Clinical and Histologic Features of Micropapillary Thyroid Carcinomas: Do They Metastasize?

The incidence of thyroid carcinomas ranges from 0.5 to 10 cases per 100,000 worldwide. By the age of 80 years, approximately 60% of the population has a thyroid nodule. With the growing use of ultrasound guided fine needle aspiration, more and more of these nodules are being biopsied. Correspondingly, there has been an increase in the diagnosis of papillary thyroid microcarcinomas. This term is defined by the WHO as a lesion less than 1 cm that is found incidentally. The treatment and prognostic significance of micropapillary carcinomas is debatable. The current AJCC cancer staging manual classifies lesions under 2 cm (not 1 cm) as T1. A recent study from the University of Ferrara in Italy by Roti et al examined the incidence and prognosis of micropapillary thyroid carcinoma. They performed a retrospective study over a 9-year period (1993 to 2002) of 615 consecutive cases of thyroid carcinoma of which 39.5% were micropapillary carcinomas less than or equal to 1 cm in greatest dimension. The majority of the papillary microcarcinomas were suspected from previous fine needle aspiration. However, 52 cases were incidental findings in the course of 1265 cases of thyroidectomy for various other reasons including Graves’ disease, nontoxic nodular goiter, and toxic adenoma. Follow-up time ranged from 2.4 to 10.6 years with a median of 5.1 years. Examinations were performed at 3, 6, and 12 months and then yearly, unless otherwise indicated owing to the clinical course. Treatment included total or neat total thyroidectomy (92% and 8%, respectively) with subsequent radioiodine therapy in 235 patients. The dosage ranged from 50 to 100mCi. There were no deaths from disease in the 243 patients during the follow-up period. Other end points that were followed included regional lymph node and distant metastasis. The most notable findings included lymph node metastasis in 32 patients (13%) and distant metastasis in 4 patients (1.6% of the patients). Two of the patients with lymph node metastases had a tumor diameter less than 5mm, whereas the 4 patients with distant metastases had tumor diameters of 8mm or greater. The cases with metastases to the lung and mediastinum included only those in which the primary tumor was equal to or greater than 8mm. These 4 patients had at least 2 of the characteristics typically considered as risk factors for progression or persistent disease; multifocal, bilateral, capsular extension, positive lymph nodes at the time of resection, or sclerosing variant of papillary thyroid carcinoma. An earlier study performed by the New York University Medical Center and the University of Pennsylvania Medical Center involved 39 cases of thyroid nodules measuring 1 cm or less which had fine needle aspiration. Of the 39 lesions tested by FNA, 22 were interpreted as papillary thyroid carcinoma, 13 as suspicious for papillary carcinoma, and 4 as medullary carcinoma. Total thyroidectomy was performed in 25 patients and loboisthmusectomy in 14. Subsequent histology showed 35 cases of papillary thyroid carcinoma and 4 medullary thyroid carcinomas. Whether or not the patients received further treatment with radioactive iodine or radiation therapy was not indicated. Among the papillary carcinomas in this cohort 11 cases were found to be multifocal, 3 of these also had lymph node metastasis. Lymph node metastasis was also identified in 5 other cases, so a total of 8 cases (20%) had lymph node metastases. Distant metastasis (pulmonary) was identified in a single case of 39 cases (2.6%). These studies show that lymph node metastasis can occasionally be seen in papillary thyroid carcinomas greater than 0.5 cm and distant metastasis may rarely be seen in cases in which the primary lesion is under 1 cm.