Seventy-year-old female patient with metastatic endometrial carcinoma (cancer of the epithelial tissue) undergoing external beam radiotherapy. Endometrial carcinoma is cancer of the endometrium (layer of tissue that lines the uterus). Cancer that spreads to other body parts is known as metastatic. The cause of this cancer is unknown, but being over the age of 50, high oestrogen levels, and obesity increase the risk of developing it. Symptoms include abnormal bleeding and lumps or swelling in the abdomen or pelvis. Treatments include surgery, radiotherapy and chemotherapy. During external beam radiotherapy beams of X-ray radiation are focused on the tumour, killing or slowing the growth of the cancerous cells by damaging their DNA (deoxyribonucleic acid).The red light is part of the machine's aligning system, which closely monitors the patient's movement to ensure precise targeting of the tumour. If the patient moves, the system detects it and stops the radiation.