Glossary of Terms

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



Adjuvant Therapy

Adjuvant therapy is a type of additional treatment that is given after the primary or main treatment for cancer. It is used to reduce the risk of cancer recurrence or to eliminate any remaining cancer cells that may not be detected by standard diagnostic methods. Adjuvant therapy is typically given when there is a high risk of cancer returning after the initial treatment.

The primary treatment for cancer is often surgery, radiation therapy, or a combination of both. After the primary treatment, adjuvant therapy may be recommended based on the characteristics of the cancer and the individual
patient’s risk factors. The goal of adjuvant therapy is to improve the chances of long-term disease control and overall survival.



In the context of cancer treatment, a “boost” refers to an additional, more focused, and higher dose of radiation therapy that is delivered to a specific area within or near the tumor. The boost is used to further increase the radiation dose to the tumor region, aiming to improve the chances of local tumor control and enhance treatment outcomes.

The boost technique is typically employed in cases where there is a high risk of cancer recurrence in a specific area
or when the tumor has not responded adequately to the initial or primary course of radiation therapy. It is often used in combination with the standard or primary radiation therapy, which is delivered to the entire tumor or a broader area.

The decision to use a boost in cancer treatment is based on various factors, including the tumor type, stage, location,
and response to the initial treatment. The radiation oncologist and the multidisciplinary cancer care team carefully assess the patient’s case and determine the most appropriate treatment plan, which may include the use of a boost if necessary.


Brachytherapy is a type of internal radiation therapy. It uses radioactive seeds to deliver a high dose of radiation to a small area internally.

Bragg Peak

The Bragg peak is a fundamental concept in radiation therapy and refers to the characteristic distribution of energy deposition by charged particles, such as protons and carbon ions, as they pass through matter. This phenomenon is particularly important in particle therapy, a specialized form of radiation therapy that uses charged particles to treat cancer.

When charged particles are accelerated to high energies and directed toward a tumor, they deposit most of their
energy at a specific depth within the tissue. This point of maximum energy deposition is known as the Bragg peak.The energy deposition increases gradually as the particle slows down while traveling through the tissue until it reaches the Bragg peak, where it releases a significant portion of its energy.

One of the key advantages of the Bragg peak is its ability to deliver a high dose of radiation precisely at the tumor site
while sparing the healthy tissues beyond the peak. Unlike conventional photon-based radiation therapy, where the
energy deposition is spread along the entire path of the beam, the Bragg peak allows for better control of the
radiation dose distribution.


CT or CAT Scan

A CT scan, also known as computed tomography or CAT scan, is a medical imaging technique that uses X-rays and advanced computer processing to create detailed cross-sectional images of the body. It provides valuable information about the internal structures, organs, and tissues, helping healthcare professionals diagnose and monitor various medical conditions.

During a CT scan, X-ray beams are directed through the body from multiple angles. Detectors on the opposite side of the patient measure the amount of X-rays that pass through the body. These measurements are then used to create detailed image slices, also called “tomographic” or “cross-sectional” images.

Cyber Knife

CyberKnife is a brand name for a type of radiation therapy system known as robotic radiosurgery. It is a non-invasive and highly precise form of radiation treatment that uses advanced technology to deliver high doses of radiation to a targeted area within the body.


A cyclotron is a type of particle accelerator used to accelerate charged particles, such as protons or ions, to very high energies. The basic principle behind a cyclotron is the application of a strong magnetic field to guide charged particles in a circular path. The cyclotron consists of two large D-shaped magnets, referred to as the “dee” magnets, placed facing each other with a gap between them. In the center of this gap is the particle source, where the charged particles are initially introduced. In proton therapy, a cyclotron is used to accelerate protons to very high energies, which are then directed at a tumor to deliver radiation treatment.



A dosimetrist is a specialized healthcare professional who plays a crucial role in radiation therapy treatment planning. They work as part of the radiation oncology team, alongside radiation oncologists and medical physicists, to ensure the safe and accurate delivery of radiation to cancer patients.

The primary responsibility of a dosimetrist is to create a customized treatment plan for each patient undergoing radiation therapy. They use advanced treatment planning software and medical imaging data, such as CT scans, MRI, or PET scans, to precisely calculate the radiation dose distribution within the patient’s body.



Fractionation is a term used in radiation therapy to describe the process of dividing the total prescribed radiation dose into smaller, individual doses, called fractions, which are delivered over several treatment sessions. The purpose of fractionation is to improve the therapeutic ratio, maximizing the effectiveness of radiation treatment while minimizing damage to healthy tissues.

