Sometimes, cells keep dividing when new cells are not needed. The mass of extra cells forms a growth or tumor . Tumors can be either benign or malignant. Benign tumors are not cancer . They often can be removed and, in most cases, they do not come back.
Malignant tumors are cancerous.
Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them.