Metastasis

Metastasis is the spread of cancer cells from a primary tumor to distant sites in the body, where they establish secondary tumors. The process typically involves local invasion of surrounding tissue, intravasation into blood or lymphatic vessels, survival in circulating vessels, extravasation into distant tissues, and colonization to form new tumors. Metastasis is the leading cause of cancer-related mortality and significantly influences prognosis and treatment choices.

Cancer cells can reach distant organs through two main routes: hematogenous spread, via the bloodstream, and lymphatic spread, via the lymphatic system. Hematogenous spread commonly involves organs with high blood flow such as the liver, lungs, bones, and brain. Lymphatic spread often first affects regional lymph nodes and can extend to other nodal basins.

Biological mechanisms underlying metastasis include genetic and epigenetic changes that promote invasion, survival, and growth in new environments. Epithelial-mesenchymal transition, angiogenesis, and evasion of the immune response contribute to progression. The “seed and soil” concept describes how certain tumor cells (the seed) preferentially colonize organs with permissive microenvironments (the soil), though metastatic spread is still a highly inefficient and selective process.

Clinically, metastases may be detected by imaging, biopsy, or tumor markers. The presence of distant metastasis is designated as stage IV in many cancers and generally indicates a poorer prognosis, though advances in targeted therapies and immunotherapies have improved outcomes for some patients.

Treatment aims to control disease, alleviate symptoms, and extend survival. Approaches include systemic therapies (chemotherapy, targeted therapy, hormonal therapy, immunotherapy) and local therapies (surgery, radiotherapy) for palliation or local control.