Cytomorphologic features of NTRK-rearranged thyroid carcinoma

Abstract

Background: NTRK-rearranged thyroid carcinomas (NRTC), though rare, harbor a potential therapeutic target. The cytomorphologic features by fine needle aspiration (FNA) and the utility of preoperative molecular testing for NRTC remain largely uncharacterized. We provide a detailed cytomorphologic analysis of an institutional NRTC cohort with clinical, radiologic, histopathologic, and molecular correlations.

Methods: Our NRTC FNA cohort included 21 specimens from 19 patients. The mean age and female-to-male ratio were 42 years and 2.2:1, respectively. Predominantly alcohol-stained Papanicolaou smears and liquid-based preparations were reviewed for 14 patients with available materials, and histologic review of subsequent resections was conducted for all 19 patients. Imaging and clinical data were accessed through electronic medical records.

Results: Sonographically, NRTC were hypoechoic (87%), predominantly solid (53%) with limited central vascularity (27%), ill-defined borders (67%), and microcalcifications (67%). Observed cytomorphologic features include mixed architectural patterns (79%), fibrosis (93%), oncocytic and vacuolated cytoplasm (36% and 43%, respectively), and abundant intranuclear pseudoinclusions (14%). Most NRTC FNAs were classified as suspicious for malignancy or malignant (89%). One case classified as atypia of uncertain significance underwent ThyroSeq sequencing where a NTRK1 fusion was identified.

Conclusion: Although NRTC did not show a consistent cytomorphologic signature, mixed architectural patterns, prominent fibrosis and distinct cytoplasmic or nuclear features should raise suspicion for NRTC and, when accompanied by negative BRAFV600E by immunohistochemistry on cell block material, aid in selecting cases for molecular testing. This algorithmic approach may help identify potential NRTC, maximizing treatment options for patients, especially in patients for whom treatment planning is complicated.

Keywords: FNA; NTRK; cytomorphology; histology; kinase; thyroid; thyroid cytology.

Figure 1.

Distinct mixed architectural patterns in NTRK-rearranged thyroid carcinomas. (A, D) Mixed papillary (A, white arrow) and micropapillary (A, black arrow) patterns in NTRK-related thyroid carcinoma (NRTC) with TPR-NTRK1 on fine needle aspiration (FNA) (Papanicolaou stain, original magnification ×200) and its corresponding histology (D, black and white arrows; hematoxylin and eosin [H&E] stain, original magnification ×100). (B, E) Unusual squamoid morule-type structures in NRTC with TPR-NTRK1 on FNA (B, black arrow; Papanicolaou stain, original magnification ×200) and possible corresponding histology (E, black arrow; H&E stain, original magnification ×200). (C, F) Micropapillary/small crowded group pattern in several NRTCs and shown here in an ETV6-NTRK3 NRTC FNA (C, Papanicolaou stain, original magnification×100), and the corresponding histologic correlate (F, black arrow; H&E stain, original magnification ×100).

Figure 2.

Fibrotic fragments are prominent in NTRK-rearranged thyroid carcinomas. Representative examples of fibrotic fragments from case 16 (A, TPM3-NTRK1) and case 14 (B, SQSTM1-NTRK1) (Papanicolaou stain, original magnification ×40 and ×200, respectively) are shown with their corresponding histologic features (C, D; hematoxylin and eosin stain, original magnification ×20).

Figure 3.

Distinct cytoplasmic features noted in few NTRK-rearranged thyroid carcinomas. (A, D) Microvacuolated and oncocytic cytoplasm noted in a SQSTM1-NTRK1 fusion NTRK-related thyroid carcinoma (NRTC) (A, black arrow; Papanicolaou stain, original magnification ×600) and its histologic correlate (D, black arrow; hematoxylin and eosin [H&E] stain, original magnification ×400). (B, E) Microvacuolated and oncocytic cytoplasm noted in a SQSTM1-NTRK3 fusion NRTC (B, Papanicolaou stain, ×200 original magnification [inset: SurePathpreparation, original magnification ×600]) and its histologic correlate (E, black arrow; H&E stain, original magnification×400). (C, F) Prominent cytoplasmic vacuoles were most notable in case 5, an ETV6-NTRK3 rearranged primary secretory carcinoma of the thyroid (C, black arrow; Papanicolaou stain, original magnification×400) and its histologic correlate (F, black arrow; H&E stain, original magnification ×40).

Figure 4.

Representative example of extensive intranuclear pseudoinclusions noted in 2 NTRK-rearranged thyroid carcinomas. Extensive intranuclear pseudoinclusions in case 1 (A, Papanicolaou stain, original magnification ×400) and its histologic correlate (B, hematoxylin and eosin stain, original magnification × ×200) are shown.

Figure 5.

Proposed algorithm for triaging potential NTRK-rearranged thyroid carcinoma fine needle aspiration (FNA). The proposed FNA NTRK algorithm parallels the recently published proposed RTK testing algorithm. If the cytomorphology on the FNA preparations demonstrates a mixed architectural pattern, fibrotic fragments, and any unusual cytoplasmic or nuclear features (eg, abundant intranuclear pseudoinclusions), a dedicated FNA pass may prove useful for BRAF testing or direct molecular testing with a larger panel to include NTRK on residual liquid-based material with platforms such as Afirma or ThyroSeq or on cell block material for patients who either have advanced disease or may benefit from neoadjuvant chemotherapy or are inoperable/suboptimal candidates for surgery.