APRIL 7, 2026 – Stanford Medicine is the first in the world to introduce ultracompact proton therapy that will make the advanced targeted radiotherapy more accessible to patients.
Stanford Medicine is opening a first-of-its-kind facility to deliver a form of radiation therapy that has been difficult for cancer patients to access.
The treatment, called proton therapy, enables oncologists to deliver cancer-killing radiation precisely to a tumor with minimal damage to healthy tissues. Although it was developed in the 1950s, proton therapy has been hard for Northern California patients to obtain. The football-field-sized footprint of traditional proton therapy equipment, and its high cost, prevents most hospitals from offering the treatment.
Stanford Medicine’s innovation — in collaboration with two medical technology companies — is to drastically shrink the size and cost of the machinery used to deliver proton therapy. They are opening the first proton therapy facility using the new equipment at the Stanford Medicine Cancer Center in Palo Alto, where leaders will cut the ribbon on April 7.
Stanford Medicine will initially be the only location in the world with the smaller equipment. The equipment is being adopted at other health care systems around the world over the next several months to years, which will eventually make proton therapy much more accessible.
“For our patients, the key is being able to eliminate their cancer without causing unacceptable collateral damage,” said Billy Loo, MD, PhD, professor of radiation oncology and co-director of particle therapy at Stanford Medicine, who played a key role in the innovation. “Proton therapy helps us balance that equation by making radiotherapy more precise.”
Especially for patients whose tumors are located near critical structures — such as the brain, heart, spinal cord, or nerves involved in speaking and swallowing — protons offer advantages over traditional radiotherapy delivered via X-rays, aka photons.
“With proton therapy, the particles are able to stop on a dime,” said Susan Hiniker, MD, associate professor of radiation oncology. “We expect similar rates of tumor control to what we see with X-rays. But with protons, the volume of tissue receiving radiation is smaller, and in many cases, this will lead to fewer long-term side effects.”
Protecting nearby tissues
Protons are the positively charged particles in the center of an atom. Their charge and mass allow them to be steered and stopped with precision.
“With protons, we can use ‘dose painting,’ using magnets to send small bits of the beam of protons in various directions to shape the dose,” said Yuan James Rao, MD, associate professor of radiation oncology. The protons can be precisely directed to tumors, even those with irregular shapes.
By adjusting their energy, protons can be targeted to stop inside a tumor, with little to no exit dose. In contrast, photons used in conventional radiotherapy travel through a tumor and keep going out of the body, exposing tissues behind the tumor to radiation.
Tightly directed protons help patients avoid consequences of off-target radiation, such as short-term nerve damage and organ damage from inflammation and scarring that develop months to years later. With minimal damage to healthy tissues near the tumor, proton therapy is often the best choice for tumors in the brain, spine, head and neck, lungs, liver, and prostate.
The minimized side effects of proton therapy are especially profound for children.
“Because kids are growing and developing, various parts of the body are more sensitive to even low doses of radiation,” said Hiniker, who treats children receiving radiotherapy at Stanford Medicine Children’s Health. Tissues such as the brain and the growth plates at the ends of bones are especially vulnerable to radiation injury, for example. In addition, because most kids with cancer are cured, they have more time than adult patients to experience long-term side effects of treatment.
As a pediatric radiation oncologist, Hiniker knows that undergoing radiotherapy isn’t always easy for kids. She developed a way to let children watch their favorite videos during treatment, a distraction technique that helps them stay still and avoid the need for anesthesia. The video distraction system, called AVATAR and supported by Stanford Medicine’s Catalyst program, has been installed in about 30 places around the world. It will be part of Stanford Medicine’s proton therapy setup.
Proton therapy isn’t the right choice for every cancer, however.
“Depending on the location and characteristics of the tumor, other radiotherapy techniques may still be best,” Hiniker said, noting that Stanford Medicine offers a wide variety of cutting-edge radiotherapy options. “Having proton therapy at Stanford Medicine gives us an important additional tool to consider as we recommend treatment in a personalized, case-by-case way.”
Expanding patient reach
Many Northern California cancer patients have been unable to access proton therapy because the nearest facility equipped to treat most cancers with protons was hundreds of miles away. (Radiotherapy treatment takes a month or more, and patients can’t always relocate for that long.)
By making proton therapy equipment smaller, less expensive and easier to install, Stanford Medicine is breaking the bottleneck. The new equipment fits into a standard linear accelerator vault of 1,200 square feet and was added in the Stanford Medicine Cancer Center without constructing a new building. Nine other medical centers around the world are also installing the new system, Loo said.
About four years ago, Loo and his Stanford Medicine colleagues realized that they knew of two medical device companies taking different spins on smaller equipment.
Mevion Medical Systems offered the most compact cyclotron in the industry, the machine that generates the protons. With these smaller machines, proton therapy facilities shrank from about the size of a football field to around half the size of a basketball court. But the smaller cyclotron still required a three-story building. This was because, during treatment, patients lay flat and the machine delivering radiation rotated on a gantry around the patient, moving one story above or below a treatment room on the building’s second floor.
Meanwhile, Leo Cancer Care Inc. took a different space-saving approach, developing a system that positioned patients upright for treatment, in a specialized treatment chair.
Upright treatment makes it easy to move the patient during treatment, enabling the proton source to stay still, Loo said. “It’s much easier to rotate the patient rather than having to rotate a big machine around the patient.”
Loo’s team wondered if the two innovations could be merged.
“Once we brought the companies together, the light bulbs went on,” he said. Combining the world’s smallest cyclotron and the rotating chair would make a very small proton therapy setup. “They said, ‘OK, we will make this product,’ and Stanford Medicine agreed to be the first customer.”
Into the future
As Stanford Medicine’s radiation oncology team prepares to treat their first patients with proton therapy, they are also figuring out how to advance the technology.
They’ll explore the advantages of delivering radiation treatment to patients sitting upright instead of lying down. Evidence suggests that for people with certain diseases, such as lung cancer, sitting up puts the organ that requires treatment in a better position to irradiate safely — the lung is more stretched out when someone is upright than lying down, which should spare damage to healthy tissue.
The upright position of patients also provides more flexibility than traditional equipment to deliver radiation from many different angles. “It’s a new mindset of planning proton therapy using continuous arcs of radiation, instead of beams from a few angles,” Loo said.
The Stanford Medicine team also plans to study different patterns of how the radiation is delivered. One of those is FLASH treatment — giving the same dose of radiation in a much shorter span of time. Stanford Medicine experts began exploring FLASH treatment to improve the precision of dose delivery, because the body shifts even when someone is holding still. While they were working on the technology to deliver radiotherapy much faster — in a fraction of a second — a team of researchers elsewhere demonstrated that very fast radiotherapy has the same impact on the tumor but causes less collateral damage to normal tissues.
“It’s a really surprising scientific finding that we’re still trying to understand,” Loo said. “FLASH techniques have not yet been implemented in human therapy, except in some limited ways, but it’s in the pipeline, and we plan to develop clinical trials of that method for our proton system.”
Loo and his colleagues are excited about what the future holds for patients. Radiotherapy is the most broadly used cancer treatment, benefiting about two-thirds of patients in the U.S., he said. “That means anything we can do to substantially improve radiotherapy is a big gain for cancer therapy overall.”
About Stanford Medicine
Stanford Medicine is an integrated academic health system comprising the Stanford School of Medicine and adult and pediatric health care delivery systems. Together, they harness the full potential of biomedicine through collaborative research, education and clinical care for patients. For more information, please visit med.stanford.edu.