Pan-TRK Immunohistochemistry: An Example-Based Practical Approach to Efficiently Identify Patients With NTRK Fusion Cancer

Abstract

Context.— Food and Drug Administration–approved TRK inhibitors with impressive overall response rates are now available for patients with multiple cancer types that harbor NTRK rearrangements, yet the identification of NTRK fusions remains a difficult challenge. These alterations are highly recurrent in extremely rare malignancies or can be detected in exceedingly small subsets of common tumor types. A 2-step approach has been proposed, involving a screening by immunohistochemistry (IHC) followed by a confirmatory method (fluorescence in situ hybridization, reverse transcriptase–polymerase chain reaction, or next-generation sequencing) in cases expressing the protein. However, there is no interpretation guide for any of the available IHC clones.

Objective.— To provide a pragmatic update on the use of pan-TRK IHC. Selected examples of the different IHC staining patterns across multiple histologies are shown.

Data Sources.— Primary literature review with PubMed, combined with personal diagnostic and research experience.

Conclusions.— In-depth knowledge of pan-TRK IHC will help pathologists implement a rational approach to the detection of NTRK fusions in human malignancies.

Several TRK inhibitors with impressive overall response rates in patients with NTRK rearrangements are currently available or under clinical development.1,2 The search for NTRK fusions should benefit from what we have learned in recent years about identifying other druggable rearrangements, mainly in lung cancer (ALK, ROS1, etc).3–7 Therefore, NTRK fusions can be detected with immunohistochemistry (IHC), fluorescence in situ hybridization (FISH), reverse transcriptase–polymerase chain reaction, or next-generation sequencing (NGS).8 If NGS is not routinely performed in all advanced malignant tumors, most proposed algorithms use IHC as a screening method, followed by orthogonal confirmation of all positive IHC cases (mainly using FISH or NGS).8–11 To further enrich for NTRK fusions, both histology-based and genomic-based triaging approaches have been proposed.12 A summary of the available evidence is presented in the Table. Until NGS becomes the main testing methodology on all advanced cancers, algorithm considerations should include feasibility, cost, sample size, and pretest probability of NTRK fusions. A review of the frequencies of NTRK fusions highlights the need to be aware of these strategies. NTRK fusions have been observed in 0.31% of adult tumors and 0.34% of pediatric tumors.13 In clinical series, the most common partners have been NTRK1 and NTRK3.14–16 The most frequent fusion is ETV6-NTRK3.16 With the exception of gliomas,13,15 NTRK2 fusions appear to be restricted to isolated examples of sarcomas or lung adenocarcinomas.15,17–19 Although it may be too soon to draw definitive conclusions on the positive and negative predictive value of pan-TRK IHC by partner and cancer type, the largest series to date showed an overall sensitivity of 87.9% and specificity of 81.1%.14 Decreased sensitivity was reported for NTRK3 fusions and sarcomas, and lower specificity involved sarcomas, breast carcinomas, and salivary gland carcinomas.