Common Histologic Type
Diagnosis is based on the presence of follicular differentiation with papillary and/or follicular structures and typical nuclear features. Nuclei are larger than normal and overlap. They may be fissured, with hypodense chromatin, and irregularly bordered with cytoplasmatic invagination (11).
Papillary thyroid cancers appear as firm, unencapsulated, or partly encapsulated tumors. Extension beyond the capsule has been reported in 8% to 32% of the cases (9). PTCs may be partly necrotic and some undergo cystic degeneration. PTC is often multifocal when it occurs in a single thyroid lobe and occurs in bilateral lobes in 20% to 80% of the cases (12). Lymph-node metastatic involvement is found in 15% to 80% of the cases depending on the extent of lymph node dissection and examination of the surgical specimen. From the ipsilateral thyroid lymph node chains they may spread to more distant lymph node groups to the upper mediastinal nodes. Vascular invasion is rare and distant metastases (mostly to lungs) are observed in 5% to 10% of the cases, mostly as a result of lymphatic spread (11). Lymph node metastases as the first clinical finding in patients with PTC are not uncommon. The mean 10-year survival rates in PTC are 80% to 90%, with optimal treatment.
Papillary Thyroid Cancer Variants
Encapsulated variant: A tumor capsule similar to that of an adenoma is present but with focal invasion. It has a good prognosis.
Follicular variant: There is total predominance of follicles over papillae. This type is found in young patients and the clinical behavior is similar to pure PTC (13,14).
There are three follicular carcinoma subtypes:
Diffuse follicular variant—this subtype is more aggressive than the others with frequent lung metastases (15,16).
Thyroid diffuse sclerosing variant: This cancer is characterized by diffuse involvement of one or both thyroid lobes, widespread lymphatic permeation, prominent fibrosis, squamous metaplasia, psammoma body formation, and lymphoid infiltration. Lymph-node metastases are almost always present and lung metastases are frequent. It occurs more frequently in children and young adults (17).
Tall cell variant: This tumor is characterized by papillae structures covered with cells that are twice as tall as they are wide. Tumors are usually large, extend beyond the capsule, show frequent vascular invasion, lymph node and distant metastases. It occurs in elderly patients (18).
Columnar cell variant: This is a more rare tumor. It is characterized his-tologically by prominent nuclear stratification of elongated cells.
Hurthle cell variant: This is another rare tumor type. It is characterized by papillary structures lined with histologically distinct oxyphilic cells.
In general, the tall and columnar cell variants are more aggressive forms of thyroid papillary cancer.
According to the World Health Organization (WHO) classification, a PTC measuring 1.0 cm or less in diameter is identified as a microcarcinoma. It has a very good prognosis and grows very slowly (19). It may present as a small classic papillary carcinoma or may appear as an unencapsulated sclerotic nodule of a few millimeters in diameter that infiltrates the thyroid tissue ("occult carcinoma"). The microcarcinoma is usually detected as an incidental nodule at ultrasonography or by surgical histopathology in the thyroid resection specimen.
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