Incidental papillary microcarcinoma of the thyroid--further evidence of a very low malignant potential: a retrospective clinicopathological study with up to 30 years of follow-up.

Background: Despite the frequent occurrence of papillary microcarcinoma (PMC) of the thyroid, no consensus on its malignant potential or its treatment exists. The objective of this study was to analyze the clinicopathological characteristics of a retrospective cohort of consecutive patients with PMC treated in a single institution during a 30-year period and to study the incidence rates of PMC in all patients operated on for thyroid diseases during this period.

Methods: Demographic data, clinical characteristics, histological workup of the resected glands, histopathological features, treatment, and follow-up data were studied.

Results: Between 1975 and 2004, [corrected] 759 PMCs were detected in 28,197 patients who received thyroidal surgery. The detection rate of PMC was significantly determined by the accuracy of the histological examination. Only 5 patients with PMC presented with clinically apparent lymph node metastases and 754 were incidental PMCs. Mean follow-up was 88±2 (range, 11-639) months. Only three patients experienced recurrence in cervical lymph nodes, all of whom presented with clinically suspect cervical nodes. No patient died of disease. Clinically apparent lymph node metastases and stage pT3 were significantly associated with recurrence.

Conclusions: Incidentally detected PMC, even when multifocal, is a biologically indolent tumor that seldom if ever progresses. In contrast, clinically occult PMC detected due to clinically suspected and histological confirmed lymph node metastases or extrathyroidal growth may show a more aggressive course with disease recurrence and an eventual poorer prognosis.