body part and a 3-D computerized treatment planning system to determine radiation dose delivery.

IMRT is being investigated in small-animal patients, but it has not been reported in horses. Auburn University's College of Veterinary Medicine has onsite computed tomography (CT), magnetic resonance imaging (MRI) and a linear accelerator with a multileaf collimator accessible for horses and can accommodate IMRT for horses, but it has not yet used the procedure, according to William Brawner, DVM, PhD, Dipl. ACVR (radiology and radiation oncology), a professor at Auburn's College of Veterinary Medicine. The challenge of doing IMRT in horses is that their body sizes limit the precise and repeatable positioning required and also make it difficult to allow the linear accelerator gantry to rotate around the patient to deliver the multiple beam angles. Linear accelerators are designed for people; none are designed for animals, and especially not for large animals. The tumor's location and size, the ability to obtain cross-sectional images and the ability to position the patient for gantry rotation will be important considerations in case selection. It's likely that only tumors of the head and extremities of horses will be accessible for IMRT.

A new equine radiation oncology unit is being built at Texas A&M College of Veterinary Medicine and Biomedical Sciences (TAMU CVM). The Diagnostic Imaging and Cancer Treatment Center (DICTC) is a state-of-the-art research and clinical facility that will contain about 9,000 square feet dedicated to advanced diagnostics, radiotherapeutics and translational research. The DICTC includes a 40-slice configuration SOMATOM Sensation Open CT scanner (Siemens), a wide-bore MAGNETOM Verio 3T MRI scanner (Siemens) and a TomoTherapy Hi·Art treatment system (TomoTherapy).

Michael Deveau, DVM, Dipl. ACVR (radiation oncology), a clinical assistant professor at TAMU CVM, says, "The facility signals a major new material and intellectual commitment on the part of TAMU CVM toward advancing research and improving veterinary patient health care."

TAMU CVM purchased a helical TomoTherapy system and has exclusively dedicated it for veterinary patient image-guided radiotherapy. The ring-based TomoTherapy platform combines integrated CT imaging with conformal IMRT to deliver sophisticated and unmatched radiation treatments with speed and precision while reducing radiation exposure to surrounding healthy tissue. "It's a flagship product in veterinary radiation oncology," says Deveau. Access to a TomoTherapy system for veterinary patients is limited to only two institutions worldwide: TAMU CVM and the University of Wisconsin-Madison Veterinary Medical Teaching Hospital. Only Texas A&M intends to treat large animals, says Deveau.

"Ring-style gantry systems like the Hi·Art treatment system aren't specifically designed for large animal use," says Deveau. "Of course, the same can be said for any of the linear accelerators. The systems are designed for attributes encompassing the human silhouette. It's fortunate we're able to translate the use to such a wide range of species and clinical presentations in veterinary medicine."

The DICTC is scheduled to start treating small-animal patients in September. Integration of large animals will be limited initially to the weight limit of the system's couch (425 to 450 lbs.). Active research and development is under way to integrate an independent couch system for large animals weighing more than 450 lb, but it's currently not ready for clinical implementation.

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.


Source: DVM360 MAGAZINE,
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