Squamous differentiation in papillary thyroid carcinoma: a rare feature of aggressive disease.

BACKGROUND: Papillary thyroid carcinoma with squamous differentiation (PTC-SD) is a poorly understood pathologic finding of unknown clinical significance. Selected case reports have suggested that PTC-SD is an aggressive tumor with a poor prognosis. Here we present the largest case series of PTC-SD reported in the United States. MATERIALS AND METHODS: The cancer registry at our tertiary care referral center was reviewed to identify all patients from 1995-2015 who had been diagnosed with PTC-SD on initial total thyroidectomy or lymph node dissection for recurrent disease. All cases were reviewed by an endocrine pathologist to confirm the diagnosis. Patient demographic, pathology, and outcomes data were collected and reviewed. RESULTS: During the study period, ten patients were diagnosed with PTC-SD, six in the primary tumor at the time of initial surgery, and four in lymph node metastases during surgery for recurrent disease. The median age at diagnosis was 56 y and half of the patients were male. Aggressive features such as multifocality (67%), extrathyroidal extension (67%), positive margin (89%), lymph node metastases (80%), and extranodal extension (60%) were far more prominent than is typically seen in classic PTC. Long-term follow-up (median 56.5 mo) demonstrated high rates of locoregional recurrence (60%), pulmonary metastases (30%), and mortality (10%). CONCLUSIONS: Squamous differentiation is a rare finding in PTC that is associated with aggressive pathologic features and poor long-term outcomes. This phenomenon may represent a step in progression toward dedifferentiation; thus, patients with PTC-SD should have close, life-long surveillance and should be treated according to evidence-based guidelines for high-risk thyroid cancers.