Abstract Introduction/Objective DICER1 mutations are associated with a host of benign and malignant neoplasms, of which the thyroid ranks among those with the highest prevalence. We present a case of a DICER1-mutant thyroid carcinoma with unusual morphologic features not described in the existing literature. Methods/Case Report A 29-year old 33 weeks pregnant female presented with swelling of the left neck. The patient subsequently underwent an ultrasound, which revealed a thyroid lesion measuring 6.2cm in greatest dimension. Labs including TSH and free T4 wree within normal 3rd trimester limits. Following an unremarkable delivery, FNA revealed loosely clustered cells with marked hyperchromasia and nuclear pleomorphism and variable microfollicular architecture in a colloid-containing background. Though definitively malignant, the anomalous cytomorphology of this neoplasm prompted molecular testing by Afirma Genomic Sequencing Classifier and Xpression Atlas next-generation sequencing, which was significant for a DICER1 hotspot mutation (p.E1705K c.5113G>A). Follow-up total thyroidectomy was pertinent for a tan-red, nodular mass involving 95% of the thyroid. Histomorphology revealed a neoplasm of primarily macrofollicular architecture with scattered microfollicles and intervening zones that appeared poorly-to-undifferentiated. Cells demonstrated bizarre hyperchromatic, pleomorphic nuclei with multinucleation and Azzopardi effect. Capsular invasion was identified. Inconsistent with a poorly-differentiated or anaplastic carcinoma, the mitotic rate and Ki-67 index were low at < 1/2mm2 and 1-2%, respectively. The tumor was consequently theorized to represent a follicular carcinoma with bizarre nuclei or other entity associated with DICER1 mutations not previously described. To date, no recurrence or metastasis has occurred. Results (if a Case Study enter NA) NA Conclusion We report an enigmatic morphology of a thyroid cancer associated with a DICER1 mutation. DICER1 mutations have been hitherto described in follicular carcinoma, papillary carcinoma, poorly-differentiated carcinoma and thyroblastoma, none of which are consistent with the present case. Thus, we conjecture that this morphomolecular profile may constitute a "follicular carcinoma with bizarre nuclei" or other novel entity not previously reported. Further investigation should be pursued to determine the nature of this entity, its pathophysiology, epidemiology, possible association with germline DICER1 syndrome, and relationship between morphology and prognosis in patients with DICER1 mutations.
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