Which results actually change treatment?

A positive result rarely cancels urgent surgery by itself; most decisions land at the systemic-therapy step. Quick map: Colorectal dMMR/MSI-H → immunotherapy; Lynch pathway Colorectal RAS/BRAF → anti-EGFR eligibility; prognosis context Gastric HER2 / CLDN18.2 / PD-L1 → advanced-disease drug selection Breast ER/PR/HER2 → endocrine, anti-HER2, chemotherapy sequencing Pancreas germline BRCA/PALB2 → platinum sensitivity, PARP options; family screening Rare NTRK fusion, BRAF V600E, MSI-H (any tumor) → targeted options when approved A negative result is also information: that drug path is not appropriate.

Can open immunotherapy discussions and Lynch evaluation. Often considered early in colorectal diagnosis.

Targetable changes are different classes

Mutations, amplifications, fusions, and protein expression are not interchangeable words. RAS/BRAF → often mutations HER2 → amplification/expression NTRK → usually fusion CLDN18.2 → protein expression

A test does not equal cure; stage, performance status, and resectability still matter.

Do not delay emergencies

Necessary emergency surgery is not postponed waiting for every molecular result; systemic steps can follow when results return.

How we use reports together

Pathology, imaging, and biomarkers are read as one story in tumor board — not as competing scores.

Written and medically reviewed by: Assoc. Prof. Cengiz Dibekoğlu, MD, General SurgeryLast updated: 8 September 2026

Biomarkers

Molecular Testing in Cancer

Name: Molecular Testing in Cancer
Creator: Assoc. Prof. Cengiz Dibekoğlu, MD

Biomarkers that guide therapy — not good/bad labels

More tests are not always better — the question comes first.

Molecular and biomarker tests describe tumor or hereditary features that guide targeted drugs, immunotherapy, surgical sequencing, and family screening. Examples include MMR/MSI, RAS/BRAF, HER2, PD-L1, NTRK fusions, and CLDN18.2 protein expression — not all are “mutations.” Somatic tumor testing answers treatment questions; germline testing answers family-risk questions — mixing them confuses patients. A positive targetable result does not erase stage, fitness, or surgical judgment. Ordering panels without a clinical question wastes time and can delay necessary care.

3D diagram: Molecular Testing in Cancer · educational illustration — Molecular Testing in Cancer · educational illustration

Educational schematic. Some structures may be simplified or emphasized for clarity; not an exact anatomical depiction.

Six terms before you read further

Germline: Inherited DNA in every cell — answers family-risk questions
Somatic: Change only in tumor tissue — answers treatment questions
Biomarker: A molecular feature that may select a therapy path
MMR/MSI: Repair-pathway status; may open immunotherapy and Lynch work-up
Targeted therapy: Drugs matched to a specific biomarker
NGS (panel): Sequencing many genes at once

Figure summary

Hero figure: molecular testing in cancer as an educational overview.
Related figures explain why biomarkers and panel testing guide oncology decisions.

Educational figures

Tap a figure to enlarge. These English illustrations mirror the Turkish hub gallery for this condition.

Overview
Educational schematic. Some structures may be simplified or emphasized for clarity; not an exact anatomical depiction.
© Cengiz Dibekoğlu — illustrative; not for unauthorized use
Colorectal
Educational schematic. Some structures may be simplified or emphasized for clarity; not an exact anatomical depiction.
© Cengiz Dibekoğlu — illustrative; not for unauthorized use
Gastric
Educational schematic. Some structures may be simplified or emphasized for clarity; not an exact anatomical depiction.
© Cengiz Dibekoğlu — illustrative; not for unauthorized use
Breast
Educational schematic. Some structures may be simplified or emphasized for clarity; not an exact anatomical depiction.
© Cengiz Dibekoğlu — illustrative; not for unauthorized use

Pathway selectors
Results choose roads; they are not moral labels.
NTRK fusions
Rare; may open TRK inhibitor options.
CLDN18.2
Protein expression — especially relevant in gastric cancer.
Germline vs somatic
Family-risk testing is a different conversation.

What to know

Six terms before you read further
Reports use many abbreviations. Start with these six — detail below is layered, not removed.
- Germline: inherited DNA in every cell — answers family-risk questions
- Somatic: change only in tumor tissue — answers treatment questions
- Biomarker: a molecular feature that may select a therapy path
- MMR/MSI: repair-pathway status; may open immunotherapy and Lynch work-up
- Targeted therapy: drugs matched to a specific biomarker
- NGS (panel): sequencing many genes at once
Which results actually change treatment?
Key note inside
Note: A negative result is also information: that drug path is not appropriate.
A positive result rarely cancels urgent surgery by itself; most decisions land at the systemic-therapy step. Quick map:
- Colorectal dMMR/MSI-H → immunotherapy; Lynch pathway
- Colorectal RAS/BRAF → anti-EGFR eligibility; prognosis context
- Gastric HER2 / CLDN18.2 / PD-L1 → advanced-disease drug selection
- Breast ER/PR/HER2 → endocrine, anti-HER2, chemotherapy sequencing
- Pancreas germline BRCA/PALB2 → platinum sensitivity, PARP options; family screening
- Rare NTRK fusion, BRAF V600E, MSI-H (any tumor) → targeted options when approved
MMR / MSI
Can open immunotherapy discussions and Lynch evaluation. Often considered early in colorectal diagnosis.
Targetable changes are different classes
Mutations, amplifications, fusions, and protein expression are not interchangeable words.
- RAS/BRAF → often mutations
- HER2 → amplification/expression
- NTRK → usually fusion
- CLDN18.2 → protein expression
Limits of testing
Key note inside
A test does not equal cure; stage, performance status, and resectability still matter.
Do not delay emergencies
Key note inside
Necessary emergency surgery is not postponed waiting for every molecular result; systemic steps can follow when results return.
How we use reports together
Pathology, imaging, and biomarkers are read as one story in tumor board — not as competing scores.

What happens next

Biomarkers answer specific treatment and hereditary questions

Oncologic surgery6 chapters

Evidence

Scientific sources

Show sources · 4

Guideline bases for when molecular / NGS and MMR-MSI testing change decisions — not “every test for everyone.”

1. Recommendations for the use of next-generation sequencing (NGS) for patients with advanced cancer in 2024 — ESMO Precision Medicine Working GroupESMO · 2024 · DOI: 10.1016/j.annonc.2024.04.005Open source →
2. Metastatic colorectal cancer: ESMO Clinical Practice Guideline for diagnosis, treatment and follow-upEuropean Society for Medical Oncology (ESMO) · 2023 · DOI: 10.1016/j.annonc.2022.10.003Open source →
3. CAP / AMP / ASCO guideline context — MMR/MSI testing in colorectal cancerCollege of American Pathologists (guideline ecosystem) · Current pathology guidelinesOpen source →
4. NCCN Clinical Practice Guidelines in Oncology: Colon CancerNational Comprehensive Cancer Network (NCCN) · Current versionOpen source →

Last reviewed: 21 August 2026. Links go to publisher pages.

Appointment / info

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This page is for education. It is not medical advice and does not replace a visit with your physician.

Medical editor: Assoc. Prof. Cengiz Dibekoğlu, MD · English patient-education hub.