(see visual). To treat tumors at greater depth, one needs a beam with higher energy, typically given in eV (electron volts). Accelerators used for proton therapy typically produce protons with energies of 70 to 250 MeV. Adjusting proton energy during the treatment maximizes the cell damage within the tumor. Tissue closer to the surface of the body than the tumor gets less radiation, and thus less damage. Tissues deeper in the body gets very few protons, so the dose becomes immeasurably small. In most treatments, protons of different energies with Bragg peaks at different depths are applied to treat the entire tumor. These Bragg peaks are shown as thin blue lines in the figure in this section. It is important to understand that, while tissues behind (or deeper than) the tumor get almost no radiation, the tissues in front of (shallower than) the tumor get radiation dosage based on the SOBP. Equipment Most installed proton therapy systems use isochronous cyclotrons. Cyclotrons are considered simple to operate, reliable and can be made compact, especially with use of superconducting magnets. Synchrotrons can also be used, with the advantage of easier production at varying energies. Linear accelerators, as used for photon radiation therapy, are becoming commercially available as limitations of size and cost are resolved. Modern proton systems incorporate high-quality imaging for daily assessment of tumor contours, treatment planning software illustrating 3D dose distributions, and various system configurations, e.g. multiple treatment rooms connected to one accelerator. Partly because of these advances in technology, and partly because of the continually increasing amount of proton clinical data, the number of hospitals offering proton therapy continues to grow. FLASH radiotherapy is a technique under development for photon and proton treatments, using very high dose rates (necessitating large beam currents). If applied clinically, it could shorten treatment time to just one to three 1-second sessions, and further reducing side effects. History The first suggestion that energetic protons could be an effective treatment was made by Robert R. Wilson in a paper published in 1946 while he was involved in the design of the Harvard Cyclotron Laboratory (HCL). The first treatments were performed with particle accelerators built for physics research, notably Berkeley Radiation Laboratory in 1954 and at Uppsala in Sweden in 1957. In 1961, a collaboration began between HCL and Massachusetts General Hospital (MGH) to pursue proton therapy. Over the next 41 years, this program refined and