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A 22-year-old woman is discovered to have a nontender thyroid nodule that is found to be a cold nodule on a radioisotope scan. The pathology report from a fine needle aspiration of the nodule describes a follicular neoplasm, the differential of which includes a benign follicular adenoma and a malignant follicular carcinoma. Which is the single best histologic criterion to differentiate between these two neoplasms?