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Lung adenocarcinoma — Molecular testing

TL;DR — Every advanced lung adenocarcinoma should undergo broad molecular profiling before first-line treatment because actionable alterations span point mutations, indels, copy-number changes, and fusions, and clinical phenotype cannot safely exclude any driver (Lindeman 2018, PMID 29398453). A DNA-plus-RNA next-generation sequencing (NGS) strategy reduces serial tissue exhaustion and improves fusion detection; PD-L1 immunohistochemistry is complementary, not a substitute for genotyping. Plasma ctDNA can return actionable results rapidly and increase yield, but low shedding creates false negatives, so a non-informative plasma result requires tissue testing when feasible (Leighl 2019, PMID 30988079; Rolfo 2021, PMID 34246791). At resistance, repeat tissue and/or plasma testing should be chosen to detect both genomic mechanisms and lineage transformation. The principal safety failure is starting immunotherapy before an actionable driver result in a clinically stable patient.

Who and when to test

Setting Minimum question Preferred specimen strategy
Newly diagnosed advanced nonsquamous NSCLC Is there an approved or trial-actionable driver? Broad tissue DNA/RNA NGS + PD-L1; plasma in parallel when tissue/turnaround is limiting
Resected stage IB–IIIA adenocarcinoma EGFR and ALK for adjuvant eligibility; broader panel increasingly useful Resection tissue, preserving blocks
Resectable neoadjuvant candidate Driver status before immune-therapy commitment Rapid broad profiling on diagnostic biopsy
Progression on targeted therapy On-target mutation, bypass pathway, amplification, or transformation? Plasma for breadth plus tissue when transformation possible
NSCLC-NOS / limited biopsy Can lineage be assigned without exhausting tissue? Minimal immunopanel, then molecular testing

The CAP/IASLC/AMP update requires EGFR and ALK testing irrespective of clinical characteristics and supports multiplex panels beyond sequential single-gene assays (Lindeman 2018, PMID 29398453). ASCO separately endorsed that guideline; the endorsement is not independent evidence and should not be counted as a second recommendation (Kalemkerian 2018, PMID 29401004).

European implementation reviews show that biomarker access and use differ across countries as therapies change, which is why a static “EGFR/ALK only” order set becomes unsafe (de Jager 2024, PMID 38476742). Testing policy should record version date, genes, variant classes, assay limits, and linked therapies.

What the assay must detect

Alteration class Representative drivers Assay challenge
Single-nucleotide variants EGFR L858R, KRAS G12C, BRAF V600E Depth, low tumor fraction, artefact filtering
Small insertions/deletions EGFR exon 19 deletions, exon 20 insertions Alignment and variant-specific sensitivity
Exon skipping MET exon 14 Diverse splice-site DNA events; RNA confirms consequence
Fusions ALK, ROS1, RET, NTRK, NRG1 Large introns and novel partners make DNA-only testing incomplete
Copy-number gain MET amplification, ERBB2 amplification Purity/ploidy and platform thresholds
Resistance alterations EGFR C797S, ALK kinase mutations Subclonality and post-treatment timing

DNA NGS is efficient for substitutions and indels; RNA sequencing directly observes expressed fusions and splice events. A combined strategy is especially important for never-smokers and driver-negative mucinous tumors, where fusion probability is enriched.

Tissue stewardship

Small biopsies must support diagnosis, staging, PD-L1, and multiplex genomics. Unnecessary serial immunostains and repeated single-gene tests consume sections and DNA, creating an avoidable “quantity not sufficient” result (Yu 2019, PMID 30243889).

Step Tissue-preserving control
Acquisition Obtain multiple cores; coordinate with interventionalist and pathology
Fixation Standard neutral-buffered formalin; avoid decalcification when possible
Lineage Use a minimal TTF-1/Napsin A/p40-oriented panel rather than broad stains
Tumor enrichment Pathologist marks viable tumor; macrodissection where useful
Molecular order Reflex comprehensive panel rather than serial clinician-triggered tests
Block management Track remaining material and reserve for confirmatory/resistance work

Reflex testing shortens the gap between diagnosis and actionable result by removing an extra ordering step. Published institutional implementations report improved completeness and turnaround, but workflow success depends on consent, payer rules, specimen triage, and clinician notification (Lim 2015, PMID 25922063; Anand 2020, PMID 32600793).

Plasma ctDNA

Plasma profiling is a genotyping test, not a histologic diagnosis. It can sample DNA from multiple metastatic sites and often returns faster than tissue, but sensitivity falls with low tumor volume, isolated CNS disease, and low-shedding biology.

