WORLDWIDE CANCER INCIDENCE
We begin by looking at cancer patterns worldwide. These are of interest because they show marked variations in the forms of the disease that afflict different populations. These differences indicate the importance of environmental factors that include lifestyle – for example, what we eat and tobacco use – in determining both the type of cancer and the frequency of occurrence. Although there is variation in cancer types, there is a broad trend of rising incidence across the world, for which a major driving force is increasing longevity. In the developed world lung, breast, bowel and prostate cancers head the mortality table. Taking all cancers together, the last 30 years has seen a gradual increase in the five-year survival rate, although there remain significant variations between nations and even within some countries. For the developing world the outlook is more depressing: not only is the annual number of new cases rising but inadequate screening programmes often mean that diagnosis is delayed until tumours have spread to secondary sites in the body and therefore become very difficult to treat. Analysis of different age groups revealed many years ago that the additive effects of about half a dozen discrete events drive cancer development – the first direct evidence that the accumulation of mutations is the underlying cause.
Every year over 12 million people worldwide are diagnosed with cancer. Europe and North America together contribute about 40% of this figure with just over half of all new cases (54%) arising in developing countries. Of the various types of cancer that contribute to these figures, lung cancer heads the list with 1.4 million new cases annually (12.5%) followed by breast (1.2 million, 10.6%) and colorectal (1 million, 9.4%) cancers (Fig. 1).
The names of these three cancers will be familiar to American and British readers because they are also in the top four of their national figures for both incidence and deaths due to cancers. The other member of the Big Four in those countries, and in most of the developed world, is prostate cancer (679,000, 6.3%). However, in the world rankings, the prostate is pushed down to sixth on the list by the stomach and cervical cancer (934,000 and 692,000 cases annually, respectively).
Incidence varies widely around the world is highest in the USA for both men and women. For all forms of cancer, the lowest rates are about five times less than the USA figure (e.g. in the Gambia), although some specific cancers show much greater variation (e.g. there are about 300 new skin cancer cases in some parts of Australia for everyone in Kuwait).
In the UK in 2008 there were 309,527 new cancer cases: for the USA in 2012, the estimate is that there will be 1,638,910 new cases. For women breast cancer is the most common: 48,788 British women were diagnosed in 2009, and one Americans will get it, which means over 226,800 cases in 2012, about the same percentage in the two populations.
Collecting numbers about the distribution of disease is called epidemiology and it has a fascination of its own but, as we shall see, when you get to the scale of the major cancers it can be very informative. Every year over 7 million people die from the disease – that’s 13% of the total of 56 million deaths in the world from all causes each year (Fig.2).
Unsurprisingly, the ten most common cancers worldwide are also among the leading causes of mortality. Bladder and non-Hodgkin’s lymphoma, however, are displaced in the mortality table by pancreatic cancers and leukaemias. In part, this reflects the fact that there are no effective treatments for tumours of the pancreas and it may be noted that this cancer joins the Big Four in the leading causes of USA and UK cancer deaths.