In the first part of the study we estimate the incidence of lung cancer in 1) HIV patients compared to population controls matched on age and gender and 2) in the parents of the HIV patients compared to the parents of the population controls. In the second part of the study we estimate the mortality of individuals diagnosed with lung cancer in 1) HIV patients compared to population controls matched on age and gender and 2) the parents of the HIV patients compared to the parents of the population controls.
Setting
Denmark had a population of 5.5 million as of 31 December 2008, with an estimated HIV prevalence of approximately 0.09% in the adult population. Patients with HIV infection are treated in the country's eight specialized medical centres, where they are seen on an outpatient basis at intended intervals of 12 weeks. As HAART is available only at these eight centres almost no HIV patients are treated elsewhere. Antiretroviral treatment is provided free of charge to all HIV-infected residents of Denmark.
Data sources
We used the unique 10-digit civil registration number assigned to all individuals in Denmark at birth or upon immigration to link the data sources described below [13].
The Danish HIV Cohort study (DHCS) is a population-based prospective nationwide cohort study of all HIV patients 16 years or older at diagnosis and who are treated at Danish HIV centres after 1 January 1995 [14]. The HIV patients are consecutively enrolled, and multiple registrations are avoided through the use of the unique civil registration number. December 31, 2009 the cohort included 5481 Danish residents. Data are updated yearly and includes demographics, smoking status, date of HIV infection, AIDS defining events, date and cause of death and antiretroviral treatment. CD4+ cell counts and HIV-RNA measurements are extracted electronically from laboratory data files. Data on smoking are not adequate in the database. Patients who were registered at least once as consuming tobacco in any quantity were considered smokers or former smokers. We calculated the distribution of smokers or former smokers among Danish HIV infected.
The Danish Civil Registration System (DCRS) was established in 1968 and stores information of vital status, residency as well as immigration and emigration on all Danish residents [13]. Since 1 January 1969 the registry also included identification of parents still alive at this date.
The Danish Cancer Register is a population-based register and contains information on incident cancers diagnosed in Danish Citizens since 1943. Details about registration can be found elsewhere [15]. In 1969 - 2003 cancers were coded according to International Classification of Disease version 7 (ICD-7) and International Classification of Diseases for Oncology 1 (ICD-O-1) with supplement from ICD-O-2 in 1990 - 2003. Since 2004 cancers have been coded according to ICD-O-3 and ICD-10 and data from the period 1978 - 2003 have been converted to ICD-O-3 and ICD-10. We do not have data on cancer staging or treatment from the Danish Cancer Register.
Data on smoking are not available in the Danish national registries.
Study populations
HIV and population control Study populations
In the first part of the study, we included all HIV patients from the Danish HIV Cohort Study without a diagnosis of cancer prior to index date, see Figure 1. The index date was defined as 1 January 1995, the date of the HIV diagnosis or date of immigration, which ever came last. For each of the HIV patients we identified 10 age- and gender matched population control subjects from the DCRS who were alive and living in Denmark at index date of the corresponding HIV patient (refereed to also as the index date of the respective population control) and not diagnosed with a cancer prior to index date.
Parent study populations
In the parent study population we included all mothers and fathers of the HIV patients and population controls included who 1) gave birth to a index patient or population control after 1 January 1952, 2) were alive and living in Denmark after 1 January 1969 and 3) who were not diagnosed with cancer prior to the parent index date, see Figure 1. The parent index date was defined as 1 January 1969, date of birth of the included HIV patient/population control or date of registration in DCRS, whichever came last. In case a parent was the father or mother of both an HIV patient and a population control, they were included in both parent populations.
Outcome
In the first study outcome was time to first diagnosis of lung cancer. We identified all primary lung cancers in the populations using cancer diagnoses as defined by NORDCAN [16]; see cancer definitions in Additional file 1. Lung cancer was defined according to NORDCAN: 1620, 1621, 1628 in ICD-7 and C34 in ICD-10. In order to investigate if there was any difference in the histopathological distribution of lung cancer perhaps due to immunodeficiency we grouped the lung cancers into five groups following the 1999 WHO classification [17] categorized by the following ICD-O-3 codes; 1) squamous cell carcinoma (M80503 - 80763, M80833), 2) small cell carcinoma (M80403 - 80453), 3) adenocarcinoma (M82303 -823133, M82503 - 82603, M84803 - 84903, M85503 - 85603, M85703 - 85723, M81403, M82113, M83233) 4) large cell carcinoma (M80123 -80313, M83103) and 5) other carcinomas (M85603, M82403, M82493, M84303, M82003, M85623, M80463) and unclassified malignant cells (M80013, M80103, M82463, M99903, M99993).
In the second part of the study outcome was time to death.
Statistics
Differences in characteristics between groups were evaluated by the χ2 test and Kruskal-Wallis Test when appropriate.
We estimated the probability of lung cancer and the relative risk of lung cancer in all study populations. Observation time was calculated from the respective index date to date of diagnosis of lung cancer or other cancers, death, 1 January 2009, emigration or lost to follow-up, whichever came first. We used cumulative incidence function to illustrate time to first lung cancer, recognizing death and diagnoses of other cancers as a competing risk [18]. Incidence rate ratios (IRR) and 95% confidence intervals for lung cancer as estimates of relative risks were calculated using Cox proportional-hazards regression. In the comparison of HIV patients and their matched population controls we used Cox regression analyses stratified according to the initial match criteria (age and gender).
In order to identify risk factors for lung cancer in the HIV population, we calculated incidence rate ratios (IRRs) stratified by gender, race (native vs. immigrant), age at index date (≤ 50 years vs. > 50 years), diagnosis of HIV before 1 January 1995, smoking (smoker or former smoker in any quantity vs. never smoked), route of infection (men who have sex with men (MSM) vs. heterosexually infected men vs. heterosexually infected women vs. injection drug user (IDU)), immunodeficiency at index date (AIDS defining event or CD4 cell count < 350 cells/μL vs. no AIDS defining event and CD4 cell count ≥ 350 cells/μL). All results were adjusted for age (continuous variable) and gender. Baseline CD4 cell count was defined as the measurement closest to index date +/- one year. Also, we included first date with a CD4 count below 350/μL or date of first AIDS defining event as a time dependent variable.
IRRs for parent populations were adjusted for age at parent index date (continuous variable) as well as year of birth of the parent divided into the following decades: - 1920, 1920-1930, 1930-1940, 1940-1950, 1950 - later. A robustness analysis was performed where we calculated IRRs for lung cancer including only parents of HIV patients and population controls born in Denmark.
In a sub analysis, we calculated IRR for the following four types of lung cancers: Squamous cell carcinoma, small cell carcinoma, adenocarcinoma and large cell carcinoma.
We computed person-years at risk of dying from lung cancer from date of the diagnosis to date of death, 1 January 2009, emigration or lost to follow up whichever came first. We used Kaplan-Meier analysis to construct survival curves and Cox regression analyses were used to estimate mortality rate ratios (MRR). In the population consisting of HIV patients and population controls all results were adjusted for age at lung cancer diagnosis (continuous variable) and gender. In the parent population all results were adjusted for age at lung cancer diagnosis (continuous variable) as well as year of birth of the parent divided into the following decades: - 1920, 1920-1930, 1930-1940, 1940-1950, 1950 - later. The study was approved by the Danish Data Protection Agency. SPSS statistical software, Version 15.0 (Norusis; SPSS Inc., Chicago, Illinois, USA) and R software, version 2.8.1, was used for data analysis.
