Breast cancer. Light micrograph of a section through breast tissue showing adenocarcinoma (purple lobes). This invasive, cancerous tumour arises from the epithelial lining of mammary lobules. This is a fairly common form of breast tumour. Breast cancer has a peak incidence in the decade before the menopause. Surgical treatment may involve lumpectomy (removal of the tumour) or mastectomy (removal of the breast) and is often combined with use of radiotherapy and/or chemotherapy. Magnification unknown.