Glossary of radiotherapy terms

Radiotherapy and oncology terms explained.

4D adaptive radiotherapy

A treatment which can be altered on a day to day basis to take account of movements of the tumour in three dimensions i.e. Anterior, posterior, superior, inferior, lateral, medial, and with time.

Adjuvant therapy

A term used to refer to additional treatment usually given after surgery where all detectable disease has been removed, but where there remains a statistical risk of relapse due to occult disease.


A set of precise rules that specify how to solve some problem or perform some task, commonly used in computing.

Anatomical imaging

Methods employed to visualise of the structure of a body.


Use of algorithms to provide three-dimensional visualisation of critical structures and pathology in a matter of minutes.


A beamlet is a single “element” of a beam, which in turn is one of hundreds of beams within a typical helical TomoTherapy delivery. A single beamlet corresponds to the radiation emitted through a single open MLC leaf, with the gantry at any given angle during rotation.


A procedure in which radioactive material is placed directly into or near the cancer. The radiation is sealed in needles, seeds, wires or catheters.

Bragg Peak

The point at which protons deposit most of their energy.

Clinical Target Volume (CTV)

A clinically defined target volume that contains the demonstrable tumour (gross tumour volume or GTV) unless it has been surgically excised and microscopic invisible tumour. This volume contains
cancer cells and must be treated with the prescribed radiation dose adequately to achieve a cure.


The process of shaping a therapeutic beam by using materials that are opaque to it. These materials are placed in front of part of the beam to define the shape of the beam.


The device used for collimation. TomoTherapy treatment systems have two collimating devices: the primary collimator, or jaws; and the multileaf collimator (MLC).

Computed tomography

An x-ray technique using a scanner which takes a series of images across the body which can be viewed in two or three dimensional form.

Cone Beam CT

A diagnostic energy X-ray machine is mounted with the linear accelerator and by rotation acquires a three dimensional image of the tumour with the patient in the treatment position.

Conformal radiotherapy (CRT)

Treatment delivery techniques which aims to shape the 3D high dose volume to the planning target volume whilst minimising dose to healthy tissue.


External shape of a part of a body or imaged organ.

Couch Indexing

A system of graduated measurements on a couch top that can be used to ensure reproducibility of patient positioning from day to day. A universal system of couch indexing would reduce errors as the patient moves from one machine to another.


The TomoTherapy treatment system’s integrated, true CT imaging technology, which can be used for every patient, every day for patient positioning, image-guided radiation therapy, dose targeting and adaptive radiotherapy.


Particle accelerator used in the production of radiopharmaceutical


Digital Imaging and Communications in Medicine. An international communications standard developed by NEMA that defines the format used to transfer medical image-related data between different pieces of medical equipment.

DICOM RT refers to the standards that are specific to radiotherapy data. TomoTherapy treatment systems use both general and RT-specific parts of the DICOM standard.

Digital subtraction

A method used to obtain information by subtraction of two images from each other.

Digitally reconstructed radiograph (DRR)

A planar radiograph made by computer-projected rays through 3D CT density information.

Dose Reconstruction

Calculating the dose deposited on the patient by using delivery verification and a CT obtained at the time of treatment (fraction CT).

Dose escalation

Increasing the total radiotherapy dose.

Dose guided radiotherapy (DGRT)

The dose delivered from each treatment session is calculated and adjusted to match the predicted dose.

Dose volume histogram (DVH)

Histogram showing the dose distribution within an outlined structure. It is usually presented as a cumulative plot i.e. volume of organ plotted against dose.


Specialist radiotherapy staff usually employed as clinical technologists. Their work includes patient immobilisation, treatment planning, in vivo dosimetry, general dosimetry, etc.

Electronic portal imaging devices (EPID)

An electronic system for acquiring verification images of the geometry of treatment during radiotherapy delivery.

Fan Beam

A narrow, slit-shaped divergent beam. The TomoTherapy Hi·Art treatment system delivers therapeutic radiotherapy in the shape of a fan beam.

Fiducial markers

Small radio-opaque markers which can be inserted directly into a tumour and then visualised by imaging and used to ensure accuracy of positioning from day to day.

Film dosimetry

Dose measurement by using radiographic film.


The schedule of treatment sessions required for a course of radiotherapy treatment.

Functional imaging

Imaging of the physiology of a body to detect disease.

Gross Tumour Volume (GTV)

The palpable or visible/demonstrable extent and location of the malignant growth.

Image guided radiotherapy (IGRT)

Imaging used to guide the radiotherapy for each treatment session.

Image registration

The process of establishing point-by-point correspondence between two images. Method of aligning two 3D image sets, e.g. CT, MRI, PET etc. Image sets may be overlaid or structures may be mapped between the sets.


