The primate's keepers noticed she had a decreased appetite and was growing lethargic, prompting a medical exam and ultrasound last week.
Medical staff found a mass in her abdomen and enlarged lymph nodes, leading to a diagnosis of lymphosarcoma, a cancer of the lymphatic system, The Arizona Republic of Phoenix reported Tuesday.
Gary West, executive vice president in charge of animal health at the zoo, said Duchess' cancer is "fairly aggressive."
"We don't think it's a long-term course," he said of her treatment.
West said the zoo could give Duchess chemotherapy treatment, but she would have to be a willing participant in her care.
"We obviously can't force her to take medicine," West said.
Duchess has been at the Phoenix Zoo since 1962, making her the oldest living Bornean orangutan in North America, the newspaper reported.
The average lifespan of an orangutan living in captivity is 40-plus years, the newspaper reported.
"I take solace in the fact she's 52 and not 25," said Denise Wagner a senior zookeeper said.
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