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 carc inoma of the thyroid MI) remain controversial, often resulting in unnecessa ry treatment. METHODS. The records of 130 patients with minimally invasive (MI) follicula r thyroid carcinoma were retrieved from the files of the Endocrine Tumor Re gistry of the Armed Forces Institute of Pathology. RESULTS. Ninety-five patients were confirmed to have MI based on the author s' criteria of small-to-medium vessel invasion, capsular invasion of up to full thickness, no parenchymal tumor extension, and no tumor necrosis (pati ents with oxyphilic tumors were excluded). The remaining 35 patients had tu mors that were reclassified as "not low grade" based on large vessel invasi on, 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 pat ients). The mean tumor size was 2.8 cm. Histologic features examined for tu mor classification included cellularity, capsule nature, capsular invasion, vascular invasion, extension into parenchyma, cytoplasmic oxyphilia, mitot ic activity, and necrosis. All patients were treated with surgical excision . Adjuvant radioactive iodine therapy was performed in 24 patients. Five pa tients developed recurrent disease: four were alive or had died without evi dence of disease after additional treatment (mean, 18.1 years), and one pat ient died with disease (MI tumor) at 15.1 years. All of the remaining patie nts were disease free (mean follow-up, 16.5 years). CONCLUSIONS. There are reproducible histologic criteria to diagnose patient s with MI follicular carcinoma. The overall excellent long term prognosis a nd a good patient outcome suggests that no additional surgery is necessary. Published 2001 by the American Cancer Society.