Merck Manual

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Overview of Cancer


Robert Peter Gale

, MD, PhD, Imperial College London

Last full review/revision Nov 2020| Content last modified Nov 2020
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Cancer is an unregulated proliferation of cells. Its prominent properties are

  • A lack of differentiation of cells

  • Local invasion of adjoining tissue

  • Metastasis, which is spread to distant sites through the bloodstream or the lymphatic system (often)

The immune system may play a role in eliminating early cancers or premalignant cells. This concept is termed immune surveillance. Patients with congenital or acquired immunodeficiency have an increased cancer risk, particularly those cancers associated with viral infection, kidney cancer, and melanoma.

Most cancers are potentially curable, particularly if detected at an early stage, and long-term remission is often possible in those detected at later stages. There is controversy whether cancers detected at an early stage, such as breast cancers detected by screening mammography or prostate cancers detected by screening prostate-specific antigen (PSA) testing, will progress during a patient's lifetime. However, for many cancers early detection increases the potential for cure.

When cure is not possible, as in many cases of advanced cancer, judicious treatment with radiation therapy, drugs, and/or surgery may improve quality of life and prolong survival. However, in other patients, particularly in older patients and in those with comorbid conditions, such treatment may be poorly tolerated, and palliative care may be appropriate.

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A 10-year-old girl is brought to the clinic by her father because she has had pain in her right upper leg for the past 3 months. On physical examination, the patient is unable to bear weight on the limb. Edema of the upper leg and tenderness to palpation are noted. X-rays show 2 bone lesions with sharp margins and a punched-out appearance. Which of the following is the most appropriate definitive step in diagnosis?
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