General · EdgepediaSeptember 17, 20267 min read

Metastasis

Metastasis is the spread of cancer cells from the place where they first formed, the primary tumor, to another part of the body, where they form new tumors called metastases. In metastasis, cancer cells break away from the primary tumor, travel through the blood or lymph system, and settle in other organs or tissues; the new tumor is the same type of cancer as the original.¹ Cancer that has spread to a distant part of the body is called metastatic cancer, and for many cancer types it is also called stage 4 cancer.²

Key fact	Detail
Definition	Spread of cancer cells from the primary tumor through blood or lymph to form new tumors elsewhere in the body¹
Naming	A metastasis keeps the name and cellular features of the primary cancer; breast cancer in the lung is metastatic breast cancer, not lung cancer²
Most common sites	Bone, liver, and lung²
Staging	In the TNM system, M1 means the cancer has spread to distant parts of the body; distant spread corresponds to stage IV³
Dormancy	Disseminated tumor cells can persist in a dormant state for months to decades before growing again⁴
Survival	In a registry study, median survival for cancer of unknown primary was 2 months for liver metastases, 3 months for bone, and 4 months for respiratory, gastrointestinal, and brain sites⁵
Prognosis driver	Among patients with bone metastases across 25 primary cancers, primary cancer type was the strongest factor associated with survival (HR 1.422–1.758, p < 0.001)⁶

How cancer spreads

Cancer cells spread through the body in a series of steps: invading nearby normal tissue; moving through the walls of nearby lymph nodes or blood vessels; traveling through the lymphatic system and bloodstream; stopping in small vessels at a distant location, invading the vessel walls, and entering the surrounding tissue; growing there until a tiny tumor forms; and causing new blood vessels to grow, which supplies the metastatic tumor so it can continue growing.²

Most spreading cancer cells die at some point in this process, and only when conditions are favorable at every step can some of them form new tumors.² The attrition is severe: circulating tumor cells suffer physical, redox, and immune stressors in the bloodstream, and disseminated tumor cells that reach distant organs face oxidative stress, lack of supportive growth factors, and immune defenses such as macrophages, natural killer cells, and T cells.⁷ Clusters of tumor cells entering the circulation show a substantially greater ability to survive and colonize distant organs than single disseminated cells.⁴

Dormancy. Cells that survive can enter a dormant state after reaching a secondary site, stopping proliferation while remaining viable. Disseminated tumor cells at primed metastatic sites may stay dormant for unspecified periods, from months to decades, before reactivating and growing into detectable lesions, sometimes years after treatment of the primary tumor.⁴^•⁸ Dormant cells and early disseminated disease are considered micrometastatic because they are undetectable by clinical imaging.⁷

Where cancer spreads

Cancer can spread to almost any part of the body, but different cancer types are more likely to spread to certain areas. The most common sites are bone, liver, and lung.² Typical patterns include breast cancer spreading to bone, brain, liver, and lung; colon, pancreatic, and ovarian cancer to liver, lung, and peritoneum; and prostate cancer mainly to bone, along with adrenal gland, liver, and lung.²

The strength of these preferences varies by primary type. In a large real-world cohort study, bone was the site of metastasis for 50 to 80 percent of metastatic breast and prostate cancer patients, but only 8 to 21 percent in lung cancer and melanoma.⁹ One reason some tissues resist colonization is emerging from basic research: skeletal muscle-derived reactive oxygen species incapacitate disseminated tumor cells, which helps explain why muscle is a rare metastatic site.⁴

The seed and soil hypothesis

In 1889, the English surgeon Stephen Paget examined postmortem data from 735 women with breast cancer and noted that the organ distribution of metastases was not random. He proposed that metastasis is not due to chance: some tumor cells, the "seed", grow preferentially in the microenvironment of select organs, the "soil", and metastases result when the appropriate seed is implanted in its suitable soil.¹⁰ Experimental work has supported the idea that the target-organ microenvironment regulates metastatic growth; for example, lung-homing melanoma cells metastasized to normal lung and to lung tissue grafted ectopically in mice, but not to other tissues placed at the same sites.¹⁰

Modern research frames the same idea as the metastatic niche: sites for future colonization are often pre-conditioned by factors arising from the primary tumor and by the recruitment of distinct non-tumor cell types, and colonization, the growth of disseminated cells into lesions, is described as the rate-limiting step of metastasis.⁴

Detection, naming, and staging

Metastatic cancer cells look, under the microscope and in other tests, like the cells of the primary cancer, not like the cells of the organ where they are found; this is how doctors identify spread.² Metastatic cancer is named and treated as the primary cancer: breast cancer that spreads to the lung is stage 4 breast cancer, not lung cancer.²

Staging describes how far a cancer has spread. In the TNM system, maintained by the American Joint Committee on Cancer and the Union for International Cancer Control, the M category records distant metastasis: M0 means no spread to other parts of the body, and M1 means the cancer has spread.³ Spread is also described by distance: local metastasis means spread to nearby tissues or lymph nodes, regional means farther spread within the same region, and distant means tumors far from the primary site. When only a few cells have spread, the term is micrometastasis, often too small to see on imaging.¹¹ Sometimes a metastasis is found before the primary tumor itself.¹¹

When doctors cannot determine where a metastatic cancer started, the diagnosis is cancer of unknown primary origin (CUP).²

Prognosis and survival

Survival with metastatic cancer depends strongly on the primary cancer type and the organ involved. In a Swedish registry study of patients diagnosed between 2002 and 2008, median survival for CUP varied by metastatic site: 2 months for liver, 3 months for bone, and 4 months for respiratory, other gastrointestinal, and brain metastases.⁵ Patients with metastatic cancer from a known primary had a lower risk of death than CUP patients overall (hazard ratio 0.69), and the direction of the difference depended on the primary site: colorectal (HR 0.61), kidney (0.71), breast (0.53), and prostate (0.24) cancers had decreased risk of death compared with CUP at corresponding locations, while pancreatic (1.71), liver (1.58), and stomach (1.16) cancers had increased risk.⁵ Bone metastases originating from the prostate showed an exceptional prognosis compared with CUP of bone.⁵

A registry-based analysis of 13,742 patients with bone metastases across 25 primary cancers stratified all cancers into three survival tiers, from prolonged median survival exceeding 15 months to markedly shorter survival of 3 to 10 months, and found that primary cancer type was the strongest factor associated with survival differences (HR 1.422–1.758, p < 0.001). Poorly differentiated or undifferentiated histology (HR 1.249) and age over 60 years were independently associated with worse survival, and no single metastatic site showed a consistent adverse association across cancer types.⁶ Site-specific outcomes also differ within a cancer type: in metastatic breast cancer, median overall survival was about 5.3 years with bone metastases but 2.35 years with multiple metastatic sites in one large cohort analysis.⁹

Treatment

There are treatments for most types of metastatic cancer. The goal is often to control the disease by stopping or slowing growth, and some people live for years with well-controlled metastatic cancer. Treatment depends on the primary cancer type, where it has spread, prior treatments, and general health; palliative care to relieve symptoms can be given at any point.² Research approaches include helping the immune system fight cancer, disrupting the steps that allow cells to spread, and targeting specific genetic changes in tumors.²

References

Topic: Encyclopedia › Life and health › Human health and medicine › Diseases and injuries › Digestive, metabolic, and endocrine conditions › Gastrointestinal cancers

Initially written Sep 17, 2026 · Reviewed: Sep 17, 2026; Sep 30, 2026; Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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