Papillary microcarcinoma of the thyroid: a clinico-pathologic and prognostic review

Papillary microcarcinoma (PMC) of the thyroid is the most common form of thyroid cancer, which usually remains clinically silent until its incidental histologic diagnosis in autopsy or surgical material. Autopsy incidence varies from 3%-36%. PMC may, however, present with clinical symptoms, the commonest of which is cervical lymphadenopathy with or without palpable thyroid nodules. Other reported presentations include cystic neck mass, pulmonary mass (es), metastases in the skull or vertebral column. The upper limit of size to define PMC is 10 mm in most studies but many studies include lesions up to 15 mm in diameter. Histologic variants include encapsulated and partially encapsulated papillary carcinoma, circumscribed microcarcinoma and occult sclerosing carcinoma. Younger age and size less than 10 mm (< 15 mm in other studies) are considered to be favorable prognostic factors. Size alone, however, cannot be regarded as a determinant of prognosis. Older age, larger tumor size, distant metastases, capsular invasion and multifocality indicate unfavorable prognosis. Loss of heterozygosity (LOH) is an infrequent finding, since small deletions may be missed by southern blot analysis. Activation of oncogenes ret and trk have been reported in papillary carcinoma. Some authors advocate conservative while others favor aggressive therapy including total thyroidectomy with or without Iodine 131ablation. Additional investigative techniques are needed to identify the subset of PMC cases with a potential for aggressive clinical course, thereby targeting more aggressive therapy to an appropriate subset of tumors.