The Denver Zoo is partnering with veterinarians from Colorado State University to give an elderly lion a never-before attempted cancer treatment.

In mid-March, zookeepers began noticing that Rian, a 15-year-old South African lion, was acting lethargic and immediate tests revealed a large mass in the lion's abdomen. After calling in CSU veterinary surgeon Dr. Dean Hendrickson to perform an exploratory surgery on the lion, it was revealed that Rian was suffering from a type of cancer known as high-grade splenic lymphoma.

During the surgery Rian's spleen was removed and was found to weigh 12 pounds, or nearly 10 times its normal size because it had become infiltrated with the cancer.

Medical oncologist Dr. Douglas Thamm recommended six months of chemotherapy, and a veterinary team decided to begin treatments in May in an effort to kill cancerous cells that had migrated to other parts of Rian's body.

"This treatment approach is a first at a zoo," said Dr. Thamm. "The veterinary team working with Rian is modeling treatment on that used with domestic cats, who often suffer from lymphoma as they age."

The zoo and veterinary team are hoping to both improve the quality of Rian's life as well as explore how chemotherapy treatments could help other zoo lions and large cats.

"Any time we're doing things in wild animals there are few established treatment protocols. So we use what works in domestic animals and adapt it to the best of our knowledge," Thamm said in a statement. "Rian's appetite has been a little better, so I hope that means he's feeling better and the drugs are doing their job."

Rian's first chemotherapy treatment was May 27 and he repeats it each week while the veterinarians monitor his comfort levels and attitude throughout the process.

"I hope we can establish a treatment protocol that can be tolerated by big cats and used as a jumping-off point so next time veterinarians see this they have a place to start," Thamm said.

Rian was born in 1998 at the Knoxville Zoo in Tennessee with his brother being his constant companion since birth. Both lions have lived at the Denver Zoo since they were cubs.

According to the zoo, the median life span of zoo lions is 16.8 years, while lions in the wild live about 15 years.