As a fundamental disorder of cellular growth and differentiation or development, cancer is essentially a genetic disorder at the cellular level. Most tumors are encapsulated and benign in behavior. Occasionally they may create symptoms from cosmetic or mass effects. In using the generic word cancer , however, we are concerned here with malignant tumors that are morphologically abnormal under the microscope. They show uncontrolled growth leading to local invasion with disruption of tissues, and later metastasis or spread to loco-regional lymphatics and later the blood stream.
Cancer kills mostly through blood-borne metastasis.