Total and partial cancer prevalence in the adult French population in 2008

Cancer is a high-burden disease because it affects a high number of individuals and has social and economic consequences. This life-threatening or long-duration disease (depending on the site) requires regular evaluation and anticipation of future needs, especially in terms of healthcare and surveillance.

In France, in 2008, the number of persons with cancer or having had a cancer was close to 3 million, which is nearly 5% of the adult population. The diagnosis of more than one-third of the prevalent cases was made 5 years before and that of 40% made 6 to 10 years before. The cancers with the highest partial and total prevalence were prostate, breast, and colon-rectum cancers. The total prevalence was high in the elderly: more than 30% of men and 11% of women aged ≥ 85 have had cancer.

The partial prevalence was estimated using a classical approach; i.e., annual estimations of incidence by age and observed survival probabilities [6]. Here, this approach had the advantage of using nationwide incidence estimations that are considered valid [14] but it assumed that the incidence/mortality ratio of the registry area is representative of the National ratio. The modeling made it also possible to take into account recent changes in trends [15], especially the stabilization of breast cancer incidence and the marked decrease in prostate cancer incidence since 2004–2005. The survival rate stemming from the registries was also assumed to be a proxy for the National rate. Indeed, the survival data stemmed from 12 Départements (13% of the French population), which is deemed to be acceptably representative.

The analyses of the case distributions according to the time since diagnosis and to the change in prevalence over time are complex because they depend on the initial levels and on the changes in incidence and overall survival, the latter depending itself on cancer-related and other death causes.

Although the prognosis influences the distribution of cases according to the time since diagnosis –a good prognosis led to a high number of prevalent cases beyond 5 years– there are nevertheless particular circumstances that may be explained by changes in incidence. For example, larynx cancer in men, that had a much lower survival rate than skin melanoma [9], had also a higher prevalence beyond 5 years. This can be explained by the inverse trends in incidence of these two cancers: a decrease in larynx cancer vs. an increase in melanoma [15] in a context of stable survival [16]; this translated into higher recent incidence cases of melanoma vs. larynx cancer. The age at incidence influences also the distribution of prevalence according to the time since diagnosis. For example, though it has a good prognosis, prostate cancer presents a distribution that is not favorable because patients are diagnosed late in life, which results in a high number of deaths from other causes than cancer.

The changes in prevalence observed between 2002 and 2008 can be explained by changes in the incidence of some cancers. Indeed, the decrease or stabilization in the incidence of alcohol- and tobacco-related tumors (lip-mouth-pharynx, lung, esophagus, and larynx cancers) in men and their increase in women [15] may explain the sex-linked changes in the prevalence of these cancers. The increase in the incidence of skin melanoma, in which the effect of early diagnosis cannot be distinguished from that of an increase in exposure [17], resulted also in an increase in prevalence. The prevalence of thyroid cancer was also on the rise, especially in women. This may be explained by the sharp increase in the incidence of this cancer, the greatest proportion of which results from the diagnosis of good-prognosis tumors in relatively young subjects [18].

The increase in cancer prevalence in France between 2002 and 2008 is largely attributable to changes in the numbers of breast and prostate cancers. The change in the prevalence of these two cancers can be explained by an increase, up to a recent period, in incidence [15] accompanied by improved survivals [9]. As in thyroid cancer, this increase concerns cases with very good prognoses. More generally, in a context of increase in incidence, De Angelis et al. [19] have shown that this increase may be mainly explained by a change in incidence. When incidence plateaus or decreases, changes in survival are the main explanations for changes in prevalence.

Recent changes in cancer incidence and mortality may explain the differences between the present partial prevalence estimations and those provided by Bray et al. [6], these differences being of various magnitudes according to the cancer site. The present study is based on observed incidence data from cancer registries up to 2008 and on cancers diagnosed between 1995 and 2004 with follow-ups until 2007 whereas, in the study of Bray et al., the incidence data did not extend beyond 2003 and the cancers were diagnosed between 1994 and 1999 with follow-ups until 2003. Besides, the longer surveillance period allowed here estimations of partial prevalence at ten years.

