People with AIDS have increased lung cancer risk, and it's not all due to smoking

People with AIDS are much more likely than the general population to develop lung cancer, American researchers report in the January 11^th edition of AIDS. Even though up to 80% of HIV-positive individuals in the US smoke (compared to 30% of the general population), the investigators found that this factor alone could not explain the elevated risk of lung cancer seen in people with AIDS.

Lung cancer, although rare, is the third most common cancer seen in HIV-positive individuals. Previous studies have suggested that it occurs two to seven times more frequently amongst people with HIV than in the general population. It tends to be diagnosed at an advanced stage and has a very poor prognosis.

Investigators have previously speculated that the high prevalence of cigarette smoking amongst HIV-positive individuals (between 60% - 80% in the US) could explain excess rates of lung cancer. But it has also been speculated that other factors may be involved. However, studies designed to determine this have been limited by their small size and have yielded contradictory results. Furthermore, the relationship between immune suppression and lung cancer risk in HIV-positive individuals has not been elucidated.

US researchers used data from the HIV/AIDS Cancer Match Study to determine patterns of lung cancer for AIDS patients. The extent to which immune suppression and smoking contributed to lung cancer risk were also investigated.

Surveillance data from almost 400,000 individuals diagnosed with AIDS between 1980 and 2002 were linked to population-based cancer registry data in six US states. These data were used to identify cases of lung cancer in the five years before and five years following an AIDS diagnosis, with particular emphasis on the four to 27 month period following the diagnosis of AIDS.

Data on patients CD4 cell counts were analysed between 1990 and 2002 to see if there was a relationship between immune suppression and lung cancer risk. The investigators also used an accepted model (that takes into account the duration and intensity of smoking to determine the lung cancer risk of smokers) to see if the elevated prevalence of smoking amongst people with HIV was a sufficient to explain the hypothesised excess rates of cancer.

In the ten year period spanning 60 months before and 60 months after an AIDS diagnosis, a total of 1489 cases of lung cancer were observed. Compared to the general population, people with AIDS had a significantly increased risk of lung cancer (standardised incidence ratio: 3.8). This elevated risk was present in all the periods analysed by the investigators, but particularly during the four to 27 months following diagnosis with AIDS (standardised risk ratio: 2.9).

The investigators then conducted further analysis of lung cancer incidence in this period. They found a lung cancer incidence of 82 cases per 100,000 person years, with elevated incidence for all demographic subgroups compared to the general population. A particularly increased risk was identified for younger people with AIDS (15 - 29 years, standardised risk ratio: 10.4; 30 - 39 years, standardised risk ratio: 6.3).

No significant trend was observed in the incidence of lung cancer across CD4 cell categories. This lack of significance persisted when the investigators conducted separate analysis for the periods before and after effective antiretroviral therapy became available.

Finally, the investigators compared the incidence of lung cancer in people with AIDS to the average yearly incidence predicted by a model that takes account of smoking duration and intensity. Assuming that between 60% and 80% of people with AIDS smoked, they found that the incidence of lung cancer was higher than would be expected particularly for patients aged between 40 - 49 years( observed standard incidence ratio: 5.3).

“These results point to the possibility that additional co-factors other than tobacco may be important in the development of this malignancy in HIV-infected individuals”, write the investigators.

Such factors could include the altered lung function and enhanced lung damage frequently seen in HIV-positive individuals. The investigators note that HIV may lead to increased oxidative stress in the lungs, and that “repeated or chronic lung infections could increase lung cancer risk through chromic inflammation.”

The investigators conclude, “the results of the present study indicate that lung cancer risk is significantly elevated among people with AIDS and that smoking does not readily explain all of the excess lung cancer risk.”