(CNN) Anal cancer cases and deaths are rising dramatically in the United States, especially among older people and young black men, a new study says.

Researchers examined trends in anal cancer cases over about 15 years, and identified about 69,000 cases of anal cancer and more than 12,000 deaths during this time.

"Our findings of the dramatic rise in incidence among black millennials and white women, rising rates of distant-stage disease, and increases in anal cancer mortality rates are very concerning," the study's lead author, Ashish A. Deshmukh, an assistant professor at UTHealth School of Public Health, said in a statement. "Given the historical perception that anal cancer is rare, it is often neglected."

Distant stage disease is when the cancer has spread to other parts of the body.

From 2001 to 2015, cases of the most common type of anal cancer increased by 2.7% per year, while anal cancer death rates increased by 3.1% per year from 2001 to 2016.

The study, published in the Journal of the National Cancer Institute, "gives numbers to a trend that seems to be happening over the last decade," said Dr. Virginia Shaffer, a colorectal surgeon and associate professor in Emory University's Winship Cancer Institute. "In that sense it gives us numbers to what we were already expecting." Shaffer was not involved in the study.

Cancer linked to HPV

Anal cancer occurs where the digestive tract ends. It is different from colon or rectal cancer and most similar to cervical cancer.

The most common subtype of anal cancer is squamous cell carcinoma, caused by human papillomavirus, known as HPV.

Over 90% of cases of anal cancers are associated with HPV, according to the US Centers for Disease Control and Prevention.

Screening for anal cancer has been implemented for some high-risk groups , but the study authors argue their findings suggest "broader screening efforts should be considered." But they also believe the increase in diagnoses isn't likely to be because of an increase in screening practices.

Since the 1950s, there have been substantial changes in risk factors for anal cancer, including shifts in sexual behaviors and an increased number of sexual partners , according to the study, both of which increase the likelihood of contracting HPV.

The emergence of the HIV epidemic , especially among men who have sex with men, may have also influenced anal cancer trends because HIV is a risk factor.

There are other risk factors , too, such as having had cervical or vulvar cancer, having received an organ transplant or being a current smoker.

Who's impacted by anal cancer?

The study found anal cancer cases have increased significantly in people aged 50 and older.

It might be because HPV vaccine guidelines are "very narrow," Shaffer said, limiting protection for older adults. When the first HPV vaccine was introduced in 2006, it was approved for people ages 9 to 26, "so these older adults were way past that cutoff when the vaccine came out," Shaffer said. "That's a large number of people who missed out on getting the vaccine."

Anal cancer rates are also rising among young black men.

HIV also disproportionately affects young black men , the study authors said, and having HIV is a risk factor for anal cancer.

The study also found the number of advanced-stage cases is rising. This could partly be because treatment for HIV has improved, Shaffer said, which means patients are living longer with compromised immune systems, and cancer may have progressed further by the time it's diagnosed.

Stopping stigma

There is still stigma around anal cancer.

"Desperate Housewives" star Marcia Cross opened up about her anal cancer diagnosis earlier this year to help destigmatize the disease, she said.

"I know that there are people who are ashamed," Cross told "CBS This Morning" in June. "You have cancer. Should you then also feel like ashamed like you did something bad because it took up residence in your anus?"

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Anal cancer has become "pretty taboo," Shaffer said, "I think because of some of the risk factors that have historically been known to be associated with it.

"If people are having symptoms they should see a doctor because I think a lot of people think, 'Oh, well it's just hemorrhoids,' and don't get things checked and that could potentially also mean that you don't get diagnosed until much, much later."

Anal cancer is preventable through HPV vaccination. The CDC recommends two doses of the vaccine one year apart for children ages 11 to 12 in the US. Young adults up to age 26 can be vaccinated as well; three doses are recommended for people 15 and older.

The vaccine is most beneficial when administered at younger ages, before a person is exposed to HPV. But the CDC says adults ages 27 through 45 who have not been adequately vaccinated can make make a decision with their doctors about HPV vaccination. Adults older than 45 who had not been vaccinated are not advised to do so, since HPV vaccines are not licensed for use in that age group.

To strengthen prevention efforts going forward, Shaffer said all people who qualify to be vaccinated should do so, and that the current vaccine guidelines should be studied to determine whether they can be expanded to other patients.