Coexisting Papillary and Anaplastic Thyroid Cancer: Elucidating the Spectrum of Aggressive Behavior.

BACKGROUND: Anaplastic thyroid carcinoma (ATC) is a rare and lethal form of thyroid cancer. Overall prognosis is unclear when it arises focally in a background of papillary thyroid cancer (PTC). Clinicopathologic features and outcomes of tumors with coexisting PTC and ATC histologies (co-PTC/ATC) were categorized. METHODS: The National Cancer Database was queried for histologic codes denoting PTC, ATC, and co-PTC/ATC, defined as Grade 4 PTC, diagnosed from 2004 to 2017. Clinicopathologic features, OS, and treatment outcomes were analyzed by histologic type. RESULTS: A total of 386,862 PTC, 763 co-PTC/ATC, and 3,880 ATC patients were identified. Patients with co-PTC/ATC had clinicopathologic features in-between those of PTC and ATC, including rates of tumor size >4 cm, extrathyroidal extension, and distant metastases. On multivariable Cox proportional hazards modeling, age >55 years, Charlson-Deyo score ≥2, positive lymph nodes, lymphovascular invasion, distant metastases, and positive surgical margins were associated with worse OS, whereas radioactive iodine (RAI) and external beam radiation therapy (EBRT) were associated with improved OS, irrespective of margin status. OS was worse for co-PTC/ATC than for PTC but better than for ATC and differed based on the presence or absence of "aggressive" tumor features, including lymph node positivity, lymphovascular invasion, distant metastases, and positive surgical margins. CONCLUSIONS: Survival of patients with co-PTC/ATC is dependent on the presence of aggressive clinicopathologic features and lies within a spectrum between that of PTC and ATC. Adjuvant RAI and EBRT treatment may be beneficial, even after R0 resection.