Active Beam

A moving proton beam. This is sometimes called a "scanning beam" and refers to moving (or scanning) a proton beam across the height and width of a tumor volume. It also moved throughout the tumor’s depth to "paint" the treatment volume. It does this either with ionizing radiation from the beam alone or with fewer shaping aids (such as apertures and boluses).

Active-beam technology is not yet used for patient treatments at Loma Linda University Medical Center, but is being developed. When it is available, the proton beam therapy will permit Loma Linda University Medical Center physicians to treat more cancers, including breast cancer and larger lung cancers, with protons.

Adjuvant Therapy

Treatment used in addition to the primary method. For example, radiation therapy often is used in addition (as an adjuvant) to surgery.


A metal block containing a hole through which the radiation (photon or proton) beam passes. Each portal for each patient requires a custom-made aperture. The shape of the hole is the approximate shape of the target being treated by the beam. Every patient has her or his own set of apertures, and no other patients use them.

Benign Tumor

A tumor that grows locally but does not spread to other parts of the body. Benign tumors can cause problems because of their spread, as they press and displace normal tissues. They can be dangerous in confined places such as the skull.

Biological Therapy

Treatment by stimulating the body's immune defense system. Biological therapy is sometimes called "immunotherapy.”


Removing a sample of tissue for examination by a pathologist.


A custom-made block positioned beyond the aperture. The contours and thickness of the block conform the beam to the shape of the far edge of the target.


Radiation therapy from radioactive sources inside the body. The radiation oncologist may implant radioactive material directly into the tumor or very close to it. Radioactive sources may also be placed within body cavities, such as the uterine cervix.

Bragg Peak

The point at which protons (and other heavily charged particles) deposit most of their energy. This point occurs at the ends of the protons' paths. By varying the beam's energy, radiation oncologists can spread this peak to match the contours of tumors or other targets.


Uncontrolled, abnormal growth of cells, which will invade and destroy healthy tissues if not controlled by effective treatment. Cancer is a general term. It covers hundreds of different diseases, including some not called "cancer" (such as Hodgkin's disease and leukemia).


Treatment with anticancer drugs.

Clinical Proton Therapy Center

A proton treatment facility located in, or in close association with, a hospital or cancer center. An accelerator and treatment delivery system are used. From 1954 to 1990, all proton therapy was delivered in physics laboratories, using accelerators designed originally for basic research. The Loma Linda University Medical Center facility, which opened in 1990, was the first clinical proton therapy center in the world. In 2001, the Northeast Proton therapy center, in Boston, became the second such facility in the United States. Clinical proton therapy centers enable patients and physicians to take advantage of a variety of medical specialist’s healthcare expertise, as needed.

Cobalt 60

A naturally radioactive substance that is used in machines to treat cancer by external beams.

Combined Proton and Photon Therapy

Using both protons and photons (x-rays or electron beams) to treat cancer or other diseases. Combined treatment is used when either cannot do the job alone. For example, protons are often used with x-rays to boost the radiation dose to specific parts of a treatment volume.

If x-rays alone were used, too much normal tissue could receive too much radiation. If conformal protons alone were used, microscopic cancer in sites distant from the cancer (in lymph nodes, for example) might be missed.

By giving some of the treatment with conformal protons the total dose of x-rays can be reduced substantially. This reduces the risk of complications and permits doctors to treat lymph nodes that might contain a tumor.

Conformal Radiation Therapy

Radiation that is shaped, or "conformed," to the shape of a tumor in all three dimensions. The ability to shape the beam (the more precisely the better) helps the physician to deliver most of the radiation to the tumor, not to surrounding normal tissue.

CT Scan

Computed tomography scan (also known as a CAT scan) is a computerized x-ray procedure. A CT scan produces cross-sectional images of the body. The images are far more detailed than x-ray films, and can reveal disease or abnormalities in tissue and bone. The procedure is usually noninvasive and brief.


A person who plans and calculates the proper radiation dose for treatment. Dosimetrists work under the supervision of the physician. The physician prescribes the proper dose for treatment; the dosimetrist makes sure that the prescribed dose is delivered by the therapy plan.

Digital Rectal Examination (DRE)

A screening procedure that occurs prior to treatment for prostate cancer and colorectal cancer. The physician feels the rectal wall and assesses its smoothness; abnormalities are evaluated by other procedures, generally including biopsy and examination by a pathologist.

Electron Beam

A stream of accelerated electrons. Electrons are negatively charged parts of atoms; they surround the atomic nucleus. They can be gathered into a beam that produces high-energy radiation to treat cancer.

External-Beam Radiation

Radiation delivered from a source outside the body.

Gamma Rays

High-energy rays that come from a radioactive source such as cobalt-60.


A device for rotating the radiation delivery apparatus around the patient during radiation therapy. This motion is designed to treat from different angles.

At Loma Linda University Medical Center's proton therapy center, the gantries are three-story-high, rotating structures that guide the proton beam from the beam transport system to the beam delivery nozzle.


A measure of absorbed radiation dose. One Gray equals 100 rads in the older terminology.

Ionizing Radiation

Radiation of sufficient energy to displace electrons from the atoms of cells and produce ions. Ionized cells are damaged and must repair themselves to remain alive. Generally, normal cells are better able to repair themselves than cancer cells.

Immobilization device

A device that prevents the patient from moving during radiation treatment. One example is a form-fitting foam liner surrounded by a rigid plastic shell. The device allows the patient to lie comfortably during treatment. A device that prevents the patient from moving during radiation treatment. One example is a form-fitting foam liner surrounded by a rigid plastic shell. The device allows the patient to lie comfortably during treatment.

