To the Editor:
Salivary gland tumors are relatively rare, accounting for 2–5% of all head and neck tumors in various countries, with the most common subtype (75%) being parotid gland tumors.1 Ionizing radiation exposure is the only established risk factor to date.2
Recent studies have reported an increased incidence of malignant parotid gland tumors during the last decades, especially in Israel,3 but also in England.4 This has been speculated to reflect the increased prevalence of mobile phone use since the 1990s, from virtually 0 to close to 100%, considering the high exposure of parotid gland during mobile phone use.
The Nordic countries were among the first to adopt mobile phone technology in the general population. We aimed to investigate incidence trends of malignant parotid gland tumors and all malignant salivary gland tumors in Sweden and in the Nordic countries, using the national registers.
Patients 20 years or older with primary diagnosis of parotid gland neoplasms (ICD-7 [International Classification of Diseases, Seventh Revision], 1420) and of salivary gland malignancies (ICD-7, 142) from 1 January 1970 to 31 December 2009 were identified from publicly available data in the Swedish Cancer Registry (http://22.214.171.124/sdb/can/val.aspx) and the NORDCAN database (http://www-dep.iarc.fr/NORDCAN/English/frame.asp).We used direct standardization to calculate annual age-standardized incidence rates (cases per 100,000 person-years) using the world standard population.
In Sweden, 3604 salivary gland tumor cases in adults 20 years or older were identified during the period 1970–2009, of whom 2624 (73%) were malignant neoplasms of the parotid gland. The incidence of parotid gland tumors in Sweden decreased slightly during the early study period (Figure), and then remained stable for both sexes (0.9/100,000 person-years in 1970 and 0.8/100,000 person-years in 2009 for men; 0.7/100,000 person-years in both 1970 and 2009 for women [Panel A]). The age-standardized rate of salivary gland tumors in the whole Nordic population during the period 1970–2009 was 1.1/100,000 for men (4440 cases) and 0.9/100,000 for women (4178 cases). No increase in incidence was observed; the annual percent change was −0.1% (95% CI = −0.4 to 0.2) for men and −0.2% (−0.5% to 0.1%) for women (Panel C).
Previous reports of an increased incidence of parotid gland tumors3,4 possibly due to exposure to mobile phone use were not supported by our results. We found a stable or declining trend in the incidence of salivary gland tumors during the last 40 years, during which there was relatively early and full adaptation of mobile phone use in Nordic countries. However, none of these incidence-trend studies had information on individual mobile phone use and therefore cannot directly link mobile phone use to parotid gland tumor risk. Data from case-control studies with individual self-reported information on mobile phone use do not generally support the hypothesis that mobile phone use increases parotid gland tumor risk.5–8 Two studies found an increased risk of parotid gland tumor on the side of the head where the mobile phone was usually used,6,7 but this was accompanied by a reduced risk on the opposite side of the head, indicating recall bias in retrospective reports of laterality of mobile phone use.
If mobile phone use were a noticeable risk factor for parotid gland tumors, some increase would have been seen in the Nordic countries unless the latency period exceeds 15–20 years. Our results suggest other explanations for increases in the registered incidence of parotid gland tumors observed in Israel and United Kingdom.
Division of Epidemiology
Institute of Environmental Medicine
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2. Horn-Ross PL, Ljung BM, Morrow M. Environmental factors and the risk of salivary gland cancer. Epidemiology. 1997;8:414–419.
3. Czerninski R, Zini A, Sgan-Cohen HD. Risk of parotid malignant tumors in Israel (1970–2006). Epidemiology. 2011;22:130–131.
4. de Vocht F. Cell phones and parotid cancer trends in England. Epidemiology. 2011;22:608–609.
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6. Lonn S, Ahlbom A, Christensen HC, et al.. Mobile phone use and risk of parotid gland tumor. Am J Epidemiol. 2006;164:637–643.
7. Sadetzki S, Chetrit A, Jarus-Hakak A, et al.. Cellular phone use and risk of benign and malignant parotid gland tumors—a nationwide case-control study. Am J Epidemiol. 2008;167:457–467.
8. Hardell L, Hallquist A, Hansson Mild K, et al.. No association between the use of cellular or cordless telephones and salivary gland tumours. Occup Environ Med. 2004;61:675–679.