Thyroid cancer is rare,there were 1,757 new cases of thyroid cancer in the United Kingdom in 2005.Women are more likely to have a malignant thyroid than men. The statistics for women in 2005 was 1,302 cases compared to 455 cases in men. Almost 50% of all case occur in people under 50 years old. Within the European Union there were 19,034 new cases in 2002 the highest incidence rate was in Malta where the rate for females was seven times greater than the rate for the lowest ranking country Bulgaria (12.6 versus 1.7 per 100.00 females). In Great Britain the age-standardised incidence has almost doubled from 1.4 to 2.6 per 100,000 persons from 1975 to 2005! The Cancer Statistics from the USA estimates 44,640 new cases of thyroid cancer in 2010. The expected incidence of all cancers in that year is 1,529560. Usually the cause of thyroid cancer is unknown, however there are four predisposing factors which are listed below. Interestingly Krukowski's group from Scotland have shown in 2009 that the need for radical surgery has increased in thyroid cancer.The reason for this is not clear. Fortunately it has also been shown that this is not associated with an increase in cancer specific mortality.Patients who have had a kidney transplant are at an increased risk of developing thyroid cancer.Yearly ultrasound of thyroid should be routine in transplant patients.
It is well documented that radiation to the neck given to children for non-cancerous skin conditions such as acne or tuberculosis results years later in the development of thyroid cancers. Nuclear accidents such as Chernobyl result in an increased incidence of thyroid cancer in the affected population.
Medullary cancer of the thyroid may arise as a familial condition due to a variety of germline mutations in the RET gene. In addition there is a genetic mutation that causes familial papillary thyroid cancer.
Follicular cancer rarely occurs in Iceland where, due to high dietary intake of fish, there is high iodide intake. In contrast, in areas of low iodide intake (mountainous regions such as the Alps, Andes and Himalayas) follicular cancer has a high incidence. Iodising salt so as to combat iodine deficiency does not change the incidence of thyroid cancer but increases the proportion of papillary cancers and reduces the incidence of follicular cancer. The overall incidence therefore remains virtually the same.
Papillary cancers may arise more often in patients with a long-term severe inflammation of their thyroid (thyroiditis). A special form of thyroiditis (Hashimoto's Thyroiditis) may develop into a form of cancer called lymphoma.It is now known that patients with Hashimoto's Thyroiditis have an significant increased incidence of papillary and follicular thyroid cancers.
In the past the presence of normal thyroid tissue even within the lateral neck (side of neck) was considered of no significance and was called a "lateral aberrant thyroid". This concept is now considered incorrect and thyroid tissue in the lateral neck,if within lymph nodes,is now thought to represent metastatic papillary cancer.In many cases the primary tumour is not obvious even with high quality ultrasound in the thyroid. The primary may be microscopic and only found by meticulous sectioning of the thyroid. The prognosis of this type of cancer is excellent and in most cases, despite the fact that the thyroid appears to be normal, the treatment is total thyroidectomy with a neck dissection.
Laterally placed true non metastatic thyroid tissue can occur if it is not within lymph nodes and is medial to the carotid vessels.Neck surgery for thyroidectomy can result in implantation of thyroid tissue quite lateral in the neck but such thyroid tissue is never within cervical lymph nodes.