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Cancer has a characteristic natural history. Healthy cells first
become dysplastic showing subtle morphological abnormalities under
the microscope which suggest the beginning of transformation.
The next step is carcinoma in situ where characteristic
abnormalities of both form and proliferation are present
but without invasion of the underlying basement membrane that
holds them in place in the tissue of origin. This earliest phase
is highly curable and is detected with screening programs, such as
the PAP smear for cervical cancer. Localized cancer is stage I
disease where the tumor exhibits invasion and disruption of local
tissues to form a primary lesion. Tumor cells then invade local
lymphatics and spread to the regional (stage II) or extended
regional (stage III) draining lymph nodes as secondary tumors.
Finally tumor cells invade into the blood stream where
characteristic patterns of blood-borne metastasis herald the onset
of stage IV disease. Particular tumors vary in the extent to which
they follow these phases in sequence: melanoma usually has a
distinct loco-regional phase, while breast cancer is systemic from
the beginning. Staging correlates with survival and provides an
essential guide both to prognosis and to the design of treatment
plans.
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