最近研究发现癌症在亚裔美国人中有五种不同类型 (图)

Researchers Find Distinctive Patterns of Cancer in 5 Groups of Asian-Americans

By DENISE GRADY
Published: July 11, 2007

Asian-Americans, both those born here and new immigrants, have distinctive patterns of cancer incidence that doctors should consider when treating them, researchers have found.

A report appearing today in the journal CA is “one of the most comprehensive summaries of cancer among Asian-Americans,” according to the American Cancer Society, which publishes the journal.

The report is based on information on cancer cases collected by California from 2000 to 2002, and focuses on five ethnic groups: Chinese, Filipino, Vietnamese, Korean and Japanese. The state has a large Asian population, 3.7 million, and carefully sorts its cancer data by ethnic group.

When all cancers are combined, Asian-Americans actually have lower rates than other groups in the United States. But cancer is still a major cause of death for Asians, killing more of them than heart disease. Different groups appear prone to different types of cancer.

Groups that have been in this country the longest are likely to develop cancers that are most common here, like breast and colorectal cancer, though their rates are still significantly below those of non-Hispanic whites. The risk of those cancers may be increased by obesity, inactivity, high alcohol intake and diets rich in fat and low in fruits and vegetables, and the rates in Asians seem to rise gradually as they adopt more and more American habits.

Recent immigrants, by contrast, tend to suffer from the same types of cancer that are predominant in their native countries, like stomach and liver cancer. In developing countries, those cancers are often caused by chronic infections with certain bacteria and viruses that are routinely treated or prevented in the United States.

“I was surprised to see the diversity in cancer among the ethnic groups,” said Melissa McCracken, an epidemiologist with the cancer society and the first author of the report. “The group is not homogeneous. Clinicians need to be aware of that and to really extend their scope of attention to cancer due to infectious agents, not just typical chronic conditions.”

Among the more striking findings are that Vietnamese men have incidence and death rates from liver cancer that are seven times the rate in non-Hispanic white men, and Korean men and women are five to seven times as likely as whites to develop stomach cancer. Other Asians are also prone to these cancers, but their rates are generally not as high.

The hepatitis B virus is endemic in Asia, Ms. McCracken said, and chronic infection is a major cause of liver cancer there and in recent immigrants. A study last year found high rates of infection in East Asian immigrants in New York.

Worldwide, the cancer society report says, 80 percent of liver cancer cases occur in developing countries, with 55 percent of the total in China. Pregnant women can transmit hepatitis B to the fetus, so the disease can persist in the population, along with the cancers that appear later.

Doctors should be aware of the problem, Ms. McCracken said, because there are treatments that can lower the risk of transmission from mother to baby. In others, antiviral drugs can fight the infection and lower the odds of cancer. Vaccination prevents the disease and has become routine for infants in the United States, but doctors may have to make a special effort to inform immigrants about it.

The high rates of stomach cancer are thought to have two causes. One is chronic infection with Helicobacter pylori bacteria, which is common in developing countries, and treatable with antibiotics. Stomach cancer rates in this country plummeted as refrigeration came into use. In Koreans, diet is also blamed, specifically foods that are preserved with nitrates and nitrites.

Stomach cancer is also a problem in Japan, and screening programs there have lowered death rates. But screening is not performed here, so doctors should be on the lookout for symptoms in Asian patients, Ms. McCracken said.

Genetic differences among ethnic groups may play some role in determining susceptibility to various cancers, said Dr. Regina M. Santella, a professor of environmental health science at the Mailman School of Public Health at Columbia University. Dr. Santella was not involved in the study.

For example, she said, not all smokers get lung cancer, but research has shown that smokers who lack a certain gene—about 50 percent of the population lacks it—have a slightly increased risk of developing the disease.

“But there may also be exposure differences,” Dr. Santella said.

Compared with other Asians, Chinese women have high incidence and death rates from lung cancer, the report notes. The reason is not known, since their smoking rates are low. But they do have high exposures to secondhand smoke at home and at work, and to cooking-oil vapors from high-temperature frying.

Vietnamese women pose the same puzzle, with even higher lung cancer rates and lower smoking rates.

Among Filipinos, men have higher rates of prostate cancer than other Asians, and women have the highest death rate from breast cancer. Risk factors for prostate cancer are not well understood, but breast cancer is linked to obesity, and 33.5 percent of Filipino women are overweight, more than in other Asian groups. Japanese Americans have high rates of colorectal, stomach, prostate and breast cancer compared with other groups, the researchers found. Obesity may play a role, they said, noting that 52.5 percent of Japanese men and 28.3 percent of women are overweight, and many say they are relatively inactive. Colorectal cancer rates have risen in Japan and are higher than in other Asian countries, maybe because eating habits have become more like those in the West.

Another concern raised in the report is that Vietnamese and Korean women have higher rates of cervical cancer than other groups, and lower rates of Pap test screening for it.

Several of the groups also had low rates of screening for breast and colorectal cancer. Language barriers, lack of insurance and cultural attitudes about the tests may all play a role, especially in newer immigrants, the report said, and it urged that doctors make an extra effort to convince those patients of the value of prevention and early detection.