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).