Breast cancer cells. Animated scanning electron micrograph (SEM) of dividing breast cancer cells. During this stage of cell division (cytokinesis) the cell's cytoplasm divides. Here, the cells remain joined by a cytoplasmic bridge. Malignant cells divide rapidly in a chaotic manner and may clump to form tumours. They often invade and destroy surrounding tissues. Breast cancer is the most common form of cancer in women. It can be treated by surgical removal of the cancerous cells, often combined with radiotherapy and chemotherapy.