What are the risks and preventive measures for thyroid cancer?

Thyroid Cancer: Risks and Preventive Measures

The primary established environmental risk factor for thyroid cancer is exposure to ionizing radiation, particularly in younger individuals, while regular screening with thyroid ultrasound and fine-needle aspiration cytology (FNAC) are the most effective preventive measures for early detection and improved outcomes. 1

Risk Factors for Thyroid Cancer

Environmental Risk Factors

• 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
• Medical radiation exposure, particularly from diagnostic procedures like CT scans, represents a significant source of radiation exposure that may contribute to thyroid cancer risk 2

Demographic Risk Factors

• 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
• Women have a two- to threefold higher background incidence of thyroid cancer than men, and also experience a higher absolute increase in risk due to radiation exposure 3

Genetic Risk Factors

• Evidence suggests familial susceptibility factors for radiation-induced thyroid neoplasms, indicating potential genetic predisposition 4
• 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 1
• Genetic mutations in BRAF, RET, NTRK, and MEK genes have been identified in thyroid cancers and are targets for newer therapies 5

Preventive Measures

Early Detection and Screening

• Thyroid ultrasound (US) is recommended as a first-line diagnostic procedure for detecting and characterizing nodular thyroid disease 1
• 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
• For individuals with PHTS or other genetic syndromes with increased thyroid cancer risk, annual thyroid ultrasound beginning at age 7 is recommended 1

Diagnostic Features to Monitor

• 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
• When multiple patterns suggestive of malignancy are simultaneously present in a nodule, the specificity of US increases but sensitivity becomes lower 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 1

Management of Thyroid Nodules

• For micropapillary carcinomas (≤1 cm), observation without surgical resection can be considered in appropriate cases 5
• For tumors larger than 1 cm with or without lymph node metastases, surgery with or without radioactive iodine is curative in most cases 5
• Post-surgery thyroid hormone therapy with levothyroxine serves both to replace thyroid hormone and to suppress potential TSH stimulation of tumor cells 1, 6

Risk Stratification and Follow-up

Risk Assessment

• Several staging systems provide good risk stratification, with the most popular being the AJCC/IUAC TNM staging system 1
• Risk categories for radioiodine ablation therapy include very low risk (unifocal T1 ≤1 cm, no aggressive histology, no metastases), low risk, and high risk 1
• Ongoing risk stratification based on response to therapy helps guide long-term follow-up protocols 1

Follow-up Protocols

• Short-term follow-up should include thyroid function tests 2-3 months after initial treatment to check adequacy of levothyroxine therapy 1
• At 6-12 months, screening should include physical examination, neck US, and basal and stimulated serum thyroglobulin measurement 1
• For patients considered disease-free, subsequent follow-up consists of annual physical examination, basal serum thyroglobulin measurement, and neck ultrasound 1

Special Considerations

Pediatric Patients

• Children are especially sensitive to the carcinogenic effects of radiation to the thyroid, with increased risk persisting for decades 3
• Radiation-induced tumors begin to appear 5-10 years after irradiation 3
• For children with PHTS, thyroid ultrasound screening should begin at age 7, with repeat ultrasounds every 2 years if the baseline is negative 1

Pregnancy

• For pregnant patients with pre-existing hypothyroidism, serum TSH and free-T4 should be measured as soon as pregnancy is confirmed and during each trimester 6
• Levothyroxine dosage may need to be increased during pregnancy to maintain TSH in the trimester-specific reference range 6

Common Pitfalls and Caveats

• 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
• There is a risk of overdiagnosis and unnecessary treatment, particularly for small, indolent tumors that may never become clinically significant 5
• 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 1
• Radiation exposure from diagnostic medical procedures, particularly CT scans, is increasing and represents a modifiable risk factor for thyroid cancer 2