Malignant neoplasms or cancers have several distinguishing features that enable the pathologist
or experimental cancer biologist to characterize them as abnormal. The most common types of human neoplasms derive from epithelium, that is, the cells covering internal or external
surfaces of the body. These cells have a supportive stroma of blood vessels and connective
tissue. Malignant neoplasms may resemble normal tissues, at least in the early phases of their
growth and development. Neoplastic cells can develop in any tissue of the body that contains
cells capable of cell division. Though they may grow fast or slowly, their growth rate frequently
exceeds that of the surrounding normal tissue. This is not an invariant property, however, because
the rate of cell renewal in a number of normal tissues (e.g., gastrointestinal tract epithelium,
bone marrow, and hair follicles) is as rapid as that of a rapidly growing tumor.
The term neoplasm, meaning new growth, is often used interchangeably with the term tumor
to signify a cancerous growth. It is important to keep in mind, however, that tumors are of two
basic types: benign and malignant. The ability to distinguish between benign and malignant tumors
is crucial in determining the appropriate treatment and prognosis of a patient who has
a tumor. The following are features that differentiate a malignant tumor from a benign tumor:
1. Malignant tumors invade and destroy adjacent normal tissue; benign tumors grow
by expansion, are usually encapsulated, and do not invade surrounding tissue.
Benign tumors may, however, push aside normal tissue and may become life threatening
if they press on nerves or blood vessels or if they secrete biologically active
substances, such as hormones, that alter normal homeostatic mechanisms.
2. Malignant tumorsmetastasize through lymphatic channels or blood vessels to lymph
nodes and other tissues in the body. Benign tumors remain localized and do not
metastasize.
3. Malignant tumor cells tend to be ‘‘anaplastic,’’ or less well differentiated than normal
cells of the tissue in which they arise. Benign tumors usually resemble normal tissue
more closely than malignant tumors do. Some malignant neoplastic cells at first
structurally and functionally resemble the normal tissue in which they arise. Later, as
themalignant neoplasmprogresses, invades surrounding tissues, and metastasizes, the
malignant cells may bear less resemblance to the normal cell of origin. The development
of a less well-differentiated malignant cell in a population of differentiated normal
cells is sometimes called dedifferentiation. This term is probably a misnomer for the
process, because it implies that a differentiated cell goes backwards in its developmental
process after carcinogenic insult. It is more likely that the anaplastic malignant cell type arises from the progeny of a tissue ‘‘stem cell’’ (one that still has a capacity for renewal and is not yet fully differentiated), which has been blocked or diverted in its pathway to form a fully differentiated cell.
Examples of neoplasms that maintain a modicumof differentiation include islet cell
tumors of the pancreas that still make insulin, colonic adenocarcinoma cells that form
glandlike epithelial structures and secrete mucin, and breast carcinomas that make
abortive attempts to form structures resembling mammary gland ducts. Hormoneproducing
tumors, however, do not respond to feedback controls regulating normal tissue
growth or to negative physiologic feedback regulating hormonal secretion. For
example, an islet cell tumor may continue to secrete insulin in the face of extreme
hypoglycemia, and an ectopic adrenocortiocotropic hormone (ACTH)-producing lung
carcinomamay continue to produce ACTH even though circulating levels of adrenocortical
steroids are sufficient to cause Cushing’s syndrome Many
malignant neoplasms, particularly the more rapidly growing and invasive ones, only
vaguely resemble their normal counterpart tissue structurally and functionally. They
are thus said to be ‘‘undifferentiated’’ or ‘‘poorly differentiated.’’
