General evaluation. The diagnosis of cancer can be simple or it can challenge all the skills of clinical investigation.
In some instances metastatic tumors may be the first indication of cancer, and the primary lesion may escape efforts at detection.
The challenge is to detect cancer as early as possible, when it is most likely to be cured.
One third of new patients have in situ or localized cancer, one quarter have regional lymph node disease,
and one third have distant disease;the remaining few per cent have disease of undefined stage.
Early detection of localized disease is aided by an awareness of risk factors in the family history, personal habits (tobacco, alcohol, sun exposure) and occupational exposure (asbestos, chromium, benzene).
It also requires attention to subtle and nonspecific symptoms of fatigue, weakness, weight loss, depression, headache, pain, changes in bowel habits,
persistent cough or hoarseness, rectal bleeding，and other clues from the history.
In evaluating a patient who presents with nonspecific symptoms that might be indicators of malignancy, such as weight loss, fever, or fatigue,
the physician should carefully examine all mucosal surfaces, the sigmoid colon, and the rectum for masses or ulcerated lesions.
In addition to elements of a routine examination, stool should be tested for occult blood.
More subtle clues may be seen in the skin with finding such as petechiae, hyper-pigmentation of skin folds (acanthosis nigricans) , or atypical moles (dysplastic nevi).
Attention should be paid to the presence of systemic cancer-associated effects, such as neuromyopathies.
Leads from laboratory testing may be found in unexplained anemia, thrombocytopenia, hypercalcemia, or elevation of serum LDH and acid or alkaline phosphatase levels.
Other frequent harbingers of cancer are pulmonary nodules or radiolucent bone lesions associated with new bone pain.
As a pathologic entity, cancer is defined by its properties of uncontrolled local proliferation of cells,
with invasion of adjacent normal structure and by distant spread, or metastasis, via the bloodstream of lymphatic or within a body cavity.
As a biologic entity, the malignant cell is defined by its ability to grow in tissue culture without the need for attachment to a firm surface
and by its loss of responsiveness to growth regulatory signals that cause differentiation and suppress proliferation.
Many malignant cells preserve the growth and antigenic properties characteristic of fetal cells，
secrete proteins characteristic of fetal tissues ( such as the a-fetoprotein of hepatocellular carcinomas and germ-cell tissues) ,
and appear to be frozen in an early state of differentiation that recapitulates a specific stage in normal organ development.
For example, malignancies arising from the lymphoid system reflect all stages and types of B-and T-lymphocyte development and preserve the same complex of immunoglobulin
and T-cell receptor gene rearrangement and cell surface proteins ( and in some cases potential for future differentiation) found in normal counterparts of the immune system.
These properties have become the basis for classification, diagnosis, and even treatment of tumors,
as, for example, classification of lymphoid tumors based on reactivity with monoclonal antibodies.
While the above characteristics are typical of most cancer cells, they are not universal.
Some endocrine-related tumors, for example, not only maintain well-differentiated morphologic features of their tissue of origin
but also retain endocrine function and produce bioactive hormonal substances typical of the mature tissue, as in pheochromocy-tomas.