Thyroid microcarcinoma: prevalence, prognosis, and management

To review the usual course of thyroid microcarcinoma (TMC) and the associated prognosis and treatment of affected patients.

We discuss predisposing factors in the formation of TMC and the modulation of its behavior, diagnostic evaluation, and management options.

TMC, generally defined as a well-differentiated thyroid cancer less than or equal to 15 mm in diameter, has an estimated prevalence (based on autopsy studies) of about 5 to 10%. Studies, however, have shown that most of these cancers are smaller than 5 mm in diameter. The high prevalence of TMC in the general population contrasts with the rarity of thyroid cancers of greater size, which constitute less than 1% of malignant neoplasms in the United States. The frequent detection of TMC as a result of routine imaging of the neck for unrelated reasons and as a incidental finding in surgical specimens has raised a question about whether the management of TMC should differ from that for thyroid cancer of appreciable size. The uncertainty about optimal management of TMC is attributable to the small number of long-term follow-up studies as well as the common observation that patients usually have an excellent prognosis. Although in most patients harboring a TMC the cancer remains quiescent and never becomes clinically significant, in some cases TMC can demonstrate an aggressive course. Several variables, such as older age, multifocality, bilateral disease, and extrathyroidal spread at initial assessment, may have some adverse prognostic significance. After a partial surgical removal of the thyroid gland for TMC, the recurrence rate may be as high as 11%. Therefore, a treatment dilemma is caused by the low propensity of TMC for progression to clinically significant disease, yet the potential for recurrence and aggressive behavior in some cases.

In general, surgical resection of TMC is based on results of fine-needle aspiration biopsy and the rate of growth of the nodule. Aggressive management seems indicated in high-risk patients, particularly older patients, those with a history of radiation exposure, and those with multifocal disease, bilateral disease, or lymph node involvement.