The ability to spread and seed new tumour growth at distant body sites, away from the original lesion, is a characteristic of malignant lesions. The effect and the process are called metastasis. This is different from the recurrence of a malignant tumour at its original site.
While most primary malignant tumours are able to metastasise, the degree and likelihood of spread varies considerably between different types of malignancies. For example, basal cell carcinoma of the skin very rarely spreads, whereas breast, prostate and lung cancers all have a high propensity to develop metastases. Sometimes metastases are detected but it is impossible to find and identify a primary malignancy; such metastases are called metastases of unknown primary or cancer of unknown primary (CUP). This can happen if the primary malignancy is too small to detect, or has resolved spontaneously but not before causing metastasis. Some primary malignancies (in particular cancers of the oesophagus (gullet) and bone (osteosarcoma) and oral cancers (squamous cell carcinoma) sometimes show so called skip lesions (also known as skip metastases) where regions of healthy tissue are interspersed with regions of malignant growth.
Different primary malignancies in different organs have different preferred locations for establishment and growth of metastases. For example, breast and prostate cancers tend to spread to bones (including the skull and facial bones) and lung and colon cancers to the liver. The establishment of metastases can be seen as an evolutionary process. Throughout this overall process there are significant differences between regional metastases (for example, metastases from oral cancer to the lymph nodes of the neck) and distant (systemic) metastases (for example, metastases from breast cancer to the brain).
Most head & neck malignancies have a relatively low likelihood for developing distant metastasis, although more are found currently as more oral cancer patients are living with and beyond cancer. Lymphatic regional metastasis to the cervical nodes (lymph nodes of the neck) occurs in roughly half of oral cancer patients.
Malignancies of the scalp skin tend to spread to lymph nodes in the parotid salivary glands as well as the neck, the parotid lymph nodes often being the first nodes these cancers reach.
Most distant metastases from primary head & neck malignancies are found in the lungs and some in bones (regional and distant metastases).
Primary malignancies that may lead to distant metastases in the head & neck region are breast (bone and soft tissue metastases, including brain and eyes) and prostate cancers (bone metastases), soft-tissue deposits from some forms of leukaemia, bone lesions from multiple myeloma (a primary malignancy affecting the bone marrow) and melanoma (an aggressive and invasive form of skin cancer) may progress to a variety of metastatic locations in soft (including brain and other nerve tissue) and hard tissues.
The development and growth of metastases from a primary malignancy occurs in stages. This process and its underlying mechanisms are well investigated (mostly for the most common malignancies) and are in parts quite well understood. This underpins the so called TNM system (T (tumour), N (lymph node), M (systemic metastases)). Using the TNM system all these aspects are assessed and staged (their advancement is ranked) at diagnosis, leading to an overall assessment and rational planning for optimal treatment.
Unfortunately, the development and growth of distant metastases are not yet well enough understood to either prevent or improve the treatment of systemic metastatic disease. Curative treatment or treatment with curative intent is very rarely undertaken if head & neck cancers have been detected systemically. If the rationale is symptom improvement at the primary site, this is sometimes called ‘radical treatment with palliative intent’. Very clearly a full understanding of what is being done and why between patient, carers and treating clinicians is essential.
Next section: Metastases