A time trend analysis of papillary and follicular cancers as a function of tumour size: A study of data from six cancer registries in France (1983–2000)

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Abstract

The incidence of thyroid cancers, and in particular the papillary forms, has been increasing sharply for many years in Western countries. However, the factors explaining this increase have not been clearly established. Some studies mention the effects of radioactive fallout, particularly after the accident in Chernobyl. Another probable cause is related to progress in medical practice, and particularly in diagnosis. In this article, we describe time trends in the incidence of papillary and follicular cancers, taking into account the size of the tumour at the time of diagnosis. The analysis was carried out on cases from six French cancer registries for the period 1983–2000. Anatomopathological reports concerning 3381 cancer cases were systematically recoded and centralised, following ICDO-3 rules. Over the whole period, the annual percent change of the incidence of papillary cancers was +8.13% and +8.98%, respectively in men and in women. For micropapillary carcinomas (⩽10 mm), this increase was respectively +12.05% and +12.85%. There is no significant effect of period apart from micropapillary carcinomas in women. However, a birth cohort effect exists for some groups. This effect corresponds to an acceleration in the risk for people born after the 1930s. For the most recent period (1998–2000), half the cases of papillary cancer were micropapillary carcinomas, and for one third of these, the tumour was ⩽5 mm. Our description of a time trend of incidence as a function of tumour size supports the hypothesis of the role of medical practice in a context of high prevalence. Obviously, these findings do not exclude the possible role of other factors.

Introduction

Many epidemiological studies have described an increase in the incidence of thyroid cancers. This increase is observed both for earlier periods1, 2, 3, 4, 5 and for more recent periods.6, 7, 8, 9, 10, 11, 12, 13 This increase does not involve all histological types; the incidence of papillary cancer only is increasing, while the incidence of other histological types either remains stable or decreases. Since exposure to ionising radiation has been shown to be a risk factor for papillary cancers, a doubt still exists about the possible role of radioactive fallout related to the accident in the Chernobyl nuclear plant. However, the fallout was relatively low in Western Europe.14 Advances in diagnostic practices can also lead to a mechanical increase in the number of cancers detected.11, 15, 16 Also, a further element to take into consideration in analysing the time trends in the incidence of papillary cancers is the effect of ‘tumour classification’. The application of the 1988 WHO classification of differentiated thyroid carcinomas probably had the effect of transferring some diagnoses of follicular carcinomas or atypical adenomas into the category of follicular variants of papillary carcinomas.11, 16 The general description of the changes in thyroid cancer incidence in France has been summarised in a descriptive analysis.9

Our aim in this article is to study changes in the incidence of papillary and follicular thyroid cancers, and also the effect of tumour size. If the increase was found to be mainly in small papillary cancers, this would be a further argument to indicate the major role of advances in both diagnostic and therapeutic practices in the increase in incidence of thyroid cancers.

Section snippets

Cancer cases

The cases of cancer diagnosed during the 1983–2000 period in seven French administrative areas (départements) (Ardennes, Bas-Rhin, Calvados, Isère, Marne, Somme and Tarn) are based on a total general population of 4.6 million inhabitants (2000 census). These data were collected by six cancer registries, since the Ardennes and the Marne were covered by a single specialised registry for thyroid tumour. The observations from Calvados cover the period 1983–1999.

So as to describe the time changes in

Cancer cases

For the 1983–2000 period, 3590 cases of differentiated thyroid cancers were recorded. There were 2901 papillary cancers (81%), 480 follicular cancers (13%) and 209 medullary cancers (6%). Only the papillary and follicular cancers were studied, i.e. 3381 cases. The anatomopathological report of the operative specimen was found for 3194 cases (95% of cases). For the remaining cases, 35 patients were not operated on (1%) and the report was not available for 152 cases (4.5%).

General characteristics of the cases studied

The median age of cases

Confirming the hypothesis of the increase of small tumours

Our study’s description of the change in incidence trends confirms the hypothesis about the major increase in small papillary tumours, which was the hypothesis that had already been put forward to explain the time trend of thyroid cancer incidence in France.9 These results are particularly robust because they concern a cohort including a large number of cases – over 3000 cases – diagnosed in a population of over 4.6 million inhabitants. The interest and scope of the study are also reinforced by

Conclusion

It is important to highlight factors that explain the increase in papillary cancer. Our observations are a contribution to the idea of a role of medical practice in the increase and acceleration of the incidence rate. This is extremely important, because any wrong interpretation of the causes of the increase can have negative consequences. For example, knowing the link between excess iodine and the frequency of these cancers,30 the increase in the incidence of papillary cancers should not be

Conflict of interest statement

None declared.

Acknowledgements

This research received funding from the Commission Epidémiologie du Conseil de Radioprotection (Epidemiological Commission of the Radioprotection Council) of EDF (Electricité de France). This article is part of a collaborative work with the International Agency for Research on Cancer, that is partly supported by a grant from the Agence Française de Sécurité Sanitaire de l’Environnement et du Travail (AFSSET) and the Région Rhône-Alpes, France.

The research was carried out in collaboration with:

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