Radiation therapy is typically given in multiple sessions over a period of days or weeks, rather than delivering the entire radiation dose all at once. Each treatment session is referred to as a fraction. The total radiation dose prescribed by the radiation oncologist is determined based on the type and stage of cancer, the location of the tumor, and the goal of treatment (e.g., curative or palliative).


Gamma Knife

Gamma Knife is a specialized medical device used for stereotactic radiosurgery (SRS), a non-invasive and highly precise form of radiation therapy. Despite its name, the Gamma Knife does not involve any surgical incisions; it delivers highly focused gamma radiation to treat tumors and other abnormal brain conditions.

Gray (Gy)

Gray is a unit of measurement used in radiation therapy and radiology to quantify the amount of absorbed radiation dose. It is a standard unit of the International System of Units (SI) for measuring ionizing radiation. One gray is defined as the absorption of one joule of radiation energy per kilogram of matter. In simpler terms, the gray measures the amount of energy deposited by ionizing radiation in a specific material or tissue. It helps to assess the potential biological effects of radiation exposure. The gray is used for both external beam radiation therapy, where radiation is delivered from outside the body, and for internal radiation sources, such as radioactive implants.

For example, in radiation therapy, the prescribed dose to the tumor and the surrounding healthy tissues is specified in
grays. The radiation oncologist carefully plans the treatment to deliver the appropriate dose to the tumor while minimizing the dose to nearby normal tissues.

Radiation dose is a critical factor in cancer treatment, as it determines the effectiveness of destroying cancer cells
while minimizing damage to healthy tissues. The total dose of radiation delivered during a course of radiation therapy
is usually measured in grays and is distributed over several treatment sessions, called fractions.


High-Dose-Rate Brachytherapy (HDR)

High-dose-rate (HDR) Brachytherapy enables a person to be treated for several minutes at a time with an effective radioactive source that’s placed in the body using an applicator. The source is removed after a short period of time, typically 10 to 20 minutes. This procedure may be repeated twice a day over a few days. It can also be done once a day over the course of a few weeks. The applicator may be left in place between treatments, or it can also be positioned before each treatment.

Hormone Therapy

Hormone therapy, also known as endocrine therapy, is a cancer treatment that targets the hormones or hormone receptors that play a significant role in the growth and spread of certain types of cancers. It is commonly used in the treatment of hormone receptor-positive breast cancer and prostate cancer, among other malignancies.

Hormone receptor-positive cancers have receptors on their cell surfaces that are sensitive to specific hormones, such
as estrogen or progesterone in breast cancer, or androgens (e.g., testosterone) in prostate cancer. These hormones
can promote the growth and division of cancer cells. Hormone therapy works by either blocking the hormones from binding to their receptors or reducing the production of hormones in the body.

Hypofractionated Radiation Therapy

Hypofractionated radiation therapy is a treatment approach in radiation oncology where the total prescribed radiation dose is delivered in larger doses (fractions) per treatment session compared to conventional fractionation. Instead of receiving smaller doses daily over several weeks, patients undergoing hypofractionated radiation therapy receive fewer but larger doses per session, typically delivered over a shorter overall treatment course.



IGRT stands for Image-Guided Radiation Therapy. It is an advanced radiation therapy technique that uses frequent imaging during treatment to precisely target the tumor and adjust for any changes in its position or shape. IGRT helps improve the accuracy of radiation delivery and enhances the ability to spare surrounding healthy tissues, leading to more effective cancer treatment with reduced side effects.

IGRT uses various imaging techniques, such as X-ray imaging, computed tomography (CT), magnetic resonance imaging (MRI), and cone-beam CT (CBCT), to visualize the tumor and surrounding structures in real-time or near-real-time.

IGRT enhances the overall precision of radiation therapy, which is particularly valuable when treating tumors in areas
that move or change shape during treatment.

Immobilization Device

An immobilization device, also known as a positioning or fixation device, is a specialized tool used in radiation therapy and other medical procedures to securely and precisely hold a patient in a specific position during treatment or imaging. Immobilization devices are designed to ensure the accurate and reproducible alignment of the patient, reducing movement during the procedure and enhancing the precision of the treatment or imaging process.

In radiation therapy, immobilization devices are crucial for maintaining the patient’s position throughout the entire treatment course, which may involve multiple sessions over several weeks. Keeping the patient in a consistent position helps ensure that the radiation is precisely delivered to the intended target area, minimizing the dose to nearby healthy tissues and critical organs.


Immunotherapy is a revolutionary form of cancer treatment that harnesses the body’s immune system to recognize, attack, and destroy cancer cells. Unlike traditional cancer treatments like chemotherapy and radiation therapy, which directly target cancer cells, immunotherapy works by boosting the body’s natural defenses to fight cancer more effectively.

