History of Proton Therapy

The proton was discovered by Ernest Rutherford in the early 1900’s. During this period, his research resulted in a nuclear reaction which led to the first ‘splitting’ of the atom, where he discovered protons. He named his discovery “protons” based on the Greek word “protos” which means first. It was also discovered that charged particles (protons and light ions) have a finite range in matter. The interaction probability to cause ionization increases as they lose velocity along their paths, so that a peak of deposited dose occurs at a depth proportional to the energy of the charged particle. Beyond this peak, no further dose is deposited. This scientific phenomenon was described by William Bragg at that time. In 1930, the American physicist Ernest O. Lawrence and his associates were the first to invent the cyclotron to accelerate proton to an energy high enough for cancer treatment applications. He invented the cyclotron in 1929 & developed it as a particle accelerator during the 1930s, winning the 1939 Nobel Prize for physics for this work. In 1931, he founded the Radiation Laboratory, later named the Lawrence Berkeley Laboratory (Figure 1). A decade later, his advanced version of the synchrocyclotron, which is 184 inches in diameter, is capable of producing 340 MeV protons (Figure 2).

Figure 1 American physicist Ernest O. Lawrence [1901-1958], photographed in 1937 adjusting the ion source of his 60-inch cyclotron. Lawrence moved to the University of California at Berkeley in 1928. He invented the cyclotron in 1929 & developed it as a particle accelerator during the 1930s, winning the 1939 Nobel Prize for physics for this work. In 1931 he founded the Radiation Laboratory, later the Lawrence Berkeley Laboratory, & directed it until his death. (Credit: LAWRENCE BERKELEY LAB/SCIENCE PHOTO LIBRARY)

Figure 2 Engineers in 1942 working on the construction of the 184-inch synchrocyclotron at the Radiation Laboratory at the University of California, Berkeley, USA. This cyclotron was developed by the laboratory’s director Ernest Orlando Lawrence. (Credit: LAWRENCE BERKELEY LAB/SCIENCE PHOTO LIBRARY)

In 1946, Dr. Robert Wilson wrote a seminal paper proposing the idea that proton beams could be used for cancer treatment while he was in the Physics Department at Harvard University. He described the fundamental physical feature of the depth-dose curve for protons and heavy-charged particles in comparison with photons or X-rays. He described the way the particle beams deposit their energy as the beam enters the body in route to the tumor: a smaller amount of energy is released first, and then a much larger amount of the beam energy is released at the end of its path (Bragg peak) and then completely stops (Figure 3).

Figure 3 Percent dose versus depth in the patient’s body. As a proton beam enter the body, it loses some energy and deposits most of its energy at the end of its range (Bragg’s peak) to the tumor.

Wilson did also play a significant role in the development of nuclear weapons during World War II (“The Manhattan Project”); but afterwards, he chose to shift his focus of nuclear physics into medical application for the betterment of mankind. In addition to Wilson being a very accomplished sculptor and architect, he was later responsible for the development of Fermi Laboratory and became its founding director (Figure 4).

Figure 4 Dr. Robert Rathburn Wilson, an American physicist, was the first to propose the use of proton beam therapy for cancer treatment in his seminal paper in 1946. He is considered to be “the father of proton therapy.” His other contributions to science included being a group leader of the Manhattan Project, a sculptor, and an architect of the Fermi National Laboratory (Fermilab), where he was also the director from 1967-1978. He is pictured here at the ground breaking ceremony of FermiLab. (Credit. Scientific American)