Benign Tumors Benign tumors are often named by adding the suffix -oma to the cell type (e.g., lipoma), but there are exceptions (e.g., lymphomas, melanomas, hepatomas). Benign tumors arising from glands are called adenomas; those from epithelial surfaces are termed polyps or papillomas.
Benign tumors are characterized by the following:
• Usually are encapsulated
• Grow slowly
• Usually are nonspreading
• Have minimal mitotic activity
• Resemble the parent tissue
Other types of tumors include non-neoplastic lesions associated with an overgrowth of tissue that is normally present in the organ (e.g., hyperplastic tissue) and choristomas, normal tissue in a foreign location (e.g., pancreatic tissue in the stomach).
Malignant Tumors
A malignant neoplasm of epithelial origin is referred to as carcinoma, or cancer. Those arising from squamous epithelium (e.g., esophagus, lung) are called squamous cell carcinomas, those arising from glandular epithelium (e.g., stomach, colon, pancreas) are called adenocarcinomas, and those arising from transitional epithelium in the urinary system are called transitional cell carcinomas.
Other types of malignant tumors include amine precursor uptake and decarboxylational tumors. These are neuroendocrine tumors that commonly develop from neural crest and neural ectoderm (e.g., small cell carcinoma of lung). Sarcomas, malignant tumors of connective tissue origin (e.g., fibrosarcoma), and teratomas are derived from all three germ cell layers (e.g., teratoma of the ovary or testis). Malignant tumors are characterized by the following:
• Increase in the number of cells that accumulate
• Usually, invasion of tissues
• Dissemination by lymphatic spread or by seeding within a body cavity
• Metastasis
• Characteristic nuclear cellular features
• Receptors for integrin molecules (e.g., fibronectin), which help malignant cells adhere to extracellular matrix, type IV collagenases, which dissolve basement membranes, and proteases
• Secretion of transforming growth factor α (TGF-α) and transforming growth factor β (TGF-β) to promote angiogenesis and collagen deposition
• Often, recurrence after attempts to eradicate the tumor by surgery, radiation, or chemotherapy
Biologically distinct and relatively rare populations of tumor-initiating cells have been identified in cancers of the hematopoietic system, brain, and breast. Cells of this type have the capacity for self-renewal, the potential to develop into any cell in the overall tumor population, and the proliferative ability to drive continued expansion of the population of malignant cells. The properties of these tumor-initiating cells closely parallel the three features that define normal stem cells. Malignant cells with these functional properties are termed cancer stem cells (Fig.1). Cancer stem cells can be the source of all the malignant cells in a primary tumor.
Fig1. Stem cell systems. Normal tissues arise from a central stem cell that grows and differentiates to create progenitor and mature cell populations. Key properties of normal stem cells are the ability to self-renew (curved arrow), multilineage potential, and extensive proliferative capacity. Cancer stem cells arise by means of mutation in normal stem cells or progenitor cells and subsequently grow and differentiate to create primary tumors (broken arrow indicates that specific types of progenitors involved in the generation of cancer stem cells are unclear). As with normal stem cells, cancer stem cells can self-renew, give rise to heterogeneous populations of daughter cells, and proliferate extensively. (Adapted from Jordan CT, Guzman ML, Noble M: N Engl J Med 355:1253-1260, 2006.)
Despite decreases in the incidence of some cancers and associated mortality, cancer remains highly lethal and very common. About 41% of Americans will develop some form of cancer, including nonmelanoma skin cancer, in their lifetime; 20% of Americans will die from cancer. Cancer is the second leading cause of death in the United States.