The immune system plays a crucial role in identifying and eliminating abnormal cells, including cancer cells. However, cancer cells can sometimes evade detection by the immune system or suppress its response, allowing them to grow and spread. Immunotherapy aims to overcome these barriers and enhance the immune system’s ability to recognize and attack cancer cells.

Intensity Modulated Radiation Therapy (IMRT)

IMRT stands for Intensity-Modulated Radiation Therapy. It is a type of advanced radiation therapy used in the treatment of cancer. IMRT is designed to deliver highly precise and targeted doses of radiation to cancerous tumors while minimizing the exposure to surrounding healthy tissues.


Linear Accelerator:

A linear accelerator, often abbreviated as LINAC, is a sophisticated medical device used in radiation oncology to deliver external beam radiation therapy to treat cancer and other medical conditions. It is one of the most common and versatile tools for administering radiation therapy.

The linear accelerator generates high-energy X-rays or electron beams that are precisely directed towards the tumor or affected area. The X-rays or electrons penetrate the patient’s body and deliver a targeted dose of radiation to the cancerous cells, damaging their DNA and preventing them from multiplying and growing. The goal is to destroy or shrink the tumor while sparing surrounding healthy tissues as much as possible.

Low-Dose Rate Brachytherapy (LDR)

Also referred to as permanent seed implants, LDR brachytherapy uses radioactive seeds permanently implanted into an organ. The seeds emit low levels of radiation for several weeks. When this radiation treatment ends, the seeds are left in place permanently. You may have to stay in bed for larger implants and keep still to ensure it does not move. Smaller implants like the seeds or pellets are left in place and never taken out. Over several weeks, they stop giving off radiation. The seeds or pellets are about the size of grains of rice and rarely cause problems.

If your implants are required to be left in, going home the same day they’re put in may be an option. Special precautions may be necessary, so be sure to speak with your cancer care team regarding this.


Medical Physicist

Medical physicists in radiation oncology work closely with radiation oncologists and dosimetrists to develop and implement radiation therapy treatment plans for cancer patients. They use advanced treatment planning software to calculate the appropriate radiation dose, determine the best treatment techniques, and ensure the accurate delivery of radiation to the tumor while minimizing exposure to surrounding healthy tissues.

Medical physicists are experts in radiation safety. They ensure that all radiation procedures are performed with minimal risk to patients, staff, and the public. They provide guidance on radiation protection measures and ensure compliance with regulatory guidelines and safety protocols.


Pencil Beam Scanning (PBS)

Pencil Beam Scanning (PBS) is a highly precise and advanced technique used in proton therapy, a type of radiation therapy that utilizes protons to treat cancer. PBS is designed to deliver a precise and conformal dose of radiation to the tumor while minimizing exposure to surrounding healthy tissues.

Photon Radiation

Photon radiation, also known as X-ray radiation, is a type of high-energy electromagnetic radiation commonly used in medical imaging and radiation therapy. It is produced by the interaction of high-speed electrons with a target material, resulting in the emission of X-ray photons.

Photon radiation is widely used in cancer treatment due to its ability to penetrate tissues and deliver high doses of radiation to the tumor while minimizing damage to surrounding normal tissues. It is effective in treating various types of cancer, including lung, breast, prostate, head and neck, and many other malignancies.

The selection of photon radiation for radiation therapy depends on the tumor type, location, and size, as well as the patient’s overall health and treatment goals. The radiation oncologist carefully plans the treatment to achieve the best
possible outcome while minimizing side effects.


Stereotactic Body Radiation Therapy (SBRT)

Stereotactic Body Radiation Therapy (SBRT) is an advanced and highly precise form of radiation therapy used to deliver high doses of radiation to a localized target in the body. SBRT is particularly well-suited for small tumors or lesions in various body regions, including the lung, liver, spine, prostate, and other areas.

SBRT typically requires only a few treatment sessions (often 1 to 5 sessions) compared to conventional radiation therapy, which may require multiple sessions over several weeks.



VMAT stands for Volumetric Modulated Arc Therapy. It is a type of advanced radiation therapy technique used in the treatment of cancer. In VMAT, the radiation treatment machine, typically a linear accelerator, rotates around the patient in an arc or multiple arcs while delivering radiation. During the arc(s), the machine continuously adjusts the intensity of the radiation beam, as well as the shape of the beam, to conform to the three-dimensional shape of the tumor. This dynamic modulation allows for precise dose delivery from multiple angles, providing better dose distribution compared to traditional radiation therapy techniques.