English: An early external beam radiotherapy (teletherapy) machine from 1951 using radium to treat cancer. It consisted of a radiation "gun" head made of lead containing 25 capsules of radium. The beam of gamma rays from each capsule passes through a hole in the front face. The 25 radiation beams converge on a single point 9 inches in front of the face of the machine. The purpose of the multiple beams was to reduce the dose of radiation to the patient's healthy tissue around the tumor. In use the machine was positioned above the patient, with the focal point of the beams at the tumor inside the patient's body. Thus the tumor received radiation from all 25 beams, while the intervening tissue only received radiation from one beam. When the treatment was over a remote control system would retract the radium capsules into a container of the liquid metal mercury in the head, blocking the beams,

Before nuclear reactors were invented in World War 2, virtually the only source of gamma rays for radiotherapy was radium, which was extremely rare and expensive. This machine used the largest amount of radium ever assembled for medical use; 50 grams, which cost 1 million dollars, equivalent to about $9 million in 2015 dollars. Within a few years nuclear reactors produced artificial radioisotopes for radiotherapy, and this machine was replaced by teletherapy machines using the much cheaper gamma ray source cobalt-60.

Alterations to image: cropped out captions and text graphics around image, and added drawing from another article showing beam paths.