Lung cancer is the most common tumour type, with an estimated 1.5 million cases diagnosed worldwide every year. In most countries it has historically been more common in men, but its incidence in women is rising, following an increase in the proportion of women smokers. There is therefore considerable interest in studying differences in incidence patterns of this tumour between the sexes. Several studies have shown that women are more likely than men to be diagnosed with adenocarcinoma of the lung, and that they tend to be diagnosed at a younger age and an earlier stage of the disease. However, most studies have been connected to clinical trials, and thus affected by the exclusion criteria set by those trials.

Camilla Sagerup from the University Hospital Oslo, Norway, with co-workers there and at the Cancer Registry of Norway, has now undertaken a large study of sex-specific trends in lung cancer in an unselected population. All cancer cases in Norway have been reported to the Cancer Registry of Norway since 1952; Sagerup and her colleagues collected records from lung cancer patients deposited in that registry between 1988 and 2007. The starting date was chosen as the time when CT scanning became widespread throughout the country, indicating that accurate diagnosis and staging would have been consistently available across Norway after that point. A total of 40 118 cases were available, but survival data could not be used in 1129 cases that were either diagnosed post-mortem or represented in the registry by death certificates only. Incidence and survival data were stratified by year of diagnosis in 5-year periods and by sex, age at diagnosis, stage (in terms of localised, regional or metastatic disease) and histology. Relative risks of death were obtained for the different groups using a Cox proportional hazards model.

The researchers found that the incidence of lung cancer in Norway rose during the period 1988-2007 for both sexes, with a much more dramatic rise in women; the age-adjusted incidence rate for women rose during that period by approximately 4.9% in women compared to only 1.4% in men. Relative survival, however, increased slowly but steadily throughout the period and was greater in women throughout the period in all age groups and for almost all recorded histologies. The one-year relative survival was 28.2% in males and 30.5% in females for all cases diagnosed between 1988 and 1992, and these figures rose to 35.3% in males and 40.9% in females for those diagnosed between 2002 and 2007. The corresponding rates for five-year survival were 7.9% in males and 10.1% in females for the earliest cohort and 11.6% in males and 15.4% in females for the latest.

The distribution of histological types was also found to differ between males and females and to change with time. Adenocarcinoma was, as expected, found to be the most common diagnosis in females throughout the study period, and its proportion rose steadily between 1988 and 2007. In men, however, squamous cell carcinoma was the most common diagnosis in 1988, but adenocarcinoma had come to predominate – although to a lesser extent than in women – by 2007. Squamous cell carcinoma was the only histology in which the survival advantage for women was not statistically significant. Women were also found to have been diagnosed on average at younger ages and with less advanced disease than men; the increase in survival noted over time for the whole cohort was particularly large in women diagnosed with adenocarcinoma.

Sagerup and her colleagues discussed their results in terms of the growing worldwide epidemic of lung cancer. Patterns in the incidence of this cancer clearly follow patterns in the prevalence of smoking, but with a time lag of decades; the rise in the proportion of women sufferers follows a rise in women smokers during the mid-twentieth century. Lung cancer incidence in males is already falling in many developed countries following a decline in smoking rates among men, and it is likely that rates in women will similarly start to fall. Further population studies monitoring these trends will be interesting, and the effect of sex on likely survival times should be considered in future clinical trial designs.

Article: Sagerup, C.M., Småstuen, M., Johannesen,T.B., Helland, Å. and Brustugun, O.T. (2011). Sex-specific trends in lung cancer incidence and survival: a population study of 40 118 cases. Thorax 66(4), 301-7. doi: 10.1136/thx.2010.151621