MKI67 and neoplasm: Herein, we describe the features of DICER1-associated thyroid carcinoma with the presence of high-grade areas within a differentiated tumor in four patients (median age 12.5 years, range 6–15 years), three of them carrying germline pathogenic variants of DICER1. A new tumor-in-tumor pattern characterized by intratumoral nodules with a higher histological grade (increased mitotic activity/Ki-67 and solid/trabecular/insular and/or microfollicular architecture) was detected in these DICER1-associated tumors.