Macrofollicular Architecture in Invasive Encapsulated Follicular Variant of Papillary Thyroid Carcinoma: A Pitfall in Thyroid Practice

Abstract

Background: Predominantly macrofollicular architecture in invasive encapsulated follicular variant of papillary thyroid carcinoma (IEFVPTC-MF) is rare and often a cause of misinterpretation during pre-operative work-up and histopathology evaluation. We comprehensively evaluated the radiological, cytological, gross, microscopic, molecular and follow-up characteristics of four such cases, intending to increase its recognition and add our experience to the limited literature available.

Methods: All such histopathologically-proven cases of IEFVPTC-MF were retrieved from the departmental archives. The clinical details, thyroid ultrasound, cytology and thyroid scan findings were reviewed. Allele-specific PCR for BRAF p.V600E, KRAS, NRAS, and HRAS mutations, and FISH assays for ETV6::NTRK3 fusion and RET fusions were performed.

Results: There were four cases of IEFVPTC-MF diagnosed between 2021 and 2022, involving two males and two females. The median age at presentation was 27 years, and the duration of the disease was 1-10 years. Thyroid ultrasound was TR1 (benign; n = 1), TR2 (not suspicious; n = 2), or TR4 (moderately suspicious; n = 1). Cytology was categorized as nondiagnostic (n = 1), benign (n = 1), and atypia of undetermined significance (n = 1). The three nodules with available cytology smears showed abundant colloid. Cells were arranged as sheets/microfollicles/clusters. Nuclei were predominantly round with minimal/focal elongation, membrane irregularity, and cellular crowding. On gross examination, cut surfaces of the tumors showed variable amounts of colloid. The tumors were solid-cystic. Histopathology revealed partially encapsulated multinodular tumors. There were prominent pseudopapillae projecting into the lumina of macrofollicles. Nuclei were predominantly round with variable nuclear atypia, including chromatin clearing and multifocal presence of nuclear grooves. Pseudoinclusions were identified in two. Molecular analysis revealed NRAS codon 61 mutation and ETV6::NTRK3 fusion in one case each. Two patients had cervical lymph node and hematogenous metastases. Post-radio-active iodine, the response was structurally incomplete (n = 2), indeterminate (n = 1) and excellent (n = 1).

Conclusions: Macrofollicular architecture in invasive encapsulated follicular variant of papillary thyroid carcinoma is a major pitfall in thyroid oncology practice. Long-standing disease, and ultrasonographic and cytological features that overlap with benign disease, often lead to underdiagnosis during pre-operative evaluation. As patients may consequently develop distant metastases and have inadequate treatment response, there is a need for more vigilant understanding of the spectrum of macrofollicular thyroid disease for accurate diagnosis. ETV6::NTRK3 or other fusions, when found, present opportunities for targeted therapy.

Keywords: Cytology; IEFVPTC; Macrofollicular invasive encapsulated follicular variant of papillary thyroid carcinoma; Molecular; Radiology; Thyroid cancer.

Fig. 1

Thyroid Ultrasound images of the cases: a Case 1- a solid-cystic lesion having focal spongiform pattern and scattered microcalcifications. b Case 2- a predominantly cystic lesion with few septations, lacking high-risk features. c Case 3- a solid-cystic lesion that is isoechoic with the surrounding thyroid parenchyma and having an incomplete halo. The lesion is wider than tall and does not show any high-risk features

Fig. 2

Cytological features of the cases: a A tight fragment of follicular epithelial cells along with cyst macrophages (arrow) in a colloid-rich background (400X, May Grunwald-Giemsa; case 2). b A sheet of follicular cells showing nuclear crowding, focal elongation and occasional grooves (400X cropped and enlarged, May Grunwald-Giemsa; case 2). c Dyscohesive clusters of follicular cells having round but mildly enlarged nuclei (200X, May Grunwald-Giemsa; case 3). d Follicular cells arranged as microfollicles and dyscohesive clusters having round, mildly enlarged nuclei with minimal overlapping (400X cropped and enlarged, May Grunwald-Giemsa; case 3)

Fig. 3

Macroscopic features of the cases: a Spongy gross appearance with large colloid-filled cysts of variable sizes, separated by fibrous septae (Case 1), b A multinodular tumor showing large colloid-filled cysts and solid grey-white haemorrhagic areas (Case 2), c A large nodular and heterogenous tumor. The nodules have a glistening, translucent cut-surface due to abundant colloid, resembling thyroid follicular nodular disease (Case 3)

Fig. 4

Histopathological features of the cases: a Multinodular growth pattern with tumor nodules separated by fibrous septae (40X, Hematoxylin & Eosin; Case 4), b Microscopic image depicting a macrofollicle measuring 1352.46 µm (40X, Hematoxylin & Eosin; Case 3), c Focal pseudopapillary arrangement of tumor cells (100X, Hematoxylin & Eosin; Case 4), d Diagnostic nuclear features of PTC including elongation, crowding, overlapping, chromatin clearing and occasional grooves, more evident in central portion of the image (arrow) (400X, Hematoxylin & Eosin; Case 3), e Predominantly round nuclei but prominent intranuclear inclusion in case 1 (arrow) (400X, Hematoxylin & Eosin; Case 1), f Nodular infiltrative edge of tumor (arrow) (100X, Hematoxylin & Eosin; Case 4), g Lymph node with metastatic disease (40X, Hematoxylin & Eosin; Case 2)

Fig. 5

Molecular characteristics of the cases: a Graph showing NRAS mutation in case 2 (arrow), as detected by allele‐specific real‐time polymerase chain reaction, b Fluorescence in situ hybridization assay, performed using dual fusion probes, showing ETV6::NTRK3 gene fusion as depicted by fused green and red signals in case 4 (arrows), c Fluorescence in situ hybridization assay, performed using break-apart probes, showing two fused red and green signals per nucleus, indicating lack of RET gene rearrangements (case 3)