Highly Accurate, Less Invasive Cancer Treatment With Proton Therapy

California Protons uses revolutionary proton beam technology to help fight cancer with unmatched precision and the utmost care.

What is Proton Therapy?

Proton therapy is a highly accurate and much less invasive form of cancer treatment. Protons are subatomic particles with a positive charge that can be manipulated and controlled to stop and deliver radiation directly at a tumor, traveling no further than the exterior tumor wall. This is particularly beneficial to those who are vulnerable to radiation or have received prior radiation either to or immediately adjacent to the area that needs to be treated.

Proton Therapy vs.
Standard X-ray Radiation

Standard X-ray and proton radiation are both types of “external beam” radiotherapy used to treat various types of cancers. However, the properties of each are very different and result in varying levels of radiation exposure. Standard X-ray treatment beams deposit energy along their paths before reaching the tumor, thus delivering unnecessary and potentially harmful radiation to healthy tissues and surrounding organs. Proton therapy doses, on the other hand, can be controlled to deposit most of the energy inside the tumor.

X-Ray Radiation

  • Contains no mass or electric charge
  • Highly penetrating rays can deliver radiation throughout any volume of tissue
  • Rays deposit most of the initial dose of radiation (entry dose) close to the patient’s skin, while interacting with the healthy cells closer to the surface, and then drop the remaining dose of ionizing radiation on the deeper diseased cells
  • Rays continue to emit radiation as they naturally pass through the body (exit dose) similar to X-ray imaging

Pencil Beam Proton Therapy Radiation

  • Proton beam made up of heavy, charged particles
  • Protons enter the body and deposit only a small dose along the way to the target and virtually none beyond it
  • The absorbed dose increases very gradually with greater dose and lower speed until the beam reaches the Bragg peak—the point at which the maximum energy is deposited—which is directed exactly within the tumor site
  • Immediately after this burst of energy, the proton beam comes to a complete stop, and any additional radiation ceases

Proton Therapy vs. Standard Radiation

Benefits of Proton Therapy from CA Protons in San Diego

Our Pencil Beam Scanning Technology

Precisely delivered within 2 millimeters, our pencil beam scanning technology (known as Intensity Modulated Proton Therapy or IMPT) releases a high dose of cancer-killing radiation that conforms to the unique shape and size of the tumor. Even in hard-to-scan places, our doctors can direct and manipulate the beam of protons to stop precisely at the edge of the tumor to spare healthy tissues and surrounding organs.

Unlike older passive-scattering proton therapy treatment, our proton center’s pencil beam scanning technology further enhances the precision of the radiation delivered. By enabling physicians to fine-tune the range and intensity of the proton beam, we can sweep a narrow proton beam across the tumor in fine strokes, while increasing intensity and dose layer by layer inside the tumor. California Protons was the first proton clinic in the nation to offer pencil beam proton technology in all five of our treatment rooms.

Cone Beam CT Imaging

California Protons Cancer Therapy Center in San Diego is the only proton therapy facility in all of California, and only one of a handful throughout the U.S., to consistently utilize Cone Beam CT (CBCT). This notable advancement in CT imaging helps us bring the most advanced technology to our patients.

Used in conjunction with our pencil beam scanning technology, Cone Beam CT enables a more compact, faster and safer version of regular CT imaging. Through the use of a cone-shaped X-ray beam, the radiation dose, size of the scanner and time needed for scanning are all dramatically reduced, while the image accuracy is improved. The time needed for a full scan is typically under one minute and the radiation dose is up to a hundred times less than that of a regular CT scanner.

Varian ProBeam Proton System

Our proton center is home to one of the largest state-of-the-art proton systems in the country, comprised of a cyclotron, beamline, two fixed beam treatment rooms and three rotational gantry treatment rooms.

How Is The Cyclotron Used in Proton Therapy?

The Cyclotron is a 90-ton device engineered to generate about 5 trillion protons per second and then accelerate protons outwards from the center along a spiral path at roughly 100,000 miles per second, or 0.61 times the speed of light, to create a beam that can precisely reach tumors. The proton beam travels from the cyclotron to the beam line, which spans the length of the building, and uses a series of electromagnets to bend and focus the beam into each treatment room. From there, the protons are delivered with mechanical precision to one of the two fixed beam treatment rooms or three rotational gantry rooms through a specialized nozzle. The 280-ton rotational gantry machine rotates 360˚ around the patient to ensure an ideal treatment angle.

7 Main Advantages of Proton Therapy Beam Technology

Proton therapy offers several benefits that other cancer treatments do not. Advantages to proton therapy treatment include:


Laser-like precision
Protons can be manipulated to stop and deliver radiation directly at a tumor and no further.

Radiation dose variation
Proton therapy allows physicians to vary the intensity of the radiation dose at any point within the tumor, which was not possible with other technologies.

Treatment for many tumor types
Our proton therapy technology expands the treatment options to treat larger and more irregularly shaped tumors. It also allows more types of tumors to be treated compared to what was possible with passively scattered protons.

Lower incidence of secondary tumors
Reduced radiation toxicity results in a lower incidence of secondary tumors compared to standard X-ray radiation.

Minimal side effects
Because proton therapy spares healthy tissue surrounding a tumor, side effects such as diarrhea, nausea, headache and loss of appetite are less likely. After treatment, most patients resume their daily routines.

Fast treatment time
We replaced the heavy, cumbersome devices of older passive-scattering proton technology with pencil beam scanning. Unlike the older technology, our treatment plan can be loaded into the computer and completed within a matter of minutes so that patients can get back to their lives quicker.

Personalized care
Our world-renowned medical team is committed to providing the most personalized, comprehensive cancer care for our patients, and offers the latest in medical advancements and technology. Learn More

Proton Therapy Treatment Details &
Life During Treatment

Proton therapy is a noninvasive and often painless treatment, during which patients feel no physical sensation from the proton beam, hear very little noise and experience minimal discomfort. Unlike the older technology, each treatment session takes around 30 minutes, with the actual treatment time taking only 1 to 3 minutes. Full two-way video and audio allow medical team members and patients to communicate throughout the process.


In many cases, side effects associated with external beam radiation therapy such as diarrhea, headache and loss of appetite are fewer with proton therapy than with X-ray therapy. Most patients are able to continue their usual activities throughout their treatment, including working, exercising and socializing. However, all cancer treatments have advantages and disadvantages. Be sure to discuss potential complications and side effects, as well as treatment options, with your oncologist.

Questions about proton therapy? We can help.

History of Proton Beam Therapy

In 1946, American physicist and former Manhattan Project group leader Robert Wilson laid the groundwork for the field of proton therapy with the publication of his landmark paper, “Radiological use of Fast Protons.” In this paper, he was the first to suggest that protons could be used clinically, sparing normal tissue and allowing for the maximum placement of the radiation dose inside the tumor.

Proton therapy has been around for over 65 years. The first proton radiation treatments were performed in the mid-1950s using a particle accelerator built for physics laboratory research at the University of California, Berkeley, and later at the Harvard Cyclotron Laboratory. The nation’s first hospital-based proton therapy center opened in 1990 at the Loma Linda University Medical Center in Southern California. Since then, more than twenty patient-focused proton therapy centers have opened in the United States, including the California Protons Cancer Therapy Center in San Diego.


Cost of Proton Therapy

The cost for proton therapy depends on the type of cancer, location of the tumor, the stage, how many treatments are necessary, and other factors.  While in some cases it is more expensive than traditional X-Ray radiation, the long-term side effects and chances for recurrence are lessened due to the more advanced, precise technology that greatly reduces radiation to nearby healthy tissues and organs.