Originally Published: 02 JAN 23 20:00 ET

Updated: 02 JAN 23 21:31 ET

By Carma Hassan, CNN

(CNN) -- Tennis superstar Martina Navratilova is starting treatment for stage I throat cancer and breast cancer this month, according to her agent.

American Cancer Society CEO Karen Knudsen, a tennis fan, said she was sorry to hear the news but emphasized, "there's some silver lining here in terms of the stage of disease that she's dealing with: both her oral cancer and the breast cancer."

"The frequency of having what we call multiple primary -- so cancers that are truly unrelated from each other in terms of their site of origin -- is more frequent than you would think," said Knudsen, who is not involved in Navratilova's care.

Meta-analyses looking at thousands of people in several countries have found that about 2% to 17% of people with cancer will have multiple primaries, or multiple kinds of cancers, she said.

Dr. Otis Brawley, an oncology professor at Johns Hopkins University, agreed that it's not especially rare.

"It is not uncommon for two cancers to be diagnosed in people at the same time. Some cancers grow very, very slowly. And sometimes, people will go to the doctor because they have symptoms for one cancer, and the doctor, being thorough, looks for other things and finds something else," said Brawley, who also is not involved with Navratilova's treatment plan.

Navratilova's agent, Mary Greenham, said the tennis legend discovered an enlarged lymph node in her neck during the WTA Finals in Fort Worth last year. After a biopsy, Navratilova was diagnosed with stage I throat cancer.

While she was undergoing throat tests, a suspicious site was found in Navratilova's breast, which was also diagnosed as cancer.

Greenham said both cancers were in the early stages, with great outcomes expected.

'Early detection is key'

Navratilova had breast cancer in 2010. About 20% of cancers happen in people who were previously diagnosed with cancer, according to the National Cancer Institute.

Knudsen praised Navratilova for listening to her body and highlighted the importance of cancer screening.

"This is something that she identified based on knowledge of her body and something feeling amiss and taking action. Being an advocate for herself there led to identification at stage I, which is so important," she said. "Early detection is key to improve outcomes."

Getting recommended cancer screenings is a good practice for anyone, Knudsen said. She advises talking to your doctor about the right screening plan for you.

"It's very important to start asking about what that screening plan looks like, because the screening plan is far beyond your age. It's your age, your risk factors for cancer, your genetic history if you know it but also your family history. So it is the combination of those factors that allow the development of the right screen plan for every individual," she said.

A localized stage I breast cancer diagnosis has a five-year survival rate of 99%, according to the American Cancer Society.

And although it's not clear what type of throat cancer the tennis star has, oral and oropharyngeal cancers diagnosed at an early stage have about an 85% five-year survival rate altogether -- another reason why it's important to ensure cancers are caught early, Knudsen said.

Throat cancer can mean a number of things because there are different areas that can become cancerous, according to Brawley.

"Head and neck cancer caused by HPV is an easier cancer to treat compared to a head and neck cancer that's caused by alcohol and smoking," he said.

There can also be differences in how these head and neck cancers are treated, according to Knudsen.

A vaccine can help protect against HPV cancers. The recommendation from the US Centers for Disease Control and Prevention is for children to get the vaccine around 11 or 12 years old.

"We do have a highly successful cancer vaccine against HPV-driven cancers. Now, like myself, Martina Navratilova was not of the age that could have benefited from this vaccine. But certainly, the generation behind us has an opportunity to prevent the vast majority of cervical cancers and up to six different head and neck cancers through HPV vaccination," Knudsen said.

The idea behind giving the vaccine to young people "is to vaccinate at a time when the immune system has the maximum capability to mount a response or resistance to HPV but also prior to the time that someone is likely to have been exposed to HPV," she said, adding that the recommendation is for people through the age of 26.

Adults between 27 and 45 should talk to their doctor about getting vaccinated.

"If someone is 45 or younger and has not yet been vaccinated, certainly [they should have] a conversation with their physician, and everyone 26 years and younger should consider vaccination. And I can't emphasize this enough: That's for men and women," Knudsen said.

Treating two cancers at the same time is complicated

Treating two stage I cancers is different from treating one type of cancer that has spread throughout the body, Brawley said.

Each cancer needs a separate medical team.

"You get your breast surgeons and your breast radiation oncologist and your breast medical oncologist, and they treat the breast cancer, and then you get the head-neck oncologists, including the radiation oncologist and the head-neck medical oncologist, and they treat the head and neck cancer," he said. "Now, those two groups of people are going to have to talk to each other, and they're going to have to work to coordinate with each other. But you don't treat the patient differently other than that coordination."

The new breast cancer diagnosis doesn't necessarily mean Navratilova had a recurrence, he said.

Typically, Brawley said, a stage I breast cancer diagnosis means "this is a new cancer only related to her previous breast cancer in that it occurred in the same person."

Most localized stage I head and neck cancers are treated with a combination of chemotherapy and radiation, although surgery can be used, Brawley said.

Although breast and head and neck cancers can be deadly if left untreated, Brawley says, it's good news that both of Navratilova's cancers are in early stages.

"The fact that they're both stage I gives her good prognosis from each one," he said.

Navratilova has highlighted the importance of preventive checkups to combat specific diseases such as breast cancer.

"I think that this is a really important lesson in the importance of cancer screening and early detection and taking action upon yourself," Knudsen said. "And also recognizing that even someone like an athlete is susceptible to cancer. It's so important to know your body and develop the screening plan that's most correct for you."