Clinically unsuspected papillary microcarcinomas of the thyroid: a common finding with favorable biology?

Background: The purpose of this study was to describe the incidence and clinical/pathologic characteristics of papillary thyroid microcarcinoma (PMC) in a community hospital setting and to evaluate the frequency and characteristics of these lesions when unsuspected preoperatively.

Methods: A total of 723 patients underwent a partial or total thyroidectomy. A retrospective review was performed.

Results: A total of 194 of the 723 patients had a final diagnosis of papillary carcinoma. Ninety-six (49%) of these tumors were PMCs defined as being 1.0 cm or less in diameter. One third (32 of 96) of these lesions were multifocal and 16.7% (16 of 96) were found to have regional lymph node metastases. The majority (58%) of PMCs were found on final pathology and were clinically unsuspected (occult). Multifocality was found in 32.1% (18 of 56) of patients with clinically unsuspected PMC, with nodal metastases in 3.6% (2 of 56). The other 40 patients with PMC had surgeries performed for a clinical reason related to that pathologic lesion. This clinically suspected group was comparably multifocal (35%), but more likely to have cervical lymph node metastasis (35%). Sixty-six percent (37 of 56) diagnosed with a clinically unsuspected PMC underwent a partial thyroidectomy at the initial surgery.

Conclusions: The prevalence of clinically unsuspected PMC in our population undergoing thyroidectomy was 7.7% (56 of 723). In our institution, this is more than half of all PMCs. The incidence of cervical lymph node metastasis in clinically unsuspected PMC was only 3.6% compared with 35% in clinically suspected disease, suggesting that the biological behavior (and possibly treatment) may be different. Long-term follow-up evaluation is needed to better evaluate the significance of these differences.