Follicular Thyroid Carcinoma

Follicular Thyroid Carcinoma (FTC)

Follicular Thyroid Carcinoma (FTC) is the second most common malignancy arising from follicular cells, which accounts for about 5% to10% of all thyroid cancers in the United States. It tends to occur less commonly nowadays than in the past. In comparison with papillary thyroid carcinoma (PTC), follicular thyroid carcinoma does not spread into the lymph nodes of the neck. It is more likely than papillary thyroid carcinoma to spread by the bloodstream into distant organs, particularly the lungs and bones.

FTC is classified into three categories based on the degree of invasion: widely, moderately invasive, and minimally invasive FTC. Widely invasive FTC is more aggressive and displays a poorer prognosis than minimally invasive FTC. Minimally invasive FTC is generally a more indolent disease. FTC is difficult to diagnose preoperatively by FNA biopsy or by imaging studies, unless distant metastases are detected. Some of the cytological features of FTC are similar to the follicular variant of papillary thyroid carcinoma (PTC) on FNA biopsy. Because of that, intraoperative frozen section can help differentiate PTC from FTC, but most of the time, it is very difficult to do so. If the patient has only one nodule on one side of the thyroid that is suspicious for Follicular neoplasm, then a diagnostic thyroid lobectomy is indicated. Completion of total thyroidectomy as a second surgery and radioactive iodine (RAI) ablation are thus recommended for invasive FTC detected during the first lobectomy procedure. The prognosis of patients with distant metastases or large tumor size (larger than 4 cm) is poor.

Diagnosis

With Follicular Thyroid Carcinoma (as well as with Hurthle Cell Carcinoma), preoperative diagnosis by FNA biopsy is very difficult to establish. The usual description of an FNA thyroid biopsy reveals findings of "follicular neoplasm". Fifteen to thirty percent of them will have a final diagnosis of Follicular Cell Carcinoma. Preoperative diagnosis by FNA biopsy is very difficult to establish, and surgery is needed for ultimate diagnosis.

Staging

Stage I follicular thyroid cancer

Stage I follicular carcinoma is localized to the thyroid gland. Follicular thyroid carcinoma must be distinguished from follicular adenomas, which are characterized by their lack of invasion through the capsule into the surrounding thyroid tissue. While follicular cancer has a good prognosis, it is less favorable than that of papillary carcinoma. The 10-year survival is better for patients with follicular carcinoma without vascular invasion than for those with vascular invasion.

Stage II follicular thyroid cancer

Stage II follicular carcinoma is defined as either a tumor that has spread distantly in patients younger than 45 years, or a tumor that is larger than 2 cm but 4 cm or smaller and is limited to the thyroid gland in patients older than 45 years. The presence of lymph node metastases does not worsen the prognosis among patients younger than 45 years. Follicular thyroid carcinoma must be distinguished from follicular adenomas, which are characterized by their lack of invasion through the capsule into the surrounding thyroid tissue. While follicular cancer has a good prognosis, it is less favorable than that of papillary carcinoma; the 10-year survival is better for patients with follicular carcinoma without vascular invasion than for patients with vascular invasion.

Stage III follicular thyroid cancer

Stage III is follicular carcinoma in patients older than 45 years, larger than 4 cm, and limited to the thyroid or with minimal extrathyroid extension, or positive lymph nodes limited to the pretracheal, paratracheal, or prelaryngeal/Delphian nodes. Follicular carcinoma invading cervical tissue has a worse prognosis than tumors confined to the thyroid gland. The presence of vascular invasion is an additional poor prognostic factor. Metastases to lymph nodes do not worsen the prognosis in patients younger than 45 years.

Stage IV follicular thyroid cancer

Stage IV is follicular carcinoma in patients older than 45 years with extension beyond the thyroid capsule to the soft tissues of the neck, cervical lymph node metastases, or distant metastases. The lungs and bone are the most frequent sites of spread. Follicular carcinomas more commonly exhibit blood vessel invasion and tend to metastasize hematogenously to the lungs and bones rather than through the lymphatic system. The prognosis for patients with distant metastases is poor.

Treatment

Because some cytological features of FTC are similar to those of the follicular variant of PTC on FNA biopsy, intraoperative frozen section can help differentiate PTC from FTC. Unfortunately, most of the time, it is very difficult on the frozen section as well. If the patient has only one nodule on one side of the thyroid that is suspicious for Follicular neoplasm, then a diagnostic thyroid lobectomy is indicated. Completion of total thyroidectomy as a second surgery and radioactive iodine (RAI) ablation are thus recommended for invasive FTC detected during the first lobectomy procedure. Thyroid lobectomy could be appropriate for minimally invasive FTC.

Prognosis

The prognosis of patients with minimally invasive FTC is good, and disease-free survival is similar to that of benign follicular adenoma. Similar to: The prognosis of patients with distant metastases or large tumors (>4 cm) is poor. Because of its tendency for hematogenous metastatic spread, 10% to 15% of patients will have metastases at the time of initial diagnosis. Lymph node metastases are rare, occurring in fewer than 10% of patients. Five-year survival rates for FTC have been reported to be between 82% and 92%, and ten-year survival rates have been reported to be between 67% and 90%.

References

Cooper DS, Doherty GM, Haugen BR, Kloos RT, Lee SL, Mandel SJ, Mazzaferri EL, McIver B, Pacini F, Schlumberger M, Sherman SI, Steward DL, Tuttle RM. Revised American Thyroid Association management guidelines for patients with thyroid nodules and differentiated thyroid cancer. American Thyroid Association (ATA) Guidelines Taskforce on Thyroid Nodules and Differentiated Thyroid Cancer, Thyroid. 2009 Nov;19(11):1167-214.

Mazafferi EL, Kloos RT. (2005) Carcinoma of follicular epithelium: radioiodine and other treatments and outcomes. In: Braverman LE and Utiger RD (eds.) The Thyroid, Lippincott Williams & Wilkins, Philadelphia, PA, p. 934-966.

DeLellis RA, Lloyd RV, Heitz PU et al. (2004) WHO Classification of Tumours, Pathology and Genetics of Tumours of Endocrine Organs. IARC Press, Lyon, p. 73-76.

Phitayakorn R, McHenry CR. Follicular and Hürthle cell carcinoma of the thyroid gland. Surg Oncol Clin N Am. 2006 Jul;15(3):603-23

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