Next-generation sequencing (NGS)

What it is

Next-generation sequencing (NGS) is a DNA or RNA testing method that reads millions of fragments in parallel to detect genetic changes. The key practical fact is that NGS can examine many genes at once in a single run, unlike older single-gene methods such as Sanger sequencing. In cancer care, this matters because one biopsy sample may need to be checked for multiple mutations, fusions, copy-number changes, or other biomarkers that can affect diagnosis, prognosis, and treatment choice.

NGS is a collective term rather than one single machine or chemistry. It includes several sequencing approaches developed after Sanger sequencing. In clinical practice, the most common forms are short-read, massively parallel sequencing, though long-read methods are also used in some settings. Depending on the question, NGS may be used on tumour tissue, blood, bone marrow, or other samples, and it may sequence a small gene panel, the whole exome, or the whole genome.

For patients in India, NGS is increasingly used in oncology centres for solid tumours, leukaemias, lymphomas, and inherited cancer risk assessment. Availability, turnaround time, and cost vary by laboratory, and the exact panel used can differ between hospitals.

Common NGS test type	What it looks at	Typical use in cancer
Targeted gene panel	Selected genes or hotspots	Finds actionable mutations with less tissue and lower cost
Whole exome sequencing	Most protein-coding regions	Broader search when panel testing is unrevealing
Whole genome sequencing	Nearly all genomic DNA	Research and selected advanced clinical use
RNA sequencing	RNA transcripts and fusions	Detects gene fusions and expression-related findings
Liquid biopsy NGS	Tumour DNA in blood	Useful when tissue is limited or for monitoring in some cases

How it works

NGS starts by extracting DNA or RNA from a sample. The genetic material is broken into fragments, adapters are added, and a sequencing instrument reads many fragments at the same time. Computer analysis then aligns those reads to a reference genome and identifies variants.

The main advantage over older methods is scale. Traditional sequencing usually checks one gene region at a time. NGS can assess many regions simultaneously, which is especially useful in cancer because tumours may carry several relevant alterations. Depending on the assay design, NGS can detect:

1. Single-nucleotide variants
2. Small insertions and deletions
3. Copy-number changes
4. Gene fusions or rearrangements
5. In some assays, broader genomic signatures such as tumour mutational burden or microsatellite instability

The result is not just a raw sequence. A clinical report interprets which findings are likely pathogenic, which may be targetable, and which are of uncertain significance. This interpretation step is important because not every mutation found is clinically useful.

Evidence and uses

NGS is now a core technology in modern cancer diagnostics because it can profile multiple biomarkers from limited tissue. It is used to help classify tumours, identify targetable alterations, estimate prognosis in some cancers, detect resistance mutations, and sometimes monitor minimal residual disease or relapse.

Examples in oncology include testing for EGFR, ALK, ROS1, BRAF, KRAS, BRCA1/2, NTRK, and many other genes depending on the cancer type. In blood cancers, NGS can help identify mutations relevant to diagnosis and risk stratification. In inherited cancer assessment, germline NGS panels may be used when family history suggests a hereditary syndrome.

Evidence supports NGS particularly when treatment decisions depend on multiple biomarkers. Compared with sequential single-gene testing, NGS can conserve tissue and may shorten the path to a complete molecular profile. It is also useful when rare but actionable alterations are possible, because broad testing can detect changes that would otherwise be missed.

That said, the value of NGS depends on context. A broad panel is not necessary for every patient or every cancer stage. Some cancers still have one or two key biomarkers better assessed by focused tests. Also, finding a mutation does not guarantee that a targeted drug will be available, affordable, or appropriate. Some reported variants are classified as variants of uncertain significance, which should not be overinterpreted.

Diagnosis / how it's measured

NGS is not a diagnosis by itself. It is a laboratory method used to measure genetic information. The quality of the result depends on several technical factors:

• Sample type and quality: Formalin-fixed tissue, fresh tissue, blood, or bone marrow may all be used, but degraded samples can reduce accuracy.
• Tumour content: If too few tumour cells are present, important variants may be missed.
• Depth of coverage: More reads over a region generally improve sensitivity for low-frequency variants.
• Assay design: A small hotspot panel will miss changes outside its target regions.
• Bioinformatics and reporting standards: Interpretation varies by lab and should follow validated methods.

Reports usually include the genes tested, important pathogenic or likely pathogenic variants, test limitations, and whether the findings are somatic or possibly germline. If a result suggests an inherited mutation, confirmatory germline testing and genetic counseling may be recommended.

In cancer care, NGS results are often interpreted alongside pathology, imaging, immunohistochemistry, fluorescence in situ hybridisation, PCR-based tests, and the patient's clinical history. A negative NGS result does not rule out cancer and does not prove that no relevant mutation exists. It may simply mean the panel did not cover that alteration, the tumour fraction was low, or the sample quality was poor.

When to see a clinician

Ask your oncologist or pathologist about NGS if your cancer type commonly uses molecular profiling, if standard treatment options are limited, if tissue is scarce and multiple biomarkers are needed, or if there is concern for a hereditary cancer syndrome. NGS is also worth discussing when cancer returns after treatment, because resistance mutations can emerge over time.

Patients should review three practical points before testing: what question the test is meant to answer, whether the result could change treatment, and whether the sample is adequate. In India, it is reasonable to ask whether the laboratory is accredited, what genes are included, how long results take, and whether the report will identify findings relevant to approved therapies or clinical trials.

Limitations and open questions

NGS is powerful, but it has limits. It can miss alterations outside the tested regions, very low-level variants, some structural rearrangements, or changes in poor-quality samples. Different platforms and panels do not perform identically, so results are not always directly comparable across laboratories.

Another limitation is interpretation. Many detected variants have uncertain clinical meaning. Even when an actionable mutation is found, the matching drug may not be indicated for that tumour type, may not be available locally, or may not improve outcomes in that specific setting. Evidence is strongest for some biomarkers and cancers, and weaker for others.

Liquid biopsy NGS is promising, but a negative blood test does not exclude a tumour mutation because circulating tumour DNA levels may be low. Whole exome and whole genome sequencing provide broader data, but they also generate more incidental findings and require more complex analysis.

As cancer genomics advances, NGS is likely to become more integrated with RNA profiling, epigenetic testing, and longitudinal monitoring. Even so, it remains one part of decision-making, not a standalone answer. Results should be interpreted by a qualified clinician, molecular pathologist, or genetics team in the context of the patient's cancer type, stage, and treatment goals.