Answer: Benign thyroid inclusion

Histology: A 2 mm lymph node is found to harbor a microscopic cluster of follicles. The follicles contain pink thyroglobulin-like material, and they are lined by relatively bland cuboidal cells that lack overt changes of malignancy. Psammoma bodies and papillary formations are not present.

Discussion: Non-indigenous cells are sometimes unexpectedly encountered during histologic evaluation of lymph nodes. Their presence can generally be attributed to aberrant migration of benign tissues during embryogenesis (e.g. epithelial rests, nevocellular inclusions), neoplastic transformation of resident cells (e.g. malignant lymphoma) or metastatic spread from some clinically silent malignancy. Discerning the true nature of thyroid follicles has proved particularly difficult when it is encountered laterally in the neck. The nature and significance of so-called “laterally aberrant thyroid tissue” has generated considerable debate. Some believe that the presence of thyroid tissue in cervical lymph nodes, no matter how microscopically banal, invariably signifies metastatic spread and warrants rigorous patient management. Others believe that non-neoplastic thyroid tissue can be ectopically displaced into the lateral neck, and in the absence of cellular atypia this finding is of no clinical significance whatsoever. In a systematic autopsy investigation, microscopically normal thyroid follicles within lateral cervical lymph nodes were discovered in 5% of autopsied patients, even though there was no synchronous primary cancer in the ipsilateral thyroid lobe in any of the case.

Rosai et al. suggests several criteria that may be useful in discriminating metastatic foci from these benign thyroid inclusions. Intranodal thyroid tissue may be regarded as a possible benign inclusion when:
1) it is limited to a few small follicles in a single lymph node;
2) it is located in or immediately beneath the nodal capsule; and
3) it does not demonstrate any cytoarchitectural features of papillary thyroid carcinoma.

In the present case, the extension of thyroid follicles into the substance of the lymph node would argue for a metastatic implant.

Importantly, these incidental metastatic implants rarely progress to the point that they become clinically apparent. When detection of these metastatic implants does prompt a rigorous investigation of the primary cancer (e.g. thyroid resection), the primary thyroid cancers tend to be trivial tumors. They tend to be confined to the thyroid, small (<1cm) or totally regressed. Given the apparent limited potential of these indolent neoplasms to progress to clinically overt disease, rigorous treatment is not compulsory although additional staging and surveillance studies (e.g. ultrasound and/or computerized tomography) may be indicated to monitor disease progression.