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

Abstract

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.

Keywords: CHEK2; DICER1; High-grade; Immunohistochemistry; Pediatric thyroid cancer; Poorly differentiated thyroid carcinoma; Thyroid; Tumor-in-tumor.

Fig. 1

Patient 1. Hematoxylin and eosin (H&E) stain. Well-differentiated thyroid carcinoma (tumor IA) with microfollicular architecture and an 8-mm intranodular area (arrow, A) with solid growth and increased mitotic activity (B, C; arrows in C indicate mitoses). In the contralateral lobe, an 11-mm tumor (IB) inside which there was a 5-mm tumor-in-tumor pattern area (arrow, D) with solid growth and increased mitotic activity (E, F; arrows in F indicate mitoses). Atrophic changes were found in the tumor as well as in nodular-shaped areas (asterisk) in the non-lesional parenchyma of both lobes (D)

Fig. 2

Molecular analysis of tumors IA and IB, with sub-analysis of the low- (top) and high-grade (bottom) components for both tumors

Fig. 3

Molecular analysis of tumor II: from top to bottom, analysis of low- and high-grade component, respectively

Fig. 4

Molecular analysis of tumors IIIA and IIIIB, with sub-analysis of the low- (top) and high-grade (bottom) components for both tumors

Fig. 5

Patient 4. H&E stain. Tumor IVA. A 43-mm paraisthmic tumor showed a 5-mm intranodular area (arrow, A), from which capsular invasion started (arrow, B). Higher cellularity and microfollicular architecture (C) characterized the intranodular area. Tumor IVB was surrounded by a thick calcific capsule (D), with evident capsular and vascular invasion (arrow, E) and solid/trabecular/insular architecture (F)

Fig. 6

Molecular analysis of tumor IVA: from top to bottom, analysis of low- and high-grade component, respectively

Fig. 7

DICER1-immunohistochemistry in DICER1-associated tumors. DICER1 expression was widespread and intense with stronger stain in the high-grade component (A, B) compared to the low-grade component (C). Tumor IVB showed uniform, intense (4+) and diffuse positivity (D, E). Faint positivity was observed in follicles within the atrophic changes (F)

Fig. 8

Atrophic changes. H&E stain. They are essentially an intranodular feature and represent ischemic-like, intensely eosinophilic areas, usually localized in the subcapsular zone (A) or more deeply within the tumor (B). In patients with DICER1 germline pathogenic variant, atrophic changes can also be observed as additional nodules besides the main tumor (C, D); sometimes, a non-atrophic component is still recognizable (E). At higher magnification, atrophic changes are characterized by “ghost follicles” lined by flattened thyrocytes immersed in a hyaline stroma (F)

Fig. 9

Involutional changes. H&E stain. Thyroid outside the main lesions of patient 1 showed a hyperplastic nodule and peripheral subcapsular rim with slightly ectatic follicles (arrows, A). Higher magnification of the areas indicated by the black (B) and red (C) arrows shows these peripheral dilated macrofollicles, lined by flattened thyrocytes. Similar findings were also observed in patient 3, with an emphysematous-like pattern (D). A higher magnification of the spot indicated by the black arrow (E) and another section from the same patient (F)

Fig. 10

Schematic representation of the background thyroid of patient with germline DICER1 pathogenic variants. Atrophic changes (A) assume a nodular configuration; a residual hyperplastic component is depicted in the nodule on the right, while a more marked atrophy affects the nodule on the left. Involutional changes (B) are ectatic follicles lined by flattened epithelium; if present at the periphery (arrows), they impart an “emphysematous-like” configuration which slightly deforms the outline of the section. A nodule with papillary centripetal growth and cystic component (asterisk) is also shown in the same plane of section