Controversies in thyroid pathology: thyroid capsule invasion and extrathyroidal extension

Endocrine pathologists, surgeons, and oncologists who manage patients with thyroid carcinomas confront many critical dilemmas. Controversies surrounding diagnostic criteria that distinguish benign from malignant thyroid follicular lesions have been brought to the attention of this community. In this article, we confront another controversy, the definition of a thyroid "capsule" to clarify what constitutes extrathyroidal extension (ETE) and its clinical significance in the management of patients with differentiated thyroid carcinomas.

Our review of the anatomy of the thyroid gland confirms that this structure has no defined anatomical fibrous capsule. Moreover, the presence of adipose tissue within the thyroid gland and its pseudocapsule implies that thyroid tumor within fat tissue cannot be accepted as a criterion of ETE by that thyroid carcinoma. While invasion of skeletal muscle is a more reliable feature of ETE, at the isthmus, these fibers can be normally present within the gland, and this criterion does not have value. This implies that anatomical localization is a critical element in the determination of ETE. Clarification of such issues should be reflected in future revisions of the UICC/AJCC staging criteria to allow more rational management of patients with these increasingly common cancers.