Breast cancer cells. Coloured scanning electron micrograph (SEM) of breast cancer cells. Due to genetic defects, cancer cells divide rapidly in a chaotic manner, leading to a ragged appearance. The cells may clump to form tumours, which 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.