Smoking causes one in five cancers in people with HIV in North America

A fifth of all cancers in people receiving HIV care in North America between 2000 and 2015 was due to smoking, according to US research published this month in advance online by the journal AIDS.

“In the United States, the prevalence of smoking among HIV-infected people is substantially higher than in the general population, and most HIV-infected individuals either currently smoke or have previously smoked,” comment the authors. “Our findings indicated that a substantial fraction of cancer diagnoses among HIV-infected individuals potentially would not have occurred if they had never smoked.”

Thanks to improvements in HIV treatment and care, most people with HIV now have a normal or near-normal life expectancy. As these people age, non-AIDS-related cancers are an important cause of death.

Smoking tobacco can cause a range of cancers including lung cancer and its role in the development of cancers of the oral cavity, larynx, liver, colon, rectum and kidney is well established. Smoking may also contribute to the development of other cancers but more research is needed to understand how it might contribute to the development of these cancers. Smoking was estimated to cause 29% of all cancer deaths in 2010 in the United States.

Research conducted in 2009 suggested that 42% of people with HIV in the United States were current smokers and a further 20% were former smokers, twice the rate seen in the general population. Given this high prevalence of smoking, a team of investigators designed a study to estimate the cancer risk associated with smoking and the proportion of cancers due to smoking.

The study sample included 52,441 people who received HIV care in North America between 2000 and 2015 and for whom records of smoking habits were available. Most were taking antiretrovirals.

The investigators first calculated the relationship between smoking and cancer risk and then examined the proportion of cancers attributable to smoking.

The patients contributed a total of 270,000 person-years of follow-up, a median of 3.8 years per person. A new cancer was diagnosed in 2306 people (4%), an incidence of 8.53 per 1000 person-years.

People with cancer were significantly more likely to have smoked (79% of people who developed cancer vs 73% of people who did not develop cancer had smoked at some time, p < 0.001).

After controlling for other risk factors, ever smoking was associated with a significant increase in the risk of any cancer (aHR = 1.33; 95% CI: 1.18-1.49) and a doubling (aHR = 2.31; 95% CI, 1.80-2.98) in the risk of a smoking-related cancer (such as cancer of the lung, larynx, liver, colon, rectum, oral cavity, kidney, cervix and leukemia). Smoking did not, however, significantly increase the risk of non-smoking-related cancers.

Of the smoking-related cancers, the association was strongest for lung cancer (aHR = 17.80; 95% CI, 5.60-56.63) and was also significant for other smoking-related cancers (aHR = 1.59; 95% CI, 1.22-2.06). Smoking also significantly increased the risk of anal cancer (aHR = 1.57; 95% 1.08-2.28).

The investigators estimated that 19% of all cancers were due to smoking, including 50% of smoking-related cancers and 9% of non-smoking-related cancers. Almost all cases of lung cancer (94%) were attributed to smoking, as were 32% of all cases of anal cancer. (In this study population anal cancer was diagnosed almost as frequently as lung cancer.)

“Compared to never smokers, the risk of a smoking-related cancer diagnosis was more than twice as high among those who ever smoked, and the risk of a lung cancer was nearly 18 times as high,” write the investigators. “Approximately one-fifth of cancer diagnoses in this population were potentially attributed to smoking, including 50% of smoking-related cancers and 94% of lung cancer diagnoses.”

Although the proportion of cancers caused by smoking is lower in people with HIV than in the general population, the authors say that this is explained by the fact that 12% of all cancers diagnosed in people with HIV were Kaposi’s sarcoma, which is very rare in people without HIV and not related to smoking.

The authors call for the prioritisation of smoking prevention and cessation services. They point out that quitting smoking is associated with a reduction in the risk of developing smoking-related cancers.

“Since many HIV-infected smokers are motived to quit, health care providers can assist HIV-infected people with smoking cessation at repeated clinical encounters by offering behavioral and pharmacologic interventions,” suggest the investigators. “High quality resources exist specifically to assist HIV care providers in helping their patients to quit smoking.”