Focal High-Grade Areas with a Tumor-in-Tumor Pattern: Another Feature of Pediatric DICER1-Associated Thyroid Carcinoma?

In the thyroid gland, during childhood or adolescence, DICER1-driven tumors include differentiated follicular thyroid carcinoma and, more rarely, poorly differentiated carcinoma. 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. In two patients, the high-grade component also demonstrated the presence of CHEK2 p.(Tyr390Cys) likely pathogenic variants, suggesting a role for this gene and more generally for the ATM-CHECK2-TP53 pathway as a mechanism of malignant progression of DICER1-associated thyroid carcinomas. One of these two patients presented lymph node recurrence 8 months after surgery. An immunohistochemical study was also performed to explore the possible contribution of anti-DICER1 antibodies as well as thyroglobulin, Ki-67, p53, and PRAME in characterizing these tumors. DICER1 proved to be strongly expressed in mutated tumors compared to a control cohort (p < 0.001), deserving further validation to define its possible diagnostic role. Finally, well-demarcated ischemic-like areas with ghost cells embedded in a thick hyaline stroma (atrophic changes) were found within four tumors, whereas bunches of ectatic macrofollicles lined by flattened epithelium (involutional changes) were only detected in the background thyroid parenchyma of patients with germline DICER1 variants. These morphological features may alert pathologists to suspect a somatic and/or germline DICER1 alteration.