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Globally as of 2015, 3.2 million people have thyroid cancer. In 2012, 298,000 new cases occurred. It most commonly occurs between the ages of 35 and 65. Women are affected more often than men. Those of Asian descent are more commonly affected. Rates have increased in the last few decades, which is believed to be due to better detection. In 2015, it resulted in 31,900 deaths.
Signs and symptoms
Most often, the first symptom of thyroid cancer is a nodule in the thyroid region of the neck. However, up to 65% of adults have small nodules in their thyroids, but typically under 10% of these nodules are found to be cancerous. Sometimes, the first sign is an enlarged lymph node. Later symptoms that can be present are pain in the anterior region of the neck and changes in voice due to an involvement of the recurrent laryngeal nerve.
Thyroid nodules are of particular concern when they are found in those under the age of 20. The presentation of benign nodules at this age is less likely, thus the potential for malignancy is far greater.
Thyroid cancers are thought to be related to a number of environmental and genetic predisposing factors, but significant uncertainty remains regarding their causes.
The follicular and papillary types together can be classified as "differentiated thyroid cancer". These types have a more favorable prognosis than the medullary and undifferentiated types.
Papillary microcarcinoma is a subset of papillary thyroid cancer defined as measuring less than or equal to 1 cm.  43% of all thyroid cancers and 50% of new cases of papillary thyroid carcinoma are papillary microcarcinoma. Management strategies for incidental papillary microcarcinoma on ultrasound (and confirmed on FNAB) range from total thyroidectomy with radioactive iodine ablation to observation alone. Harach et al. suggest using the term "occult papillary tumor" to avoid giving patients distress over having cancer. Woolner et al. first arbitrarily coined the term "occult papillary carcinoma", in 1960, to describe papillary carcinomas 
Cancer staging is the process of determining the extent of the development of a cancer. The TNM staging system is usually used to classify stages of cancers, but not of the brain.
Thyroid cancer can spread directly, via lymphatics or blood. Direct spread occurs through infiltration of the surrounding tissues. The tumor infiltrates into infrahyoid muscles, trachea, oesophagus, recurrent laryngeal nerve, carotid sheath, etc. The tumor then becomes fixed. Anaplastic carcinoma spreads mostly by direct spread, while papillary carcinoma spreads so the least. Lymphatic spread is most common in papillary carcinoma. Cervical lymph nodes become palpable in papillary carcinoma even when the primary tumor is unpalpable. Deep cervical nodes, pretracheal, prelaryngeal, and paratracheal groups of lymph nodes are often affected. The lymph node affected is usually the same side as that of the location of the tumor. Blood spread is also possible in thyroid cancers, especially in follicular and anaplastic carcinoma. The tumor emboli do angioinvasion of lungs; end of long bones, skull, and vertebrae are affected. Pulsating metastases occur because of their increased vascularity.
Thyroidectomy and dissection of central neck compartment is initial step in treatment of thyroid cancer in the majority of cases. Thyroid-preserving operations may be applied in cases, when thyroid cancer exhibits low biological aggressiveness (e.g. well-differentiated cancer, no evidence of lymph-node metastases, low MIB-1 index, no major genetic alterations like BRAF mutations, RET/PTC rearrangements, p53 mutations etc.) in patients younger than 45 years.
If the diagnosis of well-differentiated thyroid cancer (e.g. papillary thyroid cancer) is established or suspected by FNA, then surgery is indicated, whereas watchful waiting strategy is not recommended in any evidence-based guidelines.Watchful waiting reduces overdiagnosis and overtreatment of thyroid cancer among old patients.
Radioactive iodine-131 is used in people with papillary or follicular thyroid cancer for ablation of residual thyroid tissue after surgery and for the treatment of thyroid cancer. Patients with medullary, anaplastic, and most Hurthle-cell cancers do not benefit from this therapy.
External irradiation may be used when the cancer is unresectable, when it recurs after resection, or to relieve pain from bone metastasis.
Sorafenib and lenvatinib are approved for advanced metastatic thyroid cancer. Numerous agents are in phase II and III clinical trials.
