Letter to the Editor regarding squamoid pattern of anaplastic thyroid carcinoma, a poorly recognized entity.

To the Editor: In “Collision tumors of the thyroid: A case report and review of the literature,” Ryan et al describe an interesting case of an 88-year-old woman with airway compromise because of a thyroid mass. This patient also had widespread metastases on presentation. Surgical pathology demonstrated a papillary thyroid carcinoma and a squamous cell carcinoma. This case is a common scenario that we see at our institution in our Anaplastic Thyroid Cancer (ATC) Multidisciplinary clinic. The squamoid pattern of ATC is often confused with squamous cell carcinoma, delaying diagnosis and treatment. At our institution 21% (16/75) of our patients with ATC who presented between January 2013 and November 2015, were reported to have the squamoid pattern. The immunohistochemistry is usually negative for thyroid transcription factor-1 and thyroglobulin but the tumors express cytokeratin 5/6 and p63 consistent with a squamous phenotype. Additionally, we found that of the 14 patients with the squamoid pattern of ATC whose tumors were genotyped, 56% (9/14) harbored a BRAF mutation, consistent with progression from a welldifferentiated papillary thyroid cancer to ATC. Furthermore, the World Health Organization publication on the classification of endocrine tumors for squamous cell carcinoma of the thyroid specifically notes the tumor must be entirely composed of tumor cells with squamous differentiation and there should be no evidence of undifferentiated carcinoma or papillary carcinoma, both of which may show areas of squamous differentiation. Thus, we respectfully disagree with the conclusion from the authors that this patient had a collision tumor defined as “histologically distinct and morphologically independent neoplasms.” Rather, we believe this patient likely had widely metastatic ATC. Recent reports suggest that targeted therapies may be a viable treatment for patients with ATC, therefore, arriving at the correct diagnosis of ATC is essential to aid in therapeutic considerations.