TUESDAY, Nov. 22 (HealthDay News) -- A weakened immune system and unhealthy lifestyle habits such as smoking put HIV-infected patients at increased risk for cancer, according to a new study.

The researchers also said that starting antiretroviral therapy at an earlier stage of HIV infection might reduce cancer risk.

The primary goal of the study, one of the first to compare the risk of cancer in HIV-infected patients, was to determine how much of the increased risk was the result of the disease and how much was due to other risk factors, such as smoking, the researchers said.

The study authors compared the rates of 10 types of cancer that occurred among HIV-infected patients and HIV-free patients from Kaiser Permanente in Northern and Southern California between 1996 and 2008.

Six of the cancers were more common in HIV patients than in HIV-free patients, including Kaposi's sarcoma, non-Hodgkin's lymphoma, Hodgkin's lymphoma, melanoma, anal cancer and liver cancer. Lung and oral-cavity cancers were also more common among HIV patients, but most of the risk of those cancers appeared to be associated with lifestyle habits such as smoking.

Prostate cancer was less common in HIV patients than in HIV-free patients.

Further investigation suggested that a weakened immune system was associated with the increased risk of cancer in HIV patients.

"Taken together, we believe our results support cancer-prevention strategies that combine routine prevention activities, such as smoking cessation, with earlier HIV treatment to help maintain a patient's immune system," lead author Michael Silverberg, a research scientist with the Kaiser Permanente Division of Research in Oakland, said in a Kaiser news release.

The study appears in the current issue of the journal Cancer Epidemiology, Biomarkers and Prevention.

More information

The U.S. National Cancer Institute has more about HIV and cancer risk.