This type of immobilization is generally used for patients with tumors below the neck. Special masks or "bite blocks" are made for patients with diseases of the head or neck. Investigators in the Department of Radiation Medicine developed a vacuum-assisted bite block, which is used in many head and neck cases.


A small container of radioactive material placed in or near a cancer.

A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer. A small container of radioactive material placed in or near a cancer.

Intraoperative Radiation

A type of external radiation used to deliver a single large dose of ionizing energy. The radiation is aimed directly at the tumor bed and surrounding tissue at the time of surgery.

Linear Accelerator

A machine that creates high-energy radiation to treat cancers. A linear accelerator uses electricity to form a stream of fast-moving subatomic particles. Also called a megavoltage (MeV) or "linac" (pronounced LYNN-ack).

Magnetic Resonance Imaging (MRI)

A diagnostic imaging technique that uses a magnetic field and radio waves to produce highly detailed images of the body. Both MRI and CT scans may be used in planning proton radiation therapy.


Capable of invading. Cancers spread by invading normal tissue and spreading to distant tissues (metastasis).

Medical Oncologist

A physician who uses chemotherapy to treat cancer. Medical oncologists, like radiation oncologists and surgical oncologists, receive intensive training and serve long residency periods to become experts in their specialty.


The spreading of a cancer from one part of the body to another. Cells in the second tumor are like those in the original tumor.

Modulator Wheel

A spinning, polycarbide wheel with vanes of variable depth. In proton radiation therapy, protons passing through the thinner vanes travel farther into the body than those passing through the thicker sections. Different wheels, with different vanes, can be used to shift the peak energy (the Bragg peak) to different depths of the tumor.


The device through which protons are delivered to the patient. At Loma Linda University Medical Center, proton beam delivery begins in the accelerator, where an ion source generates protons. The accelerator (synchrotron) energizes the protons to a prescribed energy and sends them to the beam transport system, which sends the beam to the treatment rooms.

Each treatment room has a nozzle, which looks much like the nozzle of a water hose and is the final element in the beam delivery system. The nozzle not only delivers the beam to the patient, but also monitors beam uniformity, alignment, and dose delivered.


A doctor who specializes in treating cancer.

Palliative Care

Treatment to relieve, rather than cure, symptoms caused by cancer. Palliative care can help people to live more comfortably.


A quantum (energy packet) of electromagnetic radiation; the elementary particle of photon radiation therapy. X rays and gamma rays are photon radiation.

Postiron Emmision Tomography (PET)

A nuclear medicine imaging procedure employing radioactive (unstable) isotopes that decay (stabilize) by emitting a positively charged electron from the nucleus. These radioactive isotopes are bound to compounds or drugs that are injected into the body and enable physicians to study the physiology of normal and abnormal tissues. The study is usually called a "PET scan." It can be used in radiation treatment planning to help identify tumor tissue by the behavior of its cells, sometimes in cases where the tumor tissue is not visible on CT scans or MRI.

Prostate-Specific Antigen (PSA)

A protein that serves as a marker for prostate cancer or benign prostatic hyperplasia. PSA levels can be used to help detect prostate cancer, to monitor prostate cancer treatment, and to warn of possible recurrence.


A positively charged particle of an atom. The charge and relatively large mass (1800 times that of an electron) of protons account for the Bragg peak effect.


"Radiation absorbed dose" or a measure of the amount of radiation absorbed by tissues (100 rad = 1 Gray).


Energy carried by waves or a stream of particles. Visible light, x-rays, and a proton beam are all examples of radiation.

Radiation Oncologist

A physician who uses high-energy radiation to treat cancer. Radiation oncologists may also use ionizing energy to treat diseases other than cancer.

Radiation Therapist

A specially trained person who operates the equipment that delivers the radiation. Sometimes, physicians who treat with radiation are called radiation therapists, but the modern term for the physician is "radiation oncologist," even though the physician may use radiation to treat conditions other than cancer. The older term for a radiation therapist is "radiation therapy technologist," and therapists are still called "techs” from time to time.

Radiation Therapy

The use of high-energy penetrating rays or subatomic particles to treat disease. Types of radiation include x-rays, electrons, protons, alpha and beta particles, and gamma rays. Radioactive substances include cobalt, radium, iridium, and cesium.


A physician specially trained to interpret diagnostic x-ray images and perform specialized x-ray procedures.


Another word for radiation therapy (see above).


The use of special x-ray pictures to plan radiation treatment. The area to be treated is located precisely and marked for treatment.

Treatment Volume

The tissue actually treated by radiation. The treatment volume may be the diseased tissue only, but in cancer treatment it usually includes the cancer and tissues around. These surrounding tissues may harbor microscopic extensions of cancer.

Treatment Port or Field

The place on the body at which the radiation beam is aimed.

Treatment Table

The table that the patient lies on during treatment. In proton radiation treatment, final patient alignment is performed by adjusting the motorized table with respect to the proton nozzle. This ensures that the treatment position matches the position the patient was in when the planning CT scans were taken.


An abnormal mass of tissue. Tumors are either benign or malignant.


High-energy, ionizing, electromagnetic radiation that can be used at low doses to diagnose disease or at high doses to treat cancer.

*Many of these definitions were adapted from the National Cancer Institute publication, “Radiation Therapy and You: a Guide to Self-help During Treatment."