A clinicopathologic study of minimally invasive follicular carcinoma of the thyroid gland with a review of the English literature.

BACKGROUND: The criteria for minimally invasive (low grade) follicular carcinoma of the thyroid (MI) remain controversial, often resulting in unnecessary treatment.
METHODS: The records of 130 patients with minimally invasive (MI) follicular thyroid carcinoma were retrieved from the files of the Endocrine Tumor Registry of the Armed Forces Institute of Pathology.
RESULTS: Ninety-five patients were confirmed to have MI based on the authors' criteria of small-to-medium vessel invasion, capsular invasion of up to full thickness, no parenchymal tumor extension, and no tumor necrosis (patients with oxyphilic tumors were excluded). The remaining 35 patients had tumors that were reclassified as "not low grade" based on large vessel invasion, extension into parenchyma, and tumor necrosis (oxyphilic cases excluded). The MI patients included 67 women and 28 men, ages 20-95 years (average, 42.0 years). Nearly all patients presented with a thyroid mass (n = 90 patients). The mean tumor size was 2.8 cm. Histologic features examined for tumor classification included cellularity, capsule nature, capsular invasion, vascular invasion, extension into parenchyma, cytoplasmic oxyphilia, mitotic activity, and necrosis. All patients were treated with surgical excision. Adjuvant radioactive iodine therapy was performed in 24 patients. Five patients developed recurrent disease: four were alive or had died without evidence of disease after additional treatment (mean, 18.1 years), and one patient died with disease (MI tumor) at 15.1 years. All of the remaining patients were disease free (mean follow-up, 16.5 years).
CONCLUSIONS: There are reproducible histologic criteria to diagnose patients with MI follicular carcinoma. The overall excellent long term prognosis and a good patient outcome suggests that no additional surgery is necessary. Copyright 2001 American Cancer Society.