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Article of the Week: Occupational variation in the incidence of testicular cancer in the Nordic countries

Every Week, the Editor-in-Chief selects an Article of the Week from the current issue of BJUI. The abstract is reproduced below and you can click on the button to read the full article, which is freely available to all readers for at least 30 days from the time of this post.

In addition to the article itself, there is an accompanying editorial written by a prominent member of the urological community. This blog is intended to provoke comment and discussion and we invite you to use the comment tools at the bottom of each post to join the conversation.

If you only have time to read one article this week, it should be this one.

Time trends and occupational variation in the incidence of testicular cancer in the Nordic countries

Outi Ylönen*, Sirkku Jyrkkiö, Eero Pukkala§, Kari Syvanen and Peter J. Bostrom

*South-Karelian Central Hospital, University Hospital of Turku, Lappeenranta, Finland, Department of Oncology, ¶Department of Urology, University Hospital of Turku, Turku, Finland, School of Health Sciences, University of Tampere, Tampere, Finland and §Finnish Cancer Registry, Helsinki, Finland

 

Abstract

Objective

To describe the trends and occupational variation in the incidence of testicular cancer in the Nordic countries utilising national cancer registries, NORDCAN (NORDCAN project/database presents the incidence, mortality, prevalence and survival from >50 cancers in the Nordic countries) and NOCCA (Nordic Occupational Cancer) databases.

Patients and Methods

We obtained the incidence data of testicular cancer for 5‐year periods from 1960–1964 to 2000–2014 and for 5‐year age‐groups from the NORDCAN database. Morphological data on incident cases of seminoma and non‐seminoma were obtained from national cancer registries. Age‐standardised incidence rates (ASR) were calculated per 100 000 person‐years (World Standard). Regression analysis was used to evaluate the annual change in the incidence of testicular cancer in each of the Nordic countries. The risk of testicular cancer in different professions was described based on NOCCA information and expressed as standardised incidence ratios (SIRs)

Fig. 2. Testicular cancer incidence time trends by age in the Nordic countries 1960-2014 (5-year floating averages).

Results

During 2010–2014 the ASR for testicular cancer varied from 11.3 in Norway to 5.8 in Finland. Until 1998, the incidence was highest in Denmark. There has not been an increase in Denmark and Iceland since the 1990s, whilst the incidence is still strongly increasing in Norway, Sweden, and Finland. There were no remarkable changes in the ratio of seminoma and non‐seminoma incidences during the past 50 years. There was no increase in the incidences in children and those of pension age. The highest significant excess risks of testicular seminoma were found in physicians (SIR 1.48, 95% confidence interval [CI] 1.07–1.99), artistic workers (SIR 1.47, 95% CI 1.06–1.99) and religious workers etc. (SIR 1.33, 95% CI 1.14–1.56). The lowest SIRs of testicular seminoma were seen amongst cooks and stewards (SIR 0.56, 95% CI 0.29–0.98), and forestry workers (SIR 0.64, 95% CI 0.47–0.86). The occupational category of administrators was the only one with a significantly elevated SIR for testicular non‐seminoma (SIR 1.21, 95% CI 1.04–1.42). The only SIRs significantly <1.0 were seen amongst engine operators (SIR 0.60, 95% CI 0.41–0.84) and public safety workers (SIR 0.67, 95% CI 0.43–0.99).

Conclusions

There have always been differences in the incidence of testicular cancer between the Nordic countries. There is also some divergence in the incidences in different age groups and in the trends of the incidence. The effect of occupation‐related factors on incidence of testicular cancer is only moderate. Our study describes the differences, but provides no explanation for this variation.

Editorial: Occupational exposure and risk of testicular cancer: what can an ecological study in the Nordic countries tell us?

Examining the association between occupational exposure and incidence rates of testicular cancer over time in several countries may provide useful insights into the relative importance of lifestyle and environmental risk factors. The study by Ylonen et al. [1] assessed an ecological, rather than biological, effect of occupational exposure in order to understand differences in testicular cancer rates among populations; therefore, the authors could not draw causal inferences about the effect of occupational exposure on testicular cancer at the individual level [2]. Ecological studies are, however, a way of performing hypothesis‐generating population‐based research. They take advantage of the natural experiment following the changes in occupational exposure across countries [3].

Keeping the strengths and limitations of an ecological study design in mind, we should consider what can we learn from the study by Ylonen et al. [1]. Firstly, it is interesting to note that the authors themselves state in their discussion that ‘occupational exposure is probably not relevant’ for testicular cancer because the disease ‘is mainly diagnosed in young adults and the duration of occupational exposure before cancer diagnosis is short’. This highlights the fact that occupational exposure should probably be considered here as a proxy variable for other risk factors of testicular cancer: environmental exposure, physical activity, education, etc. This large study based on linkages of high‐quality data registers therefore merely aims to generate hypotheses. No clear patterns were observed, however, and future studies may benefit from similar ecological approaches using risk factors with a better rationale in the context of the aetiology of testicular cancer, such as socio‐economic statuts, diet or body mass index.

Secondly, the authors also note that this lack of unambiguous risk determinants and underlying mechanisms of testicular cancer makes it difficult to explain the geographic and temporal variations observed [1]. As the study did not have a hypothesis a priori, we find ourselves in a situation where occupational exposure may not be the best risk factor to examine in relation to risk of testicular cancer in an ecological study. The rationale for choosing occupational exposure as the risk factor is weak, and perhaps the authors would have been able to observe more clear patterns if they had conducted the study to assess bladder cancer, for which occupation has been a much more established risk factor and time to exposure has been found to be more relevant.

Thirdly, a clinical understanding of testicular cancer detection may inform changes in incidence over time and between countries. No additional information was provided by the authors, but changes in raising awareness of potential symptoms may have resulted in an increased incidence in a specific age group and/or country. Moreover, it would be of interest to know about differences among countries in terms of occupational exposure groups as this may also explain some of the patterns observed. A further assessment of the characteristics of different categories of occupational exposure could inform the patterns observed in this study and may shed light on the aetiology of testicular cancer.

In conclusion, ecological studies force us to think carefully about patterns of cancer incidence over time and among countries. Unfortunately, the findings cannot always lead to further hypotheses and careful consideration about potential risk factors needs to occur before conducting analyses. Moreover, detailed (clinical) knowledge is required about changes in diagnostic activity over time as well as potential changes in risk factor exposure. Future ecological studies using highly valuable resources, such as those used by Ylonen et al. [1] can help us understand cancer aetiology and prevention in more detail. They can be considered as a natural experiment to fill the gap in our understanding of the link between potential risk factors and risk of developing cancer.

Mieke Van Hemelrijck
Translational Oncology & Urology Research, Kings College
London, London, UK

 

References

1 Ylonen O, Jyrkkio S, Pukkala E, Syvanen K, Bostrom P. Time trends and occupational variation in the incidence of testicular cancer in the Nordic countries. BJU Int 2018; 122: 384–93

2 Morgenstern H. Ecologic studies. In Rothman K, Greenland S eds, Modern Epidemiology, 2nd edn, Philadelphia, PA: Lippincott Williams &Wilkins, 1998: 511–31

3 Sedgwick P. Ecological studies: advantages and disadvantages. BMJ 2014; 348: g2979

 

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