# What is Tumor marker?

Source: https://oncologyistanbul.com/glossary/tumor-marker

> A substance — often measured in blood — that can be elevated in the presence of certain cancers, used to help monitor treatment response rather than to diagnose on its own.

Tumor markers (examples include CEA, CA 19-9, CA-125, and PSA, each associated with different cancer types) are proteins or other substances that can rise when certain cancers are present. They are rarely used alone to diagnose cancer, since many can also rise for non-cancerous reasons — their main clinical value is tracking a known tumor's response to treatment or watching for recurrence over time.

Full glossary: https://oncologyistanbul.com/glossary