The prognosis of thyroid cancer is related to the type of cancer and the stage at the time of diagnosis. For the most common form of thyroid cancer, papillary, the overall prognosis is excellent. Indeed, the increased incidence of papillary thyroid carcinoma in recent years is likely related to increased and earlier diagnosis. One can look at the trend to earlier diagnosis in two ways. The first is that many of these cancers are small and not likely to develop into aggressive malignancies. A second perspective is that earlier diagnosis removes these cancers at a time when they are not likely to have spread beyond the thyroid gland, thereby improving the long-term outcome for the patient. No consensus exists at present on whether this trend toward earlier diagnosis is beneficial or unnecessary.
The argument against early diagnosis and treatment is based on the logic that many small thyroid cancers (mostly papillary) will not grow or metastasize. This view holds the overwhelming majority of thyroid cancers are overdiagnosed that is, will never cause any symptoms, illness, or death for the patient, even if nothing is ever done about the cancer. Including these overdiagnosed cases skews the statistics by lumping clinically significant cases in with apparently harmless cancers. Thyroid cancer is incredibly common, with autopsy studies of people dying from other causes showing that more than one-third of older adults technically have thyroid cancer, which is causing them no harm. Detecting nodules that might be cancerous is easy, simply by feeling the throat, which contributes to the level of overdiagnosis. Benign (noncancerous) nodules frequently co-exist with thyroid cancer; sometimes, a benign nodule is discovered, but surgery uncovers an incidental small thyroid cancer. Increasingly, small thyroid nodules are discovered as incidental findings on imaging (CT scan, MRI, ultrasound) performed for another purpose; very few of these people with accidentally discovered, symptom-free thyroid cancers will ever have any symptoms, and treatment in such patients has the potential to cause harm to them, not to help them.
Thyroid cancer is three times more common in women than in men, but according to European statistics, the overall relative 5-year survival rate for thyroid cancer is 85% for females and 74% for males.
The table below highlights some of the challenges with decision making and prognostication in thyroid cancer. While general agreement exists that stage I or II papillary, follicular, or medullary cancer have good prognoses, when evaluating a small thyroid cancer to determine which ones will grow and metastasize and which will not is not possible. As a result, once a diagnosis of thyroid cancer has been established (most commonly by a fine needle aspiration), a total thyroidectomy likely will be performed.
This drive to earlier diagnosis has also manifested itself on the European continent by the use of serum calcitonin measurements in patients with goiter to identify patients with early abnormalities of the parafollicular or calcitonin-producing cells within the thyroid gland. As multiple studies have demonstrated, the finding of an elevated serum calcitonin is associated with the finding of a medullary thyroid carcinoma in as high as 20% of cases.
In Europe where the threshold for thyroid surgery is lower than in the United States, an elaborate strategy that incorporates serum calcitonin measurements and stimulatory tests for calcitonin has been incorporated into the decision to perform a thyroidectomy; thyroid experts in the United States, looking at the same data, have for the most part not incorporated calcitonin testing as a routine part of their evaluations, thereby eliminating a large number of thyroidectomies and the consequent morbidity. The European thyroid community has focused on prevention of metastasis from small medullary thyroid carcinomas; the North American thyroid community has focused more on prevention of complications associated with thyroidectomy (see American Thyroid Association guidelines below).
As demonstrated in the table below, individuals with stage III and IV disease have a significant risk of dying from thyroid cancer. While many present with widely metastatic disease, an equal number evolve over years and decades from stage I or II disease. Physicians who manage thyroid cancer of any stage recognize that a small percentage of patients with low-risk thyroid cancer will progress to metastatic disease.
Improvements have been made in thyroid cancer treatment during recent years. The identification of some of the molecular or DNA abnormalities has led to the development of therapies that target these molecular defects. The first of these agents to negotiate the approval process is vandetanib, a tyrosine kinase inhibitor that targets the RET proto-oncogene, two subtypes of the vascular endothelial growth factor receptor, and the epidermal growth factor receptor. More of these compounds are under investigation and are likely to make it through the approval process. For differentiated thyroid carcinoma, strategies are evolving to use selected types of targeted therapy to increase radioactive iodine uptake in papillary thyroid carcinomas that have lost the ability to concentrate iodide. This strategy would make possible the use of radioactive iodine therapy to treat "resistant" thyroid cancers. Other targeted therapies are being evaluated, making life extension possible over the next 5-10 years for those with stage III and IV thyroid cancer.
