Thyroid Cancer: Risks and Preventive Measures

Risk Factors for Thyroid Cancer

Exposure to ionizing radiation is the only established environmental risk factor for thyroid carcinoma, with greater risk for papillary carcinoma in subjects exposed at younger ages 1
Following the Chernobyl nuclear accident in 1986, there was an 80-fold increase in the incidence of thyroid tumors in children from affected regions 1
Thyroid cancer incidence rates vary by race and ethnicity; among women, papillary thyroid cancer rates are higher in Asians (10.96 per 100,000 woman-years) and lower in blacks (4.9 per 100,000 woman-years) 1
Among men, papillary thyroid cancer rates are higher in whites (3.58 per 100,000 man-years) and lower in blacks (1.56 per 100,000 man-years) 1
Specific genetic syndromes like PTEN Hamartoma Tumor Syndrome (PHTS) carry increased risk for thyroid cancer, with the youngest reported case occurring in a 7-year-old child 2

Thyroid ultrasound (US) is recommended as a first-line diagnostic procedure for detecting and characterizing nodular thyroid disease 1, 3
Fine-needle aspiration cytology (FNAC) should be performed in any thyroid nodule >1 cm and in those <1 cm if there are clinical or ultrasonographic suspicions of malignancy 1, 3
For individuals with PHTS or other genetic syndromes with increased thyroid cancer risk, annual thyroid ultrasound beginning at age 7 is recommended 2, 4

US features associated with malignancy include hypoechogenicity, microcalcifications, absence of peripheral halo, irregular borders, solid aspect, intranodular blood flow, and shape (taller than wide) 1
Serum calcitonin measurement is a reliable tool for the diagnosis of medullary thyroid cancer and should be part of the diagnostic evaluation of thyroid nodules 3

Post-surgery thyroid hormone therapy with levothyroxine serves both to replace thyroid hormone and to suppress potential TSH stimulation of tumor cells 5

Several staging systems provide good risk stratification, with the most popular being the AJCC/IUAC TNM staging system 5
Risk categories for radioiodine ablation therapy include very low risk (unifocal T1 ≤1 cm, no aggressive histology, no metastases), low risk, and high risk 5, 6
Ongoing risk stratification based on response to therapy helps guide long-term follow-up protocols 5
Short-term follow-up should include thyroid function tests 2-3 months after initial treatment to check adequacy of levothyroxine therapy 5
At 6-12 months, screening should include physical examination, neck US, and basal and stimulated serum thyroglobulin measurement 3
For patients considered disease-free, subsequent follow-up consists of annual physical examination, basal serum thyroglobulin measurement, and neck ultrasound 3

For children with PHTS, thyroid ultrasound screening should begin at age 7, with repeat ultrasounds every 2 years if the baseline is negative 2

The dramatic increase in thyroid cancer incidence over recent decades may be partly attributable to increased detection of small papillary carcinomas due to improved diagnostic accuracy 1
In patients with genetic syndromes like PHTS, there is a high rate of benign nodules, increasing the risk of false positive findings that may lead to unnecessarily aggressive interventions 2

The American College of Medical Genetics and Genomics recommends that individuals with PTEN Hamartoma Tumor Syndrome (PHTS) be screened for thyroid cancer, as epithelial differentiated thyroid cancer occurs in as many as one third of patients, with a strength of evidence level of high 7
The National Comprehensive Cancer Network guidelines state that DICER1 syndrome confers a 16-24 fold increased risk for differentiated thyroid cancer, with cumulative incidence of multinodular goiter or thyroidectomy of 13% in males and 32% in females by 20 years of age, based on moderate-level evidence 8, 9

For patients with a history of head, neck, or upper thorax radiation exposure, annual thyroid screening is mandatory starting at the completion of cancer therapy, according to the National Comprehensive Cancer Network 10, 11
The American College of Clinical Oncology recommends annual thyroid/neck physical examination plus TSH measurement for all patients whose radiation field included the thyroid gland 10, 11
Consider thyroid ultrasound for initial baseline assessment, particularly in patients who received cervical radiation >20 Gy, as ultrasound detects 2-3 times more thyroid cancers than palpation alone 11

The National Comprehensive Cancer Network identifies the highest risk group as those with cervical radiation >20 Gy, cranial radiation, craniospinal radiation, nasopharyngeal radiation, mantle radiation, mediastinal radiation, or total body irradiation 11
Female gender and younger age at radiation exposure (<10-19 years) confer additional risk, according to the American College of Clinical Oncology 11

Measure TSH annually in all radiation-exposed patients, as hypothyroidism is very common and hyperthyroidism is uncommon, recommends the National Comprehensive Cancer Network 10, 11
Free T4 should be measured if TSH is abnormal, particularly in females receiving oral contraceptives, according to the American College of Clinical Oncology 11