Answer: Extra-capsular extension of benign thyroid tissue

Histology: The thyroid gland showed numerous variably sized hyperplastic nodules throughout both lobes. This section taken from the isthmus demonstrates irregular nodules extending deeply into skeletal muscle. There is no associated desmoplastic stromal reaction. The lesion demonstrates a follicular growth pattern throughout. No papillary formations are identified. At high power, the cells lining the nuclear lack significant cellular atypia, and the nuclear features of papillary carcinoma (e.g. nuclear pallor, nuclear elongation with irregular contours, etc…) are not well developed.

Discussion: The thyroid is invested by a thin fibrous capsule. The notion that this capsule is complete and intact has been challenged by studies that have addressed the integrity of the capsule at the microscopic level. The presence of nodular aggregates of thyroid follicles within the pericapsular connective tissue is a frequent finding (90% of thyroids examined); and in 7% of thyroidectomies, these nodular aggregates occur within strap muscle attached to the pyramidal lobe.

As follicular thyroid carcinoma is defined by invasive tumor growth, the presence of nodular aggregates of benign thyroid follicles within strap muscle is a confusing finding that may easily trigger an erroneous diagnosis of malignancy. In contrast to invasive carcinoma, these benign aggregates do not elicit a stromal reaction, and they generally have a normal microscopic appearance. In effect, the presence of thyroid tissue outside of the thyroid gland is not, by itself, diagnostic of malignancy, even when it occurs within the strap muscles of the neck.