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The Path to Proton Therapy

The Path to Proton Therapy

Proton therapy is a precise, effective cancer treatment, and an important resource for patients and caregivers in the Pacific Northwest. The ability to offer this modality of radiation was the result of decades of work by scientists across multiple disciplines. Here’s a look at what had to happen to make proton therapy available today.

Earliest History of Radiation

No type of radiation therapy would be possible without the discovery of X-rays by Wilhelm Röntgen in 1895, who was awarded the first Nobel Prize in Physics in 1901 for his work. This discovery was met with extreme interest in the scientific world, and a number of lines of research began. In fact, within a month of Röntgen's publication, the medical community was using X-rays to help surgeons.

Wilhelm Röntgen

Scientists soon discovered that prolonged exposure to radiation had noticeable effects, such as inflammation and tissue damage. Léopold Freund and Eduard Schiff, intrigued by this, suggested X-rays could be used in the treatment of disease. As early as 1896, a doctor in Chicago used X-rays to treat a woman with recurrent breast cancer. Shortly after that, journals had published many successful treatments of different types of skin issues with this radiation. However, the scientific community still didn't know what exactly was therapeutic. They speculated that it might bethe electrons or the ozone they created. In 1900, an Austrian radiologist named Robert Kienböck demonstrated that it was the X-rays themselves.

Soon the field of röntgenotherapy was born, mainly to treat skin problems such as eczema, lupus, carcinomas, leukemia, and bacterial diseases such as tuberculosis.

To treat with X-rays, scientists used cathode tubes to accelerate electrons, which, when they hit metal at the end, created the rays. These were then directed through the skin. However, when trying to treat deeper-lying tumors, the X-rays deposited too much radiation in the skin, leading to unwanted side effects. They experimented with cobalt and other materials to create higher energy electrons, which in turn, created higher energy radiation. To put early X-ray acceleration into perspective, using the earliest tubes, physicists could create 125,000 volts. Machines now can generate 18 million volts. The case remains that, due to the nature of X-rays, most radiation is deposited at entry into the body, and decreases as it passes through the tissue. To counteract this, doctors now often use multiple rays that overlap only at the tumor site.

Discovery of Protons and Their Superpower

Around the turn of the 20th century, a series of discoveries helped us understand the various components of atoms. First JJ Thompson discovered the positive and negative charges in atoms, leading to the discovery of electrons. A few years later, Ernest Rutherford discovered the atomic nucleus and the positively-charged particles they contained – protons.

Ernest Rutherford

William Henry Bragg found that charged particles in the atom lose energy as they travel through matter. For protons, this energy loss happens suddenly, before it comes to a stop. Bragg discovered this phenomenon in 1903, and we now call it the Bragg Peak. This discovery showed great promise in reducing potential radiation side effects if used to treat conditions underneath deeper layers of healthy tissue. American physicist Robert R. Wilson was the first to suggest using protons for radiation therapy in a paper published in 1946. Berkley Radiation Laboratory, where Wilson had studied under Ernest Lawrence (see below), delivered the first proton therapy treatments for cancer in 1954. We now understand their importance in treating any tumors that are close to vital organs. 

William Henry Bragg

The Bragg Peak


A cyclotron is a type of particle accelerator that was invented by physicist Ernest O. Lawrence in 1930 at the University of California, Berkeley. Before cyclotrons, particles such as protons were accelerated in linear accelerators called Linacs. Linacs cost more to operate and require more space than Cyclotrons, which accelerate particles in a spiral path, resulting in both space and cost savings. Cyclotrons can accelerate protons to two-thirds the speed of light.

Ernest O. Lawrence


The first treatments using any radiation were done without any X-ray imaging. Physicians made the margins around a tumor sufficiently big to ensure that the cancer was completely treated. External marks placed on the patient’s skin defined the area to be treated, and patients were immobilized using various accessories to ensure they were positioned properly every day. Imaging, even now, is not necessary in some instances, such as when the tumor lies on the skin.

In 1971, engineer Godfrey Hounsfield and physicist Allan Cormack  invented the computerized tomography (CT) scan, which combines a series of X-ray images taken from different angles around the patient’s body and uses computer processing to create cross-sectional images (slices) of the bones, blood vessels and soft tissue. Shortly after, physicians treating patients with protons began using the scans to determine the precise location of the tumor as well as the beam strength necessary. It’s fair to say that CT scans were crucial to the development of proton therapy.

Today, imaging is used in two ways for radiation therapy: CT-based imaging that doctors use to plan treatment, and X-ray imaging to position the patient precisely for treatment each day.

Treatment Planning Software

Treatment planning systems are essential for all kinds of radiation therapy. These systems are highly sophisticated computer programs developed by physicists. In straightforward terms: a proton beam coming from the cyclotron has many parameters. These parameters can be optimized to make cancer treatments better by tweaking them to ensure maximum damage to cancer cells and minimized harm to healthy cells. A treatment planning system allows tweaking of these parameters and, once finalized, sends them to a proton therapy delivery system for optimum treatment.

Many of the early proton therapy centers had to write their own treatment planning software, as commercial software was not available.  Some centers continue to use non-commercial software written by physicists.

FDA Approval

Proton therapy has been used in a research setting since the 1950s. Still, before it could become commercially available for treatment through the United States, it had to get approval from the Food and Drug Administration. Usually, a new device or procedure needs an investigational device exemption which allows scientists to use the investigational device in clinical studies – first in animals and eventually in humans - in order to collect safety and effectiveness data. There must be enough evidence that a device is safe and effective for high-risk devices such as cyclotrons. FDA approval was obtained for proton therapy in 1988. Even after approval, operators, manufacturers and physicians are required to track and report operational data about the facilities and their patients.

Cost of Centers

In the beginning, only well-established, large, hospitals could afford to commit to providing proton therapy because of the high cost of construction and equipment. We’re fortunate that the Seattle Cancer Care Alliance had the foresight, interest, and ability to open a proton therapy center in 2013. Today, many hospitals and treatment centers are opting for a one-room proton treatment facility as an affordable way to offer all the advantages of proton therapy. In addition to the physical construction, proton therapy needs collaboration between many different professional groups including engineers, scientists, physicists, physicians, treatment planners (dosimetrists), nurses, radiation therapists, and other support staff.

Many events and discoveries had to fall into place to make proton therapy possible and available to a wide number of people. Thanks to all the excellent research, discovery, and exploration, we can now treat many solid tumors with minimal side effects safely, effectively and with more precision than traditional radiation methods. We are proud to have treated nearly 3,000 patients at SCCA Proton Therapy Center, and grateful to have this important resource available to everyone who needs it in the Pacific Northwest.