Differential Diagnosis for Painless Swelling in the Posterior Triangle of the Neck

• Single most likely diagnosis
  • Ectopic thyroid: The presence of a cystic component in the swelling and the finding of complete follicular cells on FNA, along with a normal thyroid gland on ultrasound, suggests the presence of thyroid tissue in an abnormal location, which is consistent with ectopic thyroid tissue.
• Other Likely diagnoses
  • Metastatic cancer: Although less likely given the painless nature and the specific finding of follicular cells, metastasis from a primary thyroid cancer to a lymph node in the posterior triangle could present similarly. However, the absence of a known primary thyroid malignancy and the cystic nature of the swelling make this less likely.
  • Thyroglossal cyst: While thyroglossal duct cysts are typically midline, they can occasionally present off midline, including in the posterior triangle. The cystic component and the presence of follicular cells could fit this diagnosis, but the location is less typical.
• Do Not Miss diagnoses
  • Metastatic papillary thyroid cancer: This is a critical diagnosis not to miss because it has significant implications for treatment and prognosis. Even though the primary thyroid is reported as normal, the possibility of a small, undetected primary papillary thyroid cancer with metastasis to the lymph node cannot be entirely ruled out without further investigation, including a thorough examination of the thyroid and possibly additional imaging or biopsy.
• Rare diagnoses
  • Apparent thyroid (thyroid in a lymph node): This is a rare condition where thyroid tissue is found within a lymph node, often in the context of a benign process. It could be considered in the differential but is less common than ectopic thyroid tissue or other diagnoses listed above.
  • Other rare cystic neck lesions: There are various rare cystic lesions that could present in the neck, including branchial cleft cysts, which are typically anterior and not associated with thyroid tissue. These would be less likely given the specific findings of follicular cells on FNA.