Plasma result Interpretation Next action
Actionable alteration detected Usually high positive predictive value if variant is tumor-consistent Treat in clinical context; correlate with tissue/pathology
No alteration; tumor fraction detectable True negative more plausible but panel limits remain Tissue testing if clinically important variants not excluded
No alteration; no evidence of shedding Uninformative, not negative Tissue required when feasible
Germline-range allele fraction Possible inherited variant Confirm in validated germline pathway; do not infer from plasma alone
Clonal-haematopoiesis gene event May originate from blood cells Paired leukocyte analysis/clinical interpretation

In the prospective NILE study, comprehensive plasma cfDNA identified guideline biomarkers at a rate non-inferior to tissue and increased overall detection when combined with tissue, with shorter turnaround (Leighl 2019, PMID 30988079). That result supports plasma-first or plasma-parallel workflows, not abandonment of tissue.

IASLC statements emphasize that liquid biopsy is particularly useful when tissue is insufficient or unsafe to obtain and at molecular progression, while negative plasma results require reflex tissue because sensitivity is incomplete (Rolfo 2018, PMID 29885479; Rolfo 2021, PMID 34246791).

Prospective assay validation shows that agreement depends on tumor fraction, disease burden, platform, and variant class; a single pooled concordance obscures the clinically important false-negative tail (Rolfo 2021, PMID 34246791; Pritchett 2019, PMID 32914040).

Pre-analytic and analytic failure modes

Failure Consequence Prevention
Low tumor cellularity False-negative tissue result Enrichment, sensitive assay, alternate block/plasma
Bone decalcification DNA/RNA damage Prefer non-bone site or EDTA methods
Delayed plasma processing Leukocyte lysis dilutes ctDNA Stabilizing tubes or prompt separation
DNA-only fusion panel Missed ALK/ROS1/RET/NTRK/NRG1 Add RNA or orthogonal assay
Narrow hotspot panel Missed uncommon EGFR/MET/ERBB2 events Broad validated coverage
No matched normal Germline/CH ambiguity Interpret allele fraction and genes; confirm separately
Long turnaround Empiric therapy before result Reflex/rapid workflow and explicit “await testing” communication

PD-L1 and tumor mutational burden

PD-L1 immunohistochemistry answers a different question from NGS. It guides checkpoint strategies in driver-negative disease but is spatially/temporally heterogeneous and can be high in oncogene-driven tumors that respond poorly to single-agent immunotherapy. Tumor mutational burden is platform-dependent and is not a substitute for driver testing.

Testing at resistance

Initial driver Genomic resistance examples Tissue-only/phenotypic mechanisms
EGFR C797S, MET amplification, acquired fusions Small-cell or squamous transformation
ALK Compound ALK kinase mutations, bypass activation Histologic change
KRAS G12C Secondary KRAS switch-pocket mutations, pathway reactivation Phenotypic adaptation
MET Kinase-domain mutations, bypass pathways Heterogeneous amplification
RET/ROS1/NTRK Solvent-front/gatekeeper mutations Bypass signalling

Plasma can reveal polyclonal resistance across lesions, while tissue preserves architecture and enables RNA/protein analysis. The optimal strategy is often both, especially when radiographic behaviour is discordant.

Reporting

A useful report includes specimen, tumor fraction, methods, genes and variant classes covered, depth/sensitivity, detected alteration with transcript and allele fraction, classification, linked therapy/guideline, resistance implications, and explicit limitations. “No mutation detected” without assay scope is clinically unsafe.

Turnaround should be measured from specimen acquisition and diagnosis—not only laboratory accession—to a treatment-ready result. Availability of genotyping results before first-line treatment is associated with overall survival, although the observational association can also capture broader care quality (Aggarwal 2023, PMID 37499192). Plasma testing can recover drivers missed by a narrow tissue sequence, but detection is not itself proof that a program improves survival (Uemura 2023, PMID 37948122).

Open questions

Programme-level quality metrics

Report tissue adequacy, complete-panel rate, RNA-fusion completion, median and 90th-percentile turnaround from acquisition, result availability before first treatment, and matched-treatment delivery. Laboratory turnaround alone can conceal pre-analytic delay and post-result failure.

  • Does plasma-first versus tissue-first profiling improve survival, quality, and cost when reflex rules are enforced (Leighl 2019, PMID 30988079)?
  • What minimum RNA coverage prevents clinically important fusion false negatives?
  • Can rapid profiling safely avoid empiric immunotherapy without delaying urgent treatment?
  • How should low-frequency resistance clones be thresholded for action?
  • Which quality metric—completion, turnaround, matched treatment, or survival—best evaluates a molecular-testing programme?

References

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