The present estimations of the total prevalence used the net risks of incidence and death per cancer, which assumes the same frequency of death from other causes than cancer in diseased and non-diseased populations [11]. A comparison with the Nordic observations [20] and the US estimations [21], based on the “partial prevalence/total prevalence” ratio (partial prevalence at 5 and 10 years; results not shown), found close values of that ratio for most cancer sites. The most marked differences in the “partial prevalence/total prevalence” ratios may have various causes. These ratios are indeed particularly low in France for lip-mouth-pharynx and esophagus cancers which are particularly impacted by the onset of a second cancer [22-24]. Therefore, the risk of cancer may not necessarily match the death from cancer because death may be attributable to the second but not the first cancer. The ways incidence changes in different countries may also explain the differences in “partial prevalence/total prevalence” ratios; e.g.; the incidence of lip-mouth-pharynx and esophagus cancers decreased in France but increased in the Nordic countries and the United States. The reduction of the input in prevalence induces mechanically an increase in the proportion of prevalent cases with old diagnosis. Although the present estimations regarding these cancers may be biased by the potential presence of multiple cancers, it is difficult to quantify this bias.

Total prevalence was not estimated for liver cancer, pancreas cancer, and acute leukemia because the net risk of death from each of these cancers was higher than that of incidence. In fact, a death/incidence ratio > 1 is associated with a poor quality of mortality data and, sometimes, incidence data. Regarding liver cancer, an overestimation of mortality is possible because a number of secondary cancers may have been considered, at death, as primary cancers [8]. An overestimation may also occur when deaths from jaundice are registered as deaths from liver or pancreas cancer. The data relative to death from pancreas cancer might have been unreliable too [25]. Similarly, a lack of specificity regarding death registration with cause “acute leukemia” may lead to an overestimation of death from leukemia. Besides, an overestimation of mortality may combine with underestimation of incidence. This may occur with liver and pancreas cancer because of the lack of clinical arguments to confirm the diagnoses of cancer [26].

There were three cancer sites for which the net risk of incidence was lower than the net risk of death from cancer in persons aged 85 years and more: lung cancer, ovarian cancer, and central nervous system cancer. This discordance between incidence and mortality may receive two explanations. The first is mechanical. Actually, there is a sharp decrease in incidence after 80 years together with a persistent increase in mortality; this leads to net risks of death higher than incidence. This occurs in a context where the prevalence relative to these three cancers is poorly fuelled by new cases because of the bad prognoses of these cancers at such old ages. The second is epidemiological. Indeed, part of the deaths attributed to lung cancer may, in fact, be due to lung metastases derived from past primary non-pulmonary cancers. In elderly people, some lung cancers are not registered as incident cancers because of insufficient clinical investigations whereas these cases may contribute to the number of deaths from lung cancer. Similarly, in elderly women, some ovarian cancers are not fully investigated, which biases their incidence [26]. Finally, the incidence of central nervous system cancers corresponds to invasive types whereas the benign types may also contribute to the number of deaths; this may induce discordance between the net risk of incidence and mortality, a discordance that may aggravate at advanced ages.

The data on incidence and mortality used here for estimating total prevalence cover the whole 1975–2008 period; that is 34 years. This period provides a sufficient hindsight on the majority of cancer sites for which the median age at diagnosis is over 60 years. However, this period may not be sufficient for cancers of good prognosis that occur mainly in young adults such as testis cancer or Hodgkin disease. The total prevalence for these two cancers is thus probably underestimated.

As already mentioned, uncertainties about the total prevalence of some cancer are linked to poor quality of incidence or mortality data registration. Thus, in case of incidence underestimation (liver or pancreas cancer), the partial prevalence may be biased and should be interpreted with caution too because it relies on incidence and survival.

In addition, in principle, an interval estimation of prevalence seems necessary. Unfortunately, such estimation requires the knowledge of observed prevalence and some approximations [27]. The methodology used here for total or partial prevalence may be further refined by future works.

The part of a partial prevalence within the total prevalence gives an indication of the proportion of patients who continue to require care and surveillance (prevalence at 5 years, nearly one third of prevalent cases), the proportion of those who require less care or surveillance, and even those who are cured (prevalence past 10 years, nearly 40% of cases).