Radiotherapy is the use of radiation to treat conditions such as cancer. The most common technique exposes tumors to high energy photons (x-rays) and/or electrons while sparing normal tissues. Other uses are to prepare a patient for organ transplantation by suppressing immune response and to calm muscle twitches. The therapy can be delivered from the outside (external beam radiotherapy), by radioactive implants placed in the body (brachytherapy), or during surgery (interoperative). Other types of radiotherapy such as proton therapy and neutron therapy -- available at very limited centers -- have their own distinct advantages and disadvantages.