Glossary

Cancer terminology glossary

Plain-language definitions of common cancer terms. Written for patients and caregivers, not clinicians. For more depth, see our patient guides.

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Adjuvant therapy: Treatment given after the main treatment (usually surgery) to lower the chance the cancer returns. Examples include adjuvant chemotherapy or adjuvant radiation.

Biomarker: A measurable characteristic of cancer cells (a protein, gene mutation, or pattern) that helps predict response to a specific therapy. Examples: HER2 in breast, EGFR in lung, MSI in colorectal.
Biopsy: A procedure that removes cells or tissue so a pathologist can examine them for cancer. May be needle, core, excisional, or endoscopic.
BRCA1 / BRCA2: Genes whose mutations substantially raise the risk of breast, ovarian, pancreatic, and prostate cancers. Patients with BRCA mutations may respond to PARP inhibitors.

Chemotherapy: Drugs that kill rapidly dividing cells, including cancer cells. Can be given by IV infusion or by mouth. Systemic — treats cancer throughout the body.
Clinical trial: A research study testing a new therapy or intervention in patients. Phases 1 (safety), 2 (efficacy signal), 3 (vs standard of care), 4 (post-approval monitoring).

ECOG Performance Status: A 0–5 scale describing how much a patient's illness affects daily life. 0 = fully active; 4 = completely disabled. Used for trial eligibility.

Grade (tumor grade): How abnormal cancer cells look under a microscope. Grade 1 (well-differentiated, slow-growing) to Grade 3 (poorly-differentiated, aggressive). Different from stage.

HER2: A protein on breast and some other cancer cells. HER2-positive tumors respond to HER2-targeted therapies like trastuzumab.

Immunotherapy: Drugs that activate or modify the immune system to attack cancer. Checkpoint inhibitors (pembrolizumab, nivolumab) and CAR-T are examples.

Lymph node: Small bean-shaped organ that filters lymph. Cancers often spread to nearby (regional) lymph nodes first. 'Node-positive' is a key staging marker.

Metastasis (metastatic): Spread of cancer from its origin to distant parts of the body. Metastatic (Stage IV) cancer is treated systemically.
MSI / MMR: Microsatellite Instability / Mismatch Repair status. MSI-High or dMMR tumors often respond to immune checkpoint inhibitor therapy.

NCI-designated Cancer Center: A U.S. cancer center recognized by the National Cancer Institute for research and clinical excellence. 71 centers across 37 states.
Neoadjuvant therapy: Treatment given before the main treatment (usually surgery) to shrink the tumor or kill micrometastatic disease.

Oncologist: A physician specializing in cancer. Medical, radiation, surgical, gynecologic, hematologic, and pediatric are subspecialties.

PARP inhibitor: A class of targeted therapy that exploits DNA-repair defects in BRCA-mutated and HRD-positive cancers.
Pathology report: A formal document from the pathologist describing the cancer's type, grade, size, margins, node involvement, and molecular features.
PD-L1: A protein on cancer cells that can indicate whether the tumor may respond to PD-1/PD-L1 immune checkpoint inhibitors.
Phase 1 / 2 / 3 / 4 trial: Stages of clinical testing: safety (1), efficacy signal (2), head-to-head vs standard (3), post-approval monitoring (4).
Primary / metastatic (site): Primary = where the cancer started (e.g., primary breast cancer). Metastatic site = where it has spread (e.g., liver metastasis from breast cancer).
Prognosis: The likely course of disease. Based on population averages, not individual predictions.

Radiation therapy: Treatment using ionizing radiation to damage cancer DNA. External beam, brachytherapy, proton therapy, and stereotactic (SBRT/SRS) are variants.
Recurrence: Return of cancer after a period of remission. Can be local, regional, or distant.
Remission: A period during which cancer is not detectable. Partial remission = some cancer remains but tumor shrunk; complete remission = no cancer detected (not necessarily cured).

SEER: Surveillance, Epidemiology, and End Results — the NCI's database of U.S. cancer incidence and survival statistics.
Staging (TNM): Tumor (T), Node (N), Metastasis (M) — the AJCC staging system. Combines into Stage I–IV.
Standard of care: The currently accepted best treatment for a particular cancer, based on evidence and guidelines.
Systemic therapy: Treatment that affects the whole body — chemotherapy, targeted therapy, hormonal therapy, or immunotherapy.

Targeted therapy: Drugs designed to act on a specific molecular feature of cancer cells (e.g., a mutated protein), sparing normal cells more than chemotherapy.