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Glossary of Proton Therapy Terms

Below are many of the terms most commonly used in the field of proton therapy.

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Adjuvant therapy: A treatment used in addition to the main course of therapy.

Anesthesia: The process where a drug is administered for medical or surgical purposes that will induce partial or total loss of sensation and may be topical, local, regional or general, depending on the method of administration and area of the body affected. At the MD Anderson Proton Therapy Center, anesthesia is administered intravenously (by IV through the vein) and patients do not require entubation which is common practice at other proton therapy centers.

Anesthesiologist: A medical doctor who specializes in administering anesthesia. At the MD Anderson Proton Therapy Center, many of the children we treat require anesthesia to help them remain still during treatment. We have a dedicated on-site anesthesia team that specializes in treating children.

Aperture: A metal block containing a hole through which the radiation (photon or proton) beam passes. Each field or area of treatment 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. At the MD Anderson Proton Therapy Center, apertures are made of brass and created in our on-site machine shop.

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

Biopsy: Removal of a tissue sample for examination by a pathologist.

Bragg peak: The point at which protons deposit most of their energy. This point occurs at the ends of the protons' paths. Through a process called modulation, radiation oncologists can spread this peak to match the contours of tumors or other targets. The flexibility of the Bragg Peak is one of the things that make protons an excellent and targeted option for the treatment of many types of cancer.

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Cancer: Uncontrolled, abnormal cell growth that invades and destroys healthy tissues if not controlled by effective treatment. Cancer is a general term that includes hundreds of different diseases, including Hodgkin's disease and leukemia.

Chemotherapy: Treatment with anti-cancer drugs that may be administered orally (by mouth) or intravenously (by IV through the vein) of a person’s body.

Cobalt-60: A naturally radioactive substance that is used in some therapy 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 neither therapy can be used alone. For example, protons are often used with X-rays to boost the radiation dose to specific parts of a treatment volume in order to provide a higher dose of radiation while protecting nearby tissues.

Used alone, X-rays can deliver too much radiation to normal tissue. If protons alone were used, microscopic cancer in sites distant from the cancer (in lymph nodes, for example) might be missed. Combining the two treatments allows optimal use of both, while reducing the risk of complications.

Compensator: A custom-made, beam-shaping device through which a proton beam is delivered. It is used to absorb some energy from the proton beam so that it stops just on the edges of the target or tumor. This keeps the normal, healthy tissues beyond the tumor from receiving radiation. This is used with an aperture. At the MD Anderson Proton Therapy Center, the compensator for each patient is made from thick acrylic and created in our on-site machine shop.

Couch: The table – often called the “couch” at MD Anderson Proton Therapy Center -- where the patient lies during treatment. In proton radiation treatment, final patient alignment is performed by adjusting the motorized couch 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.

CT Scan: Computed tomography scan (also known as a CAT scan) is a computerized X-ray procedure that 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.

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Digital Rectal Examination (DRE): A screening procedure prostate and colorectal cancers. The physician feels the rectal wall to assess its smoothness. Abnormalities are evaluated by other tests, generally including biopsy and an exam by a pathologist.

Dosimetrist: A medical professional who plans and calculates the proper radiation dose for treatment. Dosimetrists work under the supervision of the physician – who prescribes the proper treatment dose – to make sure the prescribed dose is delivered by the therapy plan. At the MD Anderson Proton Therapy Center, our team of 9 medical dosimetrists must have graduated from an accredited program and be certified by the American Association of Medical Dosimetrists.

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Electron beam: A negatively charged subatomic particle that is accelerated to different energies and used to treat cancer.

External-beam radiation: Radiation delivered from a source outside the body.

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Gamma rays: High-energy rays that come from a radioactive source such as Cobalt-60.

Gantry: A device that rotates the radiation delivery apparatus around the patient during treatment delivery. The rotation allows treatment from different angles. At MD Anderson Proton Therapy, we have three treatment rooms that house gantries that administer proton beams from 360-degree angles.

