Lung Cancer

Diagnosis

Histopathology and characteristics of NTRK+ tumours

The histopathological characteristics of NTRK-positive lung tumours remain poorly characterized due to the low prevalence of these tumours. A study of 4,872 non-small cell lung cancer (NSCLC) cases identified 11 cases with NTRK fusions [7]. Of these, 9 were adenocarcinoma, one was squamous cell carcinoma with an ETV6-NTRK3 fusion, and one was neuroendocrine carcinoma with a SQSTM1-NTRK3 fusion. Among the patients with adenocarcinoma, various histological subtypes were observed, including adenocarcinoma with neuroendocrine features (one case with a TPR-NTRK1 fusion), poorly differentiated adenocarcinoma with solid pattern and signet ring cells, and invasive mucinous adenocarcinoma. Squamous cell histology was accompanied by immunohistochemical expression of p40 and absence of TTF1. The patient with neuroendocrine carcinoma had a morphologically well-differentiated neuroendocrine tumour (equivalent to atypical carcinoid) with an increased mitotic index and a brain metastasis; this tumour was classified as large-cell neuroendocrine carcinoma in accordance with current WHO criteria.

NTRK-rearranged spindle cell neoplasms may occur in the lung, albeit are extremely rare [9]. Histologically, they were characterized by monomorphic spindle cells arranged in haphazard fascicles accompanied by variable stromal collagens. Morphologically tumour cells display mild nuclear atypia and scarce mitotic activity. Immunohistochemically they show strong and diffuse staining of CD34, pan-TRK, and TrkA with variable expression of S100 protein, while being negative for cytokeratin, SOX10, ALK, α-smooth muscle actin, desmin, and STAT6.

Current testing algorithms

The currently known NTRK gene fusions in NSCLC have been identified using classic approaches such as immunohistochemistry (IHC) and fluorescent in situ hybridization (FISH), as well as more advanced high-throughput technologies such as RNA- and DNA-based fusion-targeted next-generation sequencing (NGS). NGS results are usually verified by FISH and/or reverse transcription PCR (RT-PCR) [10]. A study of around 34,000 cancer samples indicated 100% specificity of IHC for lung cancer and a sensitivity of 87.5% [4]. Another study of 11,502 cancer cases, including around 4,073 cases of lung cancer, showed a specificity 95.9% and a sensitivity of 75% overall; however, the ability of IHC to detect NTRK3 gene fusions was much lower (only around 54% of the cases) than that to detect NTRK1 gene fusions [6].

The recommended testing algorithms for NTRK gene fusions testing propose NGS testing upfront to test for NTRK gene fusions alongside key oncogenic drivers (EGFR, ALK, ROS1, MET, BRAF, and PD-L1). Alternatively, standard testing for key oncogenic drivers can be performed first. If an oncogenic driver is absent, screening by pan-TRK IHC can be considered, followed by a confirmatory NGS. Upon disease progression, retesting for NTRK gene fusions, as well as other actionable molecular alterations, may be considered [10-12].

Challenges

Selective patient testing has hampered the accurate detection of NTRK gene fusions in NSCLC. Importantly, cancer patients selected for molecular testing may be enriched for metastatic cases considering that no established role exists for targeted therapies in early-stage lung cancer to date. Although NTRK gene fusions are rare in lung cancer, it is estimated that, among approximately 234,000 newly diagnosed patients with NSCLC annually in the United States, around 500 of those may carry NTRK gene fusions and thus be candidates for TRK inhibitor therapy [7].

FISH may detect chromosomal rearrangements; however, the detected rearrangements may not result in expression of a fusion transcript, or the fusion transcript detected may be expressed at low levels. Moreover, screening using IHC, or FISH may not be practical in NSCLC given the low frequency of NTRK gene rearrangements, limitations of tissue availability, and/or cost. In contrast, NGS-based assays such as Anchored Multiplex PCR (AMP), detect the fusion RNA transcript itself and thus provide more definitive conclusions. Thus, incorporation of NTRK rearrangement testing into a multiplexed NGS-based assay allows for simultaneous screening for NTRK gene rearrangements among other more common gene rearrangements [13].

Although universal NGS testing in lung cancer would be ideal, a major limitation is its cost as well as the long turnaround time of about 2–3 weeks to obtain results, which could delay the initiation of appropriate treatment. When NGS is inaccessible or unaffordable, single-gene testing is still a valid approach for the molecular profiling of lung cancers.

References

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9. Zhu P, Wang J. Primary NTRK-rearranged Spindle Cell Neoplasm of the Lung: A Clinicopathologic and Molecular Analysis of 3 Cases. Am J Surg Pathol. 2022 1;46(7):1007-1013.
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11. Lim KHT, Kong HL, Chang KTE et al. Recommended testing algorithms for NTRK gene fusions in pediatric and selected adult cancers: Consensus of a Singapore Task Force. Asia Pac J Clin Oncol. 2021;18(4):394-403.
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13. Farago AF, Le LP, Zheng Z et al. Durable clinical response to entrectinib in NTRK1-rearranged non-small cell lung cancer. J Thorac Oncol. 2015;10(12):1670-1674.