Metastasis — Spread of Cancer Cells

Metastasis involves a complex cascade of biological events termed the metastatic cascade. Initially, malignant cells lose adhesion to neighbouring tissue, invade the extracellular matrix, and penetrate local blood vessels or lymphatic channels in a process called intravasation. These circulating tumour cells survive the shearing forces of circulation and immune surveillance before lodging in the microvasculature of distant organs.

Once arrested in a target organ, cancer cells exit the bloodstream through extravasation and invade the foreign tissue microenvironment. To establish a viable secondary tumour, the cells must adapt to the new niche, avoid local immune destruction, and stimulate the formation of new blood vessels through angiogenesis. Because the metastatic process is biologically inefficient, only a minority of circulating cells successfully seed established distant tumours.

Detecting metastasis fundamentally changes how cancer is staged, treated, and monitored. While localised cancers are often managed with curative intent through surgery or focal radiation, metastasis generally requires systemic therapies that travel via the bloodstream. Understanding whether metastasis has occurred helps you and your clinical team set realistic, personalised goals for treatment. These goals often focus on halting disease progression, shrinking tumours, preserving quality of life, and alleviating symptoms. Knowing the locations and extent of metastasis also guides choices regarding targeted therapy, palliative radiotherapy, and supportive care.

Metastasis is primarily identified and tracked using cross-sectional imaging, including contrast-enhanced CT scans, MRI scans, and whole-body PET-CT imaging. Bone scintigraphy is often utilised for detecting skeletal metastases. When a suspicious new lesion is discovered, an image-guided core needle biopsy or fine-needle aspiration is frequently performed to confirm malignancy and evaluate if the secondary tumour's molecular markers differ from the primary tumour. Liquid biopsies analysing circulating tumour DNA (ctDNA) are also increasingly used to monitor metastatic activity in blood samples.

Metastasis can occur in nearly all invasive malignant tumours. Common solid tumours that frequently metastasise include breast, prostate, lung, colorectal, and pancreatic cancers, as well as melanoma and renal cell carcinoma. Different cancers exhibit typical patterns of organ involvement, known as organotropism. For example, prostate and breast cancers commonly spread to bones, colorectal cancer frequently metastasises to the liver and peritoneum, and lung cancer and melanoma often travel to the brain, liver, adrenal glands, and bones.

Treatment for metastatic disease is predominantly systemic, reaching cancer cells throughout the body. Regimens frequently include chemotherapy, immunotherapy, targeted therapy, or hormone-blocking agents tailored to the cancer's biological profile. Local therapies, such as stereotactic body radiation therapy (SBRT), surgical resection, or radiofrequency ablation, may be applied to oligometastatic disease (a small number of discrete metastases) or used palliatively to relieve pain, prevent fractures, or resolve spinal cord compression, alongside supportive and bone-protective medications.