Prognosis is better in younger people than older ones.
Prognosis depends mainly on the type of cancer and cancer stage.
Thyroid cancer, in 2010, resulted in 36,000 deaths globally up from 24,000 in 1990.Obesity may be associated with a higher incidence of thyroid cancer, but this relationship remains the subject of much debate.
Thyroid cancer accounts for less than 1% of cancer cases and deaths in the UK. Around 2,700 people were diagnosed with thyroid cancer in the UK in 2011, and around 370 people died from the disease in 2012.
^Durante, Cosimo; Grani, Giorgio; Lamartina, Livia; Filetti, Sebastiano; Mandel, Susan J.; Cooper, David S. (6 March 2018). "The Diagnosis and Management of Thyroid Nodules". JAMA. 319 (9): 914-924. doi:10.1001/jama.2018.0898. ISSN0098-7484. PMID29509871.
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^Dideban, S; Abdollahi, A; Meysamie, A; Sedghi, S; Shahriari, M (2016). "Thyroid Papillary Microcarcinoma: Etiology, Clinical Manifestations,Diagnosis, Follow-up, Histopathology and Prognosis". Iranian journal of pathology. 11 (1): 1-19. PMID26870138.
^Hughes, DT; Haymart, MR; Miller, BS; Gauger, PG; Doherty, GM (March 2011). "The most commonly occurring papillary thyroid cancer in the United States is now a microcarcinoma in a patient older than 45 years". Thyroid : official journal of the American Thyroid Association. 21 (3): 231-6. doi:10.1089/thy.2010.0137. PMID21268762.
^Woolner LL, Lemmon ML, Beahrs OH, Black BM, Keating FR (1960). "Occult papillary carcinoma of the thyroid gland: a study of 140 cases observed in a 30-year period". J. Clin. Endocrinol. Metab. 20: 89-105. doi:10.1210/jcem-20-1-89. PMID13845950.
^Hindié E, Zanotti-Fregonara P, Keller I, Duron F, Devaux JY, Calzada-Nocaudie M, Sarfati E, Moretti JL, Bouchard P, Toubert ME (2007). "Bone metastases of differentiated thyroid cancer: Impact of early 131I-based detection on outcome". Endocrine-Related Cancer. 14 (3): 799-807. doi:10.1677/ERC-07-0120. PMID17914109.
^Schlumberger M, Arcangioli O, Piekarski JD, Tubiana M, Parmentier C (1988). "Detection and treatment of lung metastases of differentiated thyroid carcinoma in patients with normal chest X-rays". Journal of Nuclear Medicine. 29 (11): 1790-94. PMID3183748.
^Das, Somen (2008). A concise textbook of surgery (5th. ed.). Calcutta: Dr S. Das. ISBN978-8190568128.
^ abcdefgNumbers from National Cancer Database in the US, from Page 10Archived 13 May 2016 at the Wayback Machine in: F. Grünwald; Biersack, H. J.; Grunwald, F. (2005). Thyroid cancer. Berlin: Springer. ISBN978-3-540-22309-2. (Note:Book also states that the 14% 10-year survival for anaplastic thyroid cancer was overestimated)
^By 100% minus cause-specific mortality of 17% at 5 yr, as given by Barbet J, Campion L, Kraeber-Bodéré F, Chatal JF (2005). "Prognostic Impact of Serum Calcitonin and Carcinoembryonic Antigen Doubling-Times in Patients with Medullary Thyroid Carcinoma". Journal of Clinical Endocrinology & Metabolism. 90 (11): 6077-84. doi:10.1210/jc.2005-0044. PMID16091497.
^Lozano R, Naghavi M, Foreman K, Lim S, Shibuya K, Aboyans V, et al. (15 December 2012). "Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010". Lancet. 380 (9859): 2095-128. doi:10.1016/S0140-6736(12)61728-0. hdl:10536/DRO/DU:30050819. PMID23245604.