Squamous cell carcinoma involving thyroid gland is an extremely rare condition. To label a thyroid tumor as squamous cellcarcinoma the tumor should be entirely composed of tumor cells with squamous differentiation. This condition should be differentiated from:
Papillary carcinoma thyroid containing patches of sqamous epithelium
Anaplastic carcinoma of thyroid containing patches of squamous elements
Adenosquamous carcinoma thryoid which may contain both adeno and squamous elements.
Causes of squamous cell carcinoma thyroid:
Primary squamous cell carcinoma thyroid
Secondary involvment of thyroid gland by tumor extention from adjacent structures
Metastatic involvmement of thyroid gland from adjenct sites like lungs, head and neck, GI tract
Primary squamous cell carcinoma involving thyroid gland is an extremely rare condition affecting about 1% of all the primary thryoid malignancies. Histologically the thyroid gland does not normally contain squamous epithelium that is the reason why this condition is pretty rare. Several hypothesis have been proposed to explain the genesis of squamous cell carcinoma in the thyroid gland.
Goldberg & Harvey theory:
This is one of the earliest theories proposed to account for the development of squmaous cell carcinoma in the thyroid gland. This theory is based on the concept that embryonic remnant of thyroglossal cyst contained squamous elements. Under normal circumstances thyroglossal duct involutes. Persistence of this duct and the related squamous elements happens to be the crux of this theory. Anatomically the lower portion of thyroglossal duct happens to be the pyramidal lobe of thyroid gland. If this theory is true then squamous cell carcinoma of thyroid gland should commonly involve the pyramidal lobe. In reality this tumor is commonly seen in the lateral lobes of thyroid thus puts a question mark on the validity of this theory.
Branchial arch theory:
Branchial arch elements which include ultimobranchial body and thymic epithelium could probably be the source for squamous elements in the thyroid gland.
This happens to be the more recent one. It says that squamous elements seen in the thyroid gland could have arisen due to metaplasia of normal thyroid cells. Even this theory has its own achilles tendon. The commonest cause for squamous metaplasia in thyroid gland happens to be Hashimoto's thyroiditis. Studies have shown that it is rare for these patients to develop squamous cell carcinoma of thyroid gland.
Gulisano suggested that squamous elements could reach the thyroid gland as direct invasion from adjacent areas like larynx, pharynx and oesophagus. He demonstrated this theory by injecting methylene blue dye in the pyriform fossa before performing thyroidectomy. Superior portion of the thyroid gland was found stained by the dye. This demonstrates that there is a communication between these areas for potential tumor spread.
Clinically these tumors are very aggressive, infact as aggressive as anaplastic carcinoma. These patients usually present with rapidly increasing neck mass with evidence of involvment of strap muscles, tracheal compression and oesophageal compression.
Role of Imaging:
Allows differentiation of thyroid mass from other neck lumps
Helps in assessing adjacent organs like larynx and oropharynx
Assess oesophageal and tracheal involvement
Is very poor because of its radioresistance.
Total thyroidectomy followed by irradiation is the accepted modality. Patients with secondary deposits in the cervical nodes should undergo neck dissection procedures.