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Gastric and gastro-oesophageal junction adenocarcinoma is discussed through a group of markers assessed at diagnosis: HER2, PD-L1, mismatch repair status, and increasingly Claudin 18.2 and Epstein-Barr virus status. Each is tested by a different method, and the combination rather than any single result shapes the first-line discussion.
Also called Stomach cancer, GEJ adenocarcinoma.
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Gastric cancer management turns on a small set of markers that must be tested together before first-line treatment, because each one qualifies or excludes a different treatment class.
Gastric and gastro-oesophageal junction adenocarcinoma is discussed through a group of markers assessed at diagnosis: HER2, PD-L1, mismatch repair status, and increasingly Claudin 18.2 and Epstein-Barr virus status. Each is tested by a different method, and the combination rather than any single result shapes the first-line discussion.
01Before any molecular question
Usually established first: histology and its subtype, tumour location, stage, and whether the disease is resectable. Signet ring cell and diffuse-type histology behave differently from intestinal type and are treated as a distinct discussion.
Usually already established
02
The divisions below are not labels. Each one changes what the treatment discussion is actually about.
Early stage, resectable
Surgery with perioperative treatment organises the discussion. Marker status still matters for what is given around it.
Locally advanced
Multimodality treatment, with marker status shaping the systemic component.
Metastatic
The setting where the full marker panel most directly determines the first-line regimen.
Diffuse type and signet ring cell
Distinct biology, frequently associated with CDH1 alteration, and with a distinct germline question in some presentations.
EBV-associated
A recognised molecular subgroup with distinctive features, including frequent PIK3CA alteration and immune characteristics.
03What is discussed
HER2 amplification or overexpression is the longest-established target. PD-L1 expression is assessed for immune approaches and is usually reported as a combined positive score rather than as a percentage of tumour cells alone. Mismatch repair deficiency defines a small distinct group. Claudin 18.2 expression has become a separate immunohistochemistry-based question relevant to a specific treatment class. FGFR2 amplification, MET amplification, CDH1 alteration and Epstein-Barr virus status are also discussed. HER2 heterogeneity within a single tumour is a recognised issue in this disease and affects how a negative result on a small biopsy is interpreted.
HER2 (ERBB2)
The longest-established target. Heterogeneous within tumours, so sampling matters.
PD-L1
Reported as a combined positive score in this disease, which is not the same measure as in lung cancer.
MMR / MSI
Defines a small group with a distinct treatment discussion.
Claudin 18.2
An immunohistochemistry-based marker relevant to a specific newer class.
FGFR2
Amplification is uncommon but of active therapeutic interest.
CDH1
Associated with diffuse-type disease, and with a germline syndrome in some presentations.
EBV
Defines a molecular subgroup with distinctive immune features.
This is not an exhaustive list, and which markers are relevant depends on the individual case. Not every marker listed is assessed in every patient.
04What is tested, and from what
The practical point is that several markers are assessed by different methods and are best requested together rather than sequentially, because tissue from a gastric biopsy is often limited and sequential requests can exhaust it. Comprehensive genomic profiling captures the DNA-level findings but does not replace the immunohistochemistry-based ones such as PD-L1 and Claudin 18.2.
Hereditary diffuse gastric cancer, associated with germline CDH1 alteration, is the principal inherited syndrome. It is suggested by diffuse-type or signet ring cell histology, young age, or a family history of diffuse gastric or lobular breast cancer. It is consequential enough that raising it in the right presentation matters, because it changes management for relatives substantially. Lynch syndrome also raises gastric cancer risk.
Cancer genetics and hereditary riskPlasma testing is used where tissue is insufficient and is under study for monitoring. HER2 status assessed on plasma is one area of interest, given the heterogeneity problem with tissue sampling.
More on ctDNA monitoring05The timing question
Testing that arrives after the decision it was meant to inform has already been made is a common and avoidable problem. This is the sequence in which molecular information tends to become relevant.
06Take these to your team
When a decision is being made
Questions worth raising with a treating team.
At progression, recurrence or resistance
The molecular picture at diagnosis is not always the molecular picture later.
07What shapes eligibility
Eligibility commonly specifies HER2 status, PD-L1 score with a defined threshold, Claudin 18.2 expression, FGFR2 amplification, and prior treatment lines. Because several markers are involved, incomplete marker data is a frequent reason a match cannot be assessed.
| Term | What it means |
|---|---|
| Combined positive score | A way of scoring PD-L1 that counts immune cells as well as tumour cells. Different from the score used in lung cancer. |
| Heterogeneity | When different parts of the same tumour differ. A small biopsy may not represent the whole. |
| Diffuse type | A growth pattern that spreads through the stomach wall rather than forming a discrete mass. |
| Perioperative treatment | Treatment given both before and after surgery. |
These are the services most often relevant to this cancer. Which of them applies is a clinical question, not an automatic one.
This entry is educational. It describes what is commonly discussed in this cancer, not what should happen for any individual. Which of it applies depends on the specific diagnosis, stage, prior treatment and the person themselves. Nothing here is a recommendation, and no list of biomarkers here is exhaustive. Decisions belong with a treating team who know the whole case.
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