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Is the Incidence of Thyroid Cancer Increasing in Medical Staff?

The Editor, 
 
Sir, 
 
Thyroid cancers (TC) are malignant neoplasms originating from follicular or parafollicular thyroid cells, with five-year relative survival of approximately 85% in Europe (1). Current incidence of TC is 14.3 per 100 000 individuals in the United States of America [USA] (2). 
 
While the incidence of many head or neck cancers in the USA is decreasing, a number of studies have indicated that the incidence of TC is increasing (3–6). Li et al showed that, in the past decades (1980–2008), there is an increased incidence of TC in the USA after adjustments were made to null the effect of improved diagnostic modalities (6). 
 
To examine the incidence of TC, we reviewed the records of 4009 medical staff including doctors and nurses (2236 female vs 1773 males; aged 30–60 years old), who had undergone routine physical examination in our physical examination centre from December 2010 to November 2013. All of these medical staff members were subjected to ultrasound screening and those with suspicious findings were finally confirmed with fine needle aspiration biopsy. Among the medical staff, 1.10% (44/4009) were finally confirmed with TCs at the first time. Thyroid cancers were significantly more prevalent in females (1.34%, 30/2236) than males (0.79%, 14/1773) in the medical staff. Incidence in the 40– 50-year old group was 7.23% (29/4009); higher than in the 50–60-year old group (0.03%, 12/4009) and the 30–40-year old group (0.07%, 3/4009). In thyroid cancers, 20.5% (9/44) of cases had a history of benign thyroid nodules and 4.55% (2/44) had non-toxic goitre. Pathology results showed that 0.95% (38/4009) of the cases was papillary TC while 0.15% (6/4009) was follicular TC. 
 
The best-established cause of TC is exposure to ionizing radiation, particularly in childhood and young women (7). Chronic iodine deficiency and consequent rise in thyroid-stimulating hormone (TSH) secretion were firmly established as risk factors for goitre and follicular TC (8), while some studies have suggested that iodine supplementations could increase the incidence of papillary TC (9). 
 
For the medical staff, there was no evidence that they had received excess ionizing radiation, but very low-dose radiation may be unavoidable in the working environment of a hospital. Besides, working stress is also a risk that cannot be ignored in China. Factors such as stress, environment and individual well-being could impact the pathogenesis. Currently, whether the high incidence of TC is a small probability coincidence requires research. For scientific researchers, this phenomenon should be taken seriously to give an evidence-based explanation.