A tumor manifests as an enlarging space-occupying mass. The expanding mass compresses nearby blood vessels, leading to necrosis and an area of inflammation around the tumor. Infection may develop. Malignant cells do not adhere to each other but often break loose from the mass, infiltrating into adjacent tissue. Some neoplasms develop very rapidly, whereas other remain in situ for a long time. In situ refers to neoplastic cells in a preinvasive stage of cancer that may persist for months or years and offer an excellent opportunity for early diagnosis of cervical cancer and certain oral cancers.
As a tumor mass enlarges, the inner cells are frequently deprived of blood and nutrients and die. This necrosis can lead to more inflammation and infection at the site. Many tumor cells "trap" nutrients, depriving normal cells and preventing tissue regeneration. Also, tumor cells often secrete enzymes such as collagenase, which break down protein or cells, adding to the destruction and facilitating the tumor's spread into adjacent tissue. Inflammation and the loss of normal cells lead to a progressive reduction in organ function.
Grading of tumors is based on the degree of differentiation of the malignant cells -a grade I tumor has well- differentiated cells similar to the original cells, whereas a grade IV tumor is undifferentiated with cells varying in size and shape (anaplasia); this type of tumor is considered highly malignant and likely to progress quickly.