A 58-year-old man is evaluated for a painless thyroid nodule that has enlarged over 2 months. He has no radiation exposure or family history of thyroid disease. Serum TSH is normal, and ultrasonography shows a 4-cm solid nodule in the right thyroid lobe. Fine-needle aspiration biopsy shows follicular cells dispersed in clusters and microfollicles. The patient undergoes right thyroid lobectomy. Which of the following histologic findings would confirm a diagnosis of follicular thyroid carcinoma?
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