The most important differential factor among tumors is that they may be either benign or malignant. The benign tumor is usually harmless. These innocent new growths usually have a limiting membrane, and they push into the cells of the adjacent tissues. They never spread to other parts of the body. Malignant tumors comprise the diseases commonly called cancer. Malignant tumors and cancer are synonymous terms. The cancer cells grow between normal cells and infiltrate all the adjacent tissues by means of root-like extensions. Their process is always progressive. Later in the course of the disease, cancer cells may pass to other regions of the body to grow. Cancer has no limiting membrane and has no respect for what vital structures it invades. It is most important that benign tumors and cancer be distinguished from each other.
Benign and malignant tumors may arise in any tissue. Classification of tumors is based on this, and names given to each tumor specify the tissue from which it has arisen. For example, lipoma (fat-tumor) is the name awarded the more common tumor arising from fat tissue, and osteoma (bone-tumor) means a neoplasm arising from bone. A present system of nomenclature also implies whether the tumor is benign or malignant. There are as many different kinds of tumors as there are kinds of tissue. In all cases a specific type cell in a structure begins to proliferate and determines the cell type of the tumor. The varieties of tumors which can occur are numerous. It is not within the scope of this work to describe them all, but one must realize the tremendous number of possibilities.
Either benign or malignant tumors may arise from any cell in any tissue, and each has definite characteristics. In most cases the only exact method of determining the cell type, and whether it is benign or malignant, is to prepare a section of a bit of the tumor and study it with the microscope. The tumor always grows the same cell type throughout, and since each type cell has definite characteristics, the tissue from which the tumor is arising can be ascertained. In benign tumors the cells are always mature or in the same stage of maturity, but in malignant tumors the cells may be in all stages of maturity. The extension into adjacent tissues of the malignant growth may also be seen with the microscopic analysis of the tumor. Certain tissues are more prone to give rise to neoplasms than others, and indeed tumors of some tissues are extremely rare. But any cell in any tissue may at any time initiate a benign or a malignant neoplasm.