In the first collaboration of its kind between Penn Vet and Penn Med, clinicians used a laser treatment for humans to treat cancerous tumors in the delicate area around both eyes of a horse.

Anita is a special type of Paint horse known as Medicine Hat, prized in many Native American cultures. Distinguished by their markings, Medicine Hats are mostly white with a brown “war bonnet” over the ears, a “shield” of brown on the chest, and blue eyes.

Unfortunately, Anita’s light pigmentation increased her risk for developing squamous cell carcinoma, a tumor more likely to develop in white horses due to UV light exposure.

Traditional treatment for large areas of eyelid squamous cell carcinoma is a surgical procedure to remove the masses, often transplanting skin from the face to close the wounds and regain eyelid function, according to Catherine Nunnery, large animal ophthalmologist at New Bolton Center.

In Anita’s case, that approach was impossible because the cancerous masses were too large, and using skin from her face was too risky, since nearby skin was likely to also contain cancer cells.

Omega Horse Rescue’s Kelly Smith rescued Anita from a kill pen. Usually the treatment in Anita’s case might require removal of her eyes. However, 10-year-old Anita is a rescue horse and removal of her eyes would make finding her a home nearly impossible. Saving her eyes, on the other hand, might just save her life.

“We were going to do whatever we could to try to help this horse,” said Ms. Smith, founder and director of Omega, a non-profit in Airville, Pennsylvania. Dr. Nunnery knew the University of Missouri College of Veterinary Medicine pioneered the use of photodynamic therapy (PDT), a laser treatment to kill cancer cells on the margins of tumors, on horses. The treatment uses a drug, called a photosensitizer or photosensitizing agent, and a particular type of light, most often a laser. When photosensitizers are exposed to a specific wavelength of light, they produce a form of oxygen that kills nearby cells. Enter Keith Cengel, associate professor of radiation oncology and director of the Photodynamic Therapy Program at the Perelman School of Medicine. When Dr. Nunnery asked to borrow the equipment, he not only said yes, but that he would come to perform the procedure, along with laser specialist Carmen Rodriguez. “I am pleased that we are able and allowed to do it because this horse is in bad shape and we can fix it. How cool is that?” Dr. Cengel said while preparing for the surgery. “Besides, my kids will think I’m a hero.”

Dr. Cengel has collaborated with Penn Vet before, treating cats, dogs and birds, working with Lillian Duda, an adjunct associate professor of radiation oncology at Penn Vet’s Ryan Hospital. Anita is the largest creature the team has treated. Anita’s procedure took place at New Bolton Center.

The pharmacy and ophthalmology teams worked together to procure the necessary drug. Dr. Cengel and Dr. Rodriguez provided the equipment and the know-how.

First, Dr. Nunnery and Nicole Scherrer, New Bolton Center ophthalmology resident, performed the nearly four-hour surgery to carefully remove part of each tumor from the eyelid. The photosensitizer was then injected to treat remaining tumor cells. Dr. Cengel and Dr. Rodriguez worked together to use the laser, first by fashioning a mount to hold the fiberoptic that delivers the laser beam, which made a red circle of light over the eye, for 15 minutes. The chemical in the solution reacts to that wavelength of light to kill tumor cells.

“When done correctly at the right dose, it won’t kill the collagen, so the substance and structure of the tissue won’t be fried,” Dr. Cengel said. “This will create much more pure oxidized damage to the cells, so when the tissue heals it is more normal and more functional.” The drug and the light penetrate less than an inch into the tissue. “It is self-limiting,” Dr. Cengel said. “The drug, the light and oxygen are required, and the light has a limited depth of penetration.”

The procedure doesn’t kill just the cancer cells. “The drug may have some preference for cancer cells, but that is not proven,” he said. The chemical reacts where it is injected. “I just make sure the light covers the whole area I want to treat.”

The beam of light doesn’t burn, Dr. Rodriguez said. It’s a photochemical reaction with the drug injected under the skin. People in the room wear dark glasses as a safety precaution, but the light is at a low dose that doesn’t hurt skin. The procedure was then repeated on the other side.

“This is much larger than usual, compared to what they do at Missouri,” Dr. Nunnery said during the procedure. “We are pushing the limits, but why not try?”

Dr. Cengel believes that human medicine can learn from equine medicine, since the skin of a horse is very similar to that of a human.“Half a centimeter of light penetration is similar to what we would use for a human,” he said.

“Life span and other characteristics of horses are similar to cancer in humans in many ways. We have a lot of our DNA in common with horses. A lot of diseases are very similar,” he continued.

This collaborative effort is one of many between Penn Vet and Penn Med that highlight the importance of One Health, the concept that human, animal and environmental health are inextricably linked.

Anita was discharged ten days later to recuperate at Nicky Manfredi’s farm, behind New Bolton Center.

Anita’s eyes fully healed from the cancer, with minimal scarring. The left eye responded particularly well, Dr. Nunnery said.

The tumor on the right eye was larger and more complex. Biopsy results after surgery showed that the right eye also had sarcoid cells.

“PDT seems to be a very good option for squamous cell carcinoma,” Dr. Nunnery said. “Early studies out of Missouri show sarcoid in horses is not as responsive to PDT, and is subject to recurrence.”

The best treatment for sarcoid is chemotherapy, which Anita started in September. The drug is the same used in human medicine.

“It is a small sarcoid, and we are very hopeful chemotherapy will cure the recurrent tumor,” Dr. Nunnery said. “Anita is a fighter.”

To prevent cancers in the future, Anita has to wear a mask to protect her eyes from the sun when outside.

Ms. Smith said Anita—which means grace— may become the “spokeshorse” for Omega.

“The day I saw her in the kill pen, she had given up,” Ms. Smith said. “You can’t help but feel a lot of sympathy and compassion.

“In Anita, I see such a willing patient, trying to get well. It speaks volumes about the animal she is inside.”

With the success of the laser treatment on Anita’s eyes, Dr. Nunnery hopes to see future collaboration between New Bolton Center and Penn Med to treat horses. “We can do the surgery and inject the photosensitizer, but we don’t have a light source to activate the drug. For this procedure to be successful, it will have to be a Penn Vet-Penn Med team effort,” she said, adding that her goal is to someday have the equipment on site at New Bolton Center. “This is a first step.”