Lung cancer statistics

Abstract

Objective and design To relate UK national trends since 1950 in smoking, in smoking cessation, and in lung cancer to the contrasting results from two large case-control studies centred around 1950 and 1990.

Setting United Kingdom.

Participants Hospital patients under 75 years of age with and without lung cancer in 1950 and 1990, plus, in 1990, a matched sample of the local population: 1465 case-control pairs in the 1950 study, and 982 cases plus 3185 controls in the 1990 study.

Main outcome measures Smoking prevalence and lung cancer.

Results For men in early middle age in the United Kingdom the prevalence of smoking halved between 1950 and 1990 but the death rate from lung cancer at ages 35-54 fell even more rapidly, indicating some reduction in the risk among continuing smokers. In contrast, women and older men who were still current smokers in 1990 were more likely than those in 1950 to have been persistent cigarette smokers throughout adult life and so had higher lung cancer rates than current smokers in 1950. The cumulative risk of death from lung cancer by age 75 (in the absence of other causes of death) rose from 6% at 1950 rates to 16% at 1990 rates in male cigarette smokers, and from 1% to 10% in female cigarette smokers. Among both men and women in 1990, however, the former smokers had only a fraction of the lung cancer rate of continuing smokers, and this fraction fell steeply with time since stopping. By 1990 cessation had almost halved the number of lung cancers that would have been expected if the former smokers had continued. For men who stopped at ages 60, 50, 40, and 30 the cumulative risks of lung cancer by age 75 were 10%, 6%, 3%, and 2%.

Conclusions People who stop smoking, even well into middle age, avoid most of their subsequent risk of lung cancer, and stopping before middle age avoids more than 90% of the risk attributable to tobacco. Mortality in the near future and throughout the first half of the 21st century could be substantially reduced by current smokers giving up the habit. In contrast, the extent to which young people henceforth become persistent smokers will affect mortality rates chiefly in the middle or second half of the 21st century.

Introduction

Medical evidence of the harm done by smoking has been accumulating for 200 years, at first in relation to cancers of the lip and mouth, and then in relation to vascular disease and lung cancer.[1] The evidence was generally ignored until five case-control studies relating smoking, particularly of cigarettes, to the development of lung cancer were published in 1950, one in the United Kingdom[2] and four in the United States.[3-6] Cigarette smoking had become common in the United Kingdom, firstly among men and then among women, during the first half of the 20th century. By 1950 lung cancer rates among men in the United Kingdom had already been rising steeply for many years, but the relevance of smoking was largely unsuspected.[2 7] At that time about 80% of men and 40% of women smoked (fig 1 and BMJ's website, table A). But few of the older smokers had smoked substantial numbers of cigarettes throughout their adult life, so even male lung cancer rates were still far from their maximum (except in younger men), and rates in women were much lower. Over the next few decades, a substantial decrease occurred in the United Kingdom in the prevalence of smoking (fig 1), in cigarette tar yields, and, eventually, in

lung cancer rates (fig 2), and by 1990 male lung cancer mortality, although still high, was decreasing rapidly.[8-12]

[Figures 1-2 ILLUSTRATION OMITTED]

In this paper we relate the UK national trends in smoking, in smoking cessation, and in lung cancer to the contrasting results from two large case-control studies of smoking and lung cancer in the United Kingdom that were conducted 40 years apart, centred on the years 1950[2 7] and 1990.[8] The 1950 study was concerned with identifying the main causes of the rise in lung cancer and showed the predominant role of tobacco. The 1990 study was concerned not just with reconfirming the importance of tobacco but also with assessing the lesser effects of indoor air pollution of some houses by radon.[8] Because there has been widespread cessation of smoking (indeed, above age 50 there are now twice as many former cigarette as current cigarette smokers in the United Kingdom[10]), the second study was able to assess the long term effects of giving up the habit at various ages.

Participants and methods

The 1950 study was conducted in London and four other large towns during 1948-52, and its methods have been described elsewhere.[2 7] It involved interviewing, as potential "cases," patients younger than 75 years of age in hospital for suspected lung cancer and, as "controls," age matched patients in hospital with various other diseases (some of which would, in retrospect, have been conditions associated with smoking). After patients in whom the initial diagnosis of lung cancer was eventually refuted were excluded from the cases, 1465 cases and 1465 controls remained. A preliminary report on 709 case-control pairs was published in 1950, and the full results were published two years later.[2 7]

The 1990 study was conducted during 1988-93 in a part of southwest England that had not been included in the 1950 study. Potential cases were patients younger than 75 who were referred with suspected lung cancer to the five hospitals in Devon and Cornwall that investigated lung cancer. For each case a population control was obtained, selected randomly either from lists of the local family health services authority or from electoral rolls, and a hospital control was selected from patients whose current admission was for a disease not thought to be related to smoking. Controls were matched for age, sex, and broad area of residence to the patients with suspected lung cancer. Cases and controls were eligible for the 1990 study only if they were current residents of Devon or Cornwall, had lived in one of these two counties for at least 20 years, and could be interviewed in person by research assistants about smoking habits and other relevant characteristics. The final diagnosis of cases was sought; those who had a smoking related disease other than lung cancer were excluded; and the few who had a disease not known to be associated with smoking were transferred to the hospital control group. Similarly, in 1990 (although not in 1950) the final diagnosis of all the hospital controls was sought, and those whose main reason for being in hospital was a disease known to be related to smoking were excluded from the study.

The distributions of the smoking habits of the population controls and hospital controls in 1990 were closely similar, and the results are presented here with these two control groups combined. Further details of the study design and methods of data collection and analysis have been given elsewhere.[8] Information was obtained in the 1990 study about the smoking habits of 667 men and 315 women with a confirmed diagnosis of lung cancer and of 2108 male and 1077 female controls.

Statistical methods

Relative and cumulative risks

Relative risks for men and women comparing particular categories of smoker with lifelong non-smokers in the 1990 study (and the ratios of the risks in former smokers to those in continuing smokers) were calculated by logistic regression with adjustment for age.[13] Further adjustment for social class, radon exposure, and county of residence made no material difference. Relative risks for men and women in the 1950 study were taken as the odds ratios indicated by the published frequency distributions of the age matched cases and controls.[7] Relative risks from the studies were then combined with national lung cancer mortality rates from 1950 and 1990 respectively to estimate the absolute hazards in various categories of smoker, former smoker, and non-smoker. Because they are linked to known national rates, these absolute risks are statistically stable among smokers (and among former smokers), even though the risks relative to lifelong non-smokers would not be stable as so few non-smokers develop the disease. Such calculations of absolute risk allow comparisons between different categories of smoker not only within this study but also between this and other studies that report absolute risks.