Thyroid cancer

Published on 02/03/2015 by admin

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CHAPTER 37

Thyroid cancer

1. What are the different types of thyroid cancers?

2. Describe the epidemiology of thyroid cancer.

Thyroid cancer is one of the few cancers that have increased in both absolute incidence and mortality over the past several decades; an estimated 56,400 new cases were diagnosed and 1780 deaths occurred in 2012. However, the relative survival is actually improved compared with the 1970s, with an average 5-year survival in 97% of patients. Many new thyroid cancer diagnoses have resulted from increased imaging. The detection of tumors smaller than 1.0 cm has accounted for 50% of the increase since the late 1990s. However, up to 20% of the increase in diagnoses is for tumors larger than 2.0 cm, a finding suggesting that enhanced detection of incidental cancers is not the sole cause of the increased incidence. Thyroid cancer is the fifth most common cancer in women, and it affects women three times as often as it does men. However, the mortality rate in men and women is similar, indicating that thyroid cancer tends to be more aggressive in men.

3. What are the risk factors for thyroid cancer?

Differentiated thyroid cancer

4. What are the different forms of DTC?

5. Which is easier to diagnose based on thyroid fine-needle aspiration (FNA), PTC or FTC?

6. How do molecular markers play a role in the diagnosis or prognosis of thyroid cancer?

The discovery and utilization of molecular markers that can be assessed in thyroid biopsy aspirates have enhanced the ability of practitioners to predict malignancy in thyroid nodules that have indeterminate FNA cytology (average malignancy risk ∼25%). Two commercial tests that use different analyses to predict malignancy risk are available. The Afirma test by Veracyte, Inc., uses a microarray analysis on a gene set that has a high negative predictive value (NPV) of 93% and a 40%-50% PPV for suspicious nodules. This test is very useful in predicting benign lesions and for avoiding unnecessary diagnostic surgery. The miRInform test by Asuragen, Inc., evaluates thyroid nodule aspirates for specific DNA mutation markers (KRAS, HRAS, NRAS, and BRAF mutations) and RNA fusion transcripts (RET/PTC1, RET/PTC3, and PAX8/PPARγ) that are specific for thyroid cancer. RAS mutations carry about an 85% PPV for thyroid cancer, but they can also be present in benign follicular adenomas. The other markers of this panel are functionally 100% predictive of malignancy. Their sensitivity is relatively poor, however, because thyroid cancers may harbor genetic alterations not found in this test. BRAF is present in approximately 30% to 60% of PTCs and predicts greater local invasion, lymph node metastases, radioiodine resistance, and an overall worse prognosis than do other mutations found in DTC.

7. Describe the staging of DTC.

The American Thyroid Association (ATA) recommends the American Joint Commission on Cancer (AJCC) staging system. Thyroid cancer is the only cancer that has age as a component of stage (Table 37-1). According to the AJCC system, if a patient is less than 45 years old, stage II disease is the highest stage possible, and then only if distant metastases are present outside the neck. Conversely, in patients who are 45 years old or older, intrathyroidal tumors up to 2 cm are stage I and tumors 2 to 4 cm are stage II. Any locoregional metastases raise the stage to so-called high-risk disease at stage III. Stage IV tumors either have gross invasion into extrathyroidal neck structures or distant metastases.

TABLE 37-1.

STAGING SYSTEM FOR DIFFERENTIATED THYROID CANCER

image

From American Joint Committee on Cancer (AJCC): Cancer staging manual, ed 6, New York, 2002, Springer.

8. How do PTC and FTC generally metastasize?

9. How often do metastases occur?

10. What is the primary treatment for thyroid cancer?

11. What determines the extent of the initial surgical procedure?

Ideally, the first thyroid cancer operation is the last. Preoperative neck ultrasound (US) is an invaluable tool for identifying the extent of lymph node metastases in the anterior lateral cervical lymph node chains. US is superior to computed tomography (CT) and magnetic resonance imaging (MRI) because it identifies malignant features of lymph nodes beyond size alone. For known thyroid cancer larger than 1 cm, or when lymph node metastases are detected preoperatively, a near-total thyroidectomy with lymph node resection is the procedure of choice. If the primary tumor is smaller than 1 cm, a hemithyroidectomy may be adequate. The need for prophylactic central neck lymph node dissection (prophylactic because central neck lymph nodes cannot be visualized with an intact thyroid in place) is controversial because of the lack of studies demonstrating improved survival with prophylactic central neck dissection.

12. What is the role of radioactive iodine in thyroid cancer therapy?

13. Should all patients with thyroid cancer receive radioiodine?

14. How are patients prepared for radioactive iodine therapy?