A biopsy is a medical examination that involves removing a portion of tissue from an organ, so that it can be studied. Its main interest is to give objective results in certain diseases that require a definite diagnosis. In the case of cancer, microscopic analysis of the samples taken makes it possible to see the cancer cells, and therefore to start a treatment that is known to be justified. Most biopsies are taken painlessly during local anaesthesia or, especially with children, under sedation or general anaesthesia. Some biopsies are deep and therefore must be performed under ultrasound or scan control.