Radiotherapy by X-rays or by the gamma rays of radioactive substances may be used for certain benign conditions or for malignant disease. Because of its possible ill effects it should be advised only with caution for benign lesions, but its use may occasionally be justified in the treatment of ankylosing spondylitis and in cases of giant-cell tumour of bone that are unsuitable for local excision. In malignant disease radiotherapy is usually palliative rather than curative. In conditions such as malignant bone tumours, for which a tumour dose in the order of7000 roentgens may be required, only the penetrating rays produced by a super-voltage X-ray plant or by a radio-active cobalt unit should be used. With such apparatus a high dose can be delivered to the tumour with the least possible damage to the skin.