Excess risk of male breast cancer in the Norwegian Offshore Petroleum Workers (NOPW) cohort: a possible link to extreme night shift work?

© The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. LETTER-TO-THE–EDITOR Male breast cancer (MBC) is a rare disease with some recognised risk factors, such as age, genetic disorders, family history, hormonal influence and radiation [1, 2]. Night shift work (NSW) disrupts the circadian rhythm and may lead to sleep disturbances and changes in sex hormone levels [3], which has been suggested to increase breast cancer risk in female nurses [4]. There is, however, a lack of knowledge regarding the relationship between NSW and MBC. The Norwegian Offshore Petroleum Workers (NOPW) cohort was established in 1998 by recruiting 25,347 male workers who answered a questionnaire. Each such worker had at least one employment in oil and gas operations on the Norwegian continental shelf 1965–1998. All workers experienced a schedule with 12-h workdays for two weeks; consisting of either day work only, seven day and seven night shifts, or night shifts only; followed by a four-week off-duty period. Ever exposure to NSW was defined as having ever worked night shift during employment. We previously reported a twofold excess MBC incidence in the NOPW cohort compared to the general Norwegian population (standardized incidence ratio [SIR] = 2.18, 95% confidence interval [CI] 1.13–3.81) [5]. By December 31st, 2019, the number of prospectively recorded MBC cases had increased from 12 to 14, giving SIRs of 2.10 (95% CI 1.15–3.53) overall, 2.49 (1.14–4.73) in ever exposed, and 1.77 (0.57–4.12) in those never exposed to NSW (Table 1). We conducted internal comparisons, using a Cox proportional hazards model adjusted for age (timescale) and education. Males ever exposed to NSW were at 33% increased risk compared to those never exposed, although the estimate was not statistically significant (Table 1). Analyses were performed with Stata version 17 (StataCorp, College Station, TX, USA). We report this apparent occupational risk of MBC to inspire further research. In our cohort, cancers were prospectively recorded, and analyses were controlled for age and education. In separate analyses, we explored the association with exposure to benzene and chlorinated degreasers and found no sign of increased MBC risk. Lifestyle factors could possibly contribute to breast cancer risk among shift workers, though some of the effect would be adjusted for by education as a proxy variable. With these reservations, our data suggest that extreme NSW may contribute to MBC risk. Open Access


LETTER-TO-THE-EDITOR
Male breast cancer (MBC) is a rare disease with some recognised risk factors, such as age, genetic disorders, family history, hormonal influence and radiation [1,2]. Night shift work (NSW) disrupts the circadian rhythm and may lead to sleep disturbances and changes in sex hormone levels [3], which has been suggested to increase breast cancer risk in female nurses [4]. There is, however, a lack of knowledge regarding the relationship between NSW and MBC.
The Norwegian Offshore Petroleum Workers (NOPW) cohort was established in 1998 by recruiting 25,347 male workers who answered a questionnaire. Each such worker had at least one employment in oil and gas operations on the Norwegian continental shelf 1965-1998. All workers experienced a schedule with 12-h workdays for two weeks; consisting of either day work only, seven day and seven night shifts, or night shifts only; followed by a four-week off-duty period. Ever exposure to NSW was defined as having ever worked night shift during employment. We previously reported a twofold excess MBC incidence in the NOPW cohort compared to the general Norwegian population (standardized incidence ratio [SIR] = 2.18, 95% confidence interval [CI] 1.13-3.81) [5]. By December 31st, 2019, the number of prospectively recorded MBC cases had increased from 12 to 14, giving SIRs of 2.10 (95% CI 1.15-3.53) overall, 2.49 (1.14-4.73) in ever exposed, and 1.77 (0.57-4.12) in those never exposed to NSW (Table 1). We conducted internal comparisons, using a Cox proportional hazards model adjusted for age (timescale) and education. Males ever exposed to NSW were at 33% increased risk compared to those never exposed, although the estimate was not statistically significant (Table 1). Analyses were performed with Stata version 17 (StataCorp, College Station, TX, USA).
We report this apparent occupational risk of MBC to inspire further research. In our cohort, cancers were prospectively recorded, and analyses were controlled for age and education. In separate analyses, we explored the association with exposure to benzene and chlorinated degreasers and found no sign of increased MBC risk. Lifestyle factors could possibly contribute to breast cancer risk among shift workers, though some of the effect would be adjusted for by education as a proxy variable. With these reservations, our data suggest that extreme NSW may contribute to MBC risk.

Funding
The research project has been reviewed and granted funding by the Research Council of Norway (Grant No. 280537).

Availability of data and materials
The data that support the findings of this study are available from the CRN (cohort data and cancer data) and the National Population Register (death and emigration data) but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Requests for data sharing/case pooling for projects with necessary approvals and legal basis according to the EU General Data Protection Regulation (GDPR) may be directed to principal investigator Dr. Tom K. Grimsrud; email: tom.k.grimsrud@kreftregisteret.no.

Declarations Ethics approval and consent to participate
This study was performed at the Cancer Registry of Norway (CRN) after legal and ethical approvals were obtained from the Norwegian Data Inspectorate, the Regional Committee for Medical Research Ethics South-East (2018/1162), and the Norwegian Directorate of Health. Each study participant signed an informed consent.

Consent for publication
Not applicable.

Competing interests
TKG and JSS note that the Research Council of Norway (governmental agency) awarded an industry-collaborative grant to the CRN (governmental agency) in 2019 to establish a cohort of offshore petroleum workers. A condition pertaining to such industry-collaborative grants is that 20% (US $175,000) of the grant was provided by the petroleum industry and 80% (US $700,000) by the Research Council itself with the intention of joining forces for the common interest of improved occupational health among petroleum workers. The grant-application process was governed by the Research Council without any involvement from the petroleum industry. TKG and JSS are affiliated and employed by the CRN, but the grant does not cover their salary. The remaining authors declare that there are no conflicts of interest.