Gray: A measure of absorbed radiation dose. One Gray equals 100 rads , which is an older term used to describe this.

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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 stay alive. Normal cells are usually better able to repair themselves than cancer cells.

Immobilization device: A device – mask for the face or a cradle for the body, leg or arm, depending on the area that will receive treatment – used to help prevent the patient from moving during radiation treatment. Some patients who are receiving proton therapy will use these devices during their treatments.

Implant: The process of placing a small source of radioactive material in or near a cancer.

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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 "linac" (pronounced LYNN-ack).

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Magnetic Resonance Imaging (MRI): A diagnostic imaging technique that uses a magnetic field and radiowaves to produce highly detailed images of the body. Both MRI and CT scans may be used in planning proton therapy.

Malignant: Cancers that are capable of spreading and invading normal tissue and 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.

Metastasis: 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.

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Nozzle: The device through which protons are delivered to the patient. Proton beam delivery begins in the accelerator, where an ion source generates protons. At the MD Anderson Proton Therapy Center, 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.

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Oncologist: A doctor who specializes in treating cancer.

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Pencil beam scanning: A very precise form of proton therapy treatment that uses protons to deliver radiation treatment across the height and width of a tumor. It can be directed to move throughout the tumor’s depth to "paint" the treatment volume with radiation from the beam. MD Anderson Proton Therapy Center is one of the only centers in the world to use pencil beam scanning, also called spot scanning or active beam, to treat patients.

Photon: A quantum (energy packet) of electromagnetic radiation; the elementary particle of photon radiation therapy. X-rays and gamma rays are photon radiation (sometimes called “traditional” or “conventional” radiation).

Positron Emission Tomography (PET): A nuclear medicine imaging procedure that can identify areas of cancerous tissue based on their higher than normal metabolic activity. 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.

Proton: A positively charged particle found in the nucleus of an atom. Protons used in proton therapy come from stripping a hydrogen atom of its electron. They can be accelerated and controlled to release their energy within a well-defined range in tissues, such as a tumor.

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Rad: "Radiation absorbed dose" or a measure of the amount of radiation absorbed by tissues. This term has been replaced by the Gray (100 rad = 1 Gray).

Radiation: Energy carried by waves or a stream of particles. Visible light, X-rays and proton beams all are examples of radiation.

Radiation oncologist: A physician who uses high-energy radiation, including protons, to treat cancer. Radiation oncologists also may use ionizing energy to treat diseases other than cancer. At the MD Anderson Proton Therapy Center, patients meet with their radiation oncologist before treatment begins, weekly during the course of treatment and for follow-up as needed.

Radiation therapist: A specially trained medical professional who deliver the ionizing radiation with specialized treatment machines. At the MD Anderson Proton Therapy Center, patients will work with a team of specialized radiation therapists each day during proton therapy treatment.

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.

Radiologist: A physician specially trained to interpret diagnostic X-ray images and perform specialized X-ray procedures.

Radiotherapy: Another word for radiation therapy. 

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Simulation: The use of X-ray pictures to plan radiation treatment. The area to be treated is located precisely and marked for treatment. At MD Anderson Proton Therapy Center, all patients who receive proton therapy will first go through a simulation.

Snout: The part of the nozzle closest to the patient. The snout supports the aperture and compensator.

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Target volume: Often used to describe a tumor or area of concern that will receive proton treatment.

Treatment volume: Generally a bit larger than the target volume, the treatment volume surrounds the tartget with an additional margin to include the cancer and surrounding tissues, which may harbor microscopic extensions of cancer. With proton therapy, treatment of surrounding tissues is limited to what is absolutely necessary in order to achieve the result of destroying cancer cells.

Treatment port or field: The place in the body at which the radiation beam is aimed.

Treatment table: The table – often called the “couch” at MD Anderson Proton Therapy Center -- 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.

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

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

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© 2015 The University of Texas MD Anderson Cancer Center