Equipment of techniques designed to reduce patient movement.

In vivo dosimetry (IVD)

Equipment of techniques designed to reduce patient movement.


All couches used for treatment planning and delivery are marked with the same scale so that consistency of position can be easily achieved. Immobilisation systems should ideally attach to the couch top in a unique position.

Intensity modulated radiotherapy (IMRT)

Treatment delivery technique that modulates the intensities of the beams, as well as geometrically shaping them. IMRT provides improved dose deposition avoiding critical structures, delivering complex dose distributions designed using forward or inverse planning.


Occurring as the radiotherapy is delivered.

Irradiated Target Volume (ITV)

The volume that is actually treated.

Linear accelerator (Linac)

A treatment machine generating megavoltage x-rays or electrons.

Magnetic resonance imaging (MRI)

An imaging technique based on the differences in magnetic properties of protons within living cells. It provides superior soft tissue definition of many tumours compared with CT.

Magnetic resonance spectroscopy (MRS)

A technique that potentially provides excellent spatial information about the functionally active parts of tumours.

Molecular markers

Markers on the genetic chromosome.

Monte Carlo

Algorithm used in radiotherapy treatment planning systems.

Multileaf collimator (MLC)

A collimation system on a linear accelerator which uses a number of “leaves” to create an irregular shaped radiation beam. It is used to shape the beam to the target volume geometrically (CRT) and is also used to modulate intensity of the beam (IMRT).

On-treatment verification

Verifying the radiotherapy in real-time as or before the treatment is delivered.

Optical imaging systems

Use of non-ionising light in the visible spectrum to image patient surfaces.

Optimized Treatment Planning

A process in which the appropriate beam pattern, position, and intensity are calculated based on the prescription for how much radiation the tumour should receive, as well as acceptable levels for surrounding structures.

From the clinicians’ perspective, treatment optimization is easier and more accurate than conventional treatment planning. It is fundamental to the TomoTherapy treatment system.

Organs at risk (OAR)

As is the case with the planning target volume (PTV), the organs at risk during treatment may also move and an integrated margin must be added to the organs at risk volume to compensate for these variations and uncertainties using the same principles as for the PTV.

Patient immobilisation techniques

The methods used for immobilising and positioning patients during radiotherapy treatment in order to reduce the risk of movement of the tumour out of the high dose region, either during treatment or between treatment fractions.


Quality control device used in verifying radiotherapy doses within patients

Planned target volume (PTV)

Geometrical concept defined to ensure that appropriate beam sizes and arrangements are chosen so that the prescribed dose is actually absorbed in the CTV taking into consideration the net effect of
all possible geometrical variations and changes in tumour volume.

Portal dosimetry

The calculation of the radiation dose distribution within an area of the body determined from portal images.

Positron emission tomography

Imaging using a radionuclide which is a positron emitter. It provides functional information about the tumour and its site and size.


Positive charged particles in an atom. Used in radiotherapy to treat tumours by making use of the Bragg peak, the point at which protons 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 tumours or other targets.


Not transparent to X-rays or other forms of radiation.


Radioactively-tagged compound necessary to produce a nuclear medicine image.


Substance given to increase the radio-sensitivity of a cell or tissue.


Surgical technique involving the use of narrow beams of radiation that are precisely targeted by stereotactic methods to destroy tumours or lesions especially of the brain.


The medical use of ionizing radiation as part of cancer treatment to control malignant cells. Also known as radiation therapy or radiation oncology.

Respiratory gating

A system where delivery of radiotherapy to the patient can be limited to a particular phase of the respiratory cycle. This is used to minimise the effects of movement during breathing.

Robot-mounted linear accelerators

A robotic image guidance radiotherapy system that tracks patient and lesion positions during the entire treatment process. The system continuously scans and detects any patient or lesion
movement and makes any necessary corrections.

Stereotactic Body Radiotherapy (SBRT)

Stereotactic body radiotherapy is a technique designed to deliver radiation therapy very precisely to tumours anywhere in the body. The word stereotactic pertains to the precise positioning of a tumour in relationship to the body. The technology used in SBRT allows external beam radiation to be delivered with pinpoint accuracy.


An instrument for measuring the volume of air entering and leaving the lungs.

Stereotactic Radiosurgery (SRS)

Stereotactic radiosurgery treats brain disorders with a precise delivery of a single, high dose of radiation in a one-day session.

Stereotactic system

Type of imaging-guided system where the position of a point in space is identified and related to an external co-ordinate reference system, directing a radiation beam in three dimensions to reach a specific, localised area of the body