Prognostic Characteristics of Primary Squamous Cell Carcinoma of the Thyroid: A National Cancer Database Analysis

Primary squamous cell carcinoma of the thyroid (ThySCC) is a rare cancer, primarily described only in case reports. We aimed to characterize the prognosis of ThySCC and compare its oncologic behavior to other thyroid malignancies.

The National Cancer Database was queried to identify patients with ThySCC, papillary, tall cell variant, poorly differentiated (PDTC), and anaplastic (ATC) subtypes of thyroid cancer treated from 2004 to 2015. Demographics, tumor characteristics, and treatments were compared by tumor type and assessed to identify independent predictors of overall survival (OS).

Of 123,684 patients included, 314 had ThySCC. ThySCC patients had a 5-year OS of 17.7%, more closely resembling ATC (8.3%) than PDTC or PTC. ThySCC and ATC patients had similar demographics, except ThySCC patients who were younger (68.0 vs. 70.2 years, p < 0.01). ThySCC tumors were smaller (mean 53.5 ± 45.7 vs. 69.5 ± 75.2 mm) with less frequent extrathyroidal extension (ETE) (64.0% vs. 73.8%), lymphovascular invasion (8.3% vs. 12.3%), and positive margins (22.0% vs. 28.3%) compared to ATCs (p < 0.05). ETE [HR 3.1 (95% CI 1.5-6.4), p < 0.05] and lymph node metastases [HR 2.2 (95% CI 1.2-4.0) p < 0.05] were independently associated with worse OS for ThySCC patients. Both ThySCC and ATC patients had similar surgical success rates, with R1 resection possible in 37.3% versus 35.0% of patients, respectively (p < 0.05). However, in contrast to ATC patients, adjuvant therapy was not associated with improved OS for ThySCC patients after complete resection.

ThySCC has a poor prognosis, similar to ATC, but with fewer aggressive features and no apparent survival benefit with adjuvant therapy after complete macroscopic surgical resection.