Neoplasia literally means “ new growth”. A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues and persists in the same excessive manner after the cession of the stimuli which evoke the change. All tumors, benign and malignant, have two basic components: parenchyma and supportive stroma. Based on the parenchyma component, tumor is classified into benign tumor, attaching the suffix –oma to the cell of origin, and malignant tumor. Malignant tumors arising in mesenchymal tissue are called sarcoma, while of epithelial cell origin are called carcinoma. The difference between benign and malignant tumors are degree of differentiation and anaplasia; rate of growth; local invasion and metastasis.
Molecular basis of cancer includes (1) nonlethal genetic damage lies at the heart of carcinogenesis. (2) three classes of normal regulatory genes—the growth promoting proto－oncogenes, the growth-inhibiting tumor suppressor genes, and genes that regulate programmed cell death, or apoptosis— are the principal targets of genetic damage. (3) Genes that regulate repair of damaged DNA, is also pertinent in carcinogenesis. (4) Carcinogenesis is a multipstep process at both the phenotypic and the genetic level.
Multiple factors influence tumor growth , such as kinetics of tumor cell growth, tumor angiogenesis and tumor progression and heterogeneity. Invasion and metastasis are biologic hallmark of malignant tumors, the metastatic cascade can be divided into two phases: invasion of the extracellular matrix and vascular dissemination and homing of tumor cells.
Chemical carcinogens; radiant energy are documented causes of cancer in humans, and the evidence linking certain viruses to human cancer grows ever stronger. Tumor antigens that elicit an immune response have been demonstrated in many experimentally induced tumors and in some human cancers.
Clinically, tumor may have local and hormonal effects, while cancers may also be responsible for cachexia or paraneoplastic syndromes. Grading of a cancer is based on the degree of differentiation of tumor cells and the number of mitoses within the tumor as presumed correlates of the neoplasm’s aggressiveness. The staging of cancer is based on the size of the primary lesion, its extent of spread to regional lymph nodes, and the presence or absence of blood-borone metastasis.
In this chapter, both benign tumors and cancer are discussed; the latter receive more attention. The focus is on the basic morphologic and behavioral characteristics and present understanding of the molecular basis of carcinogenesis. Also, the interactions of the tumor with the host and the host response to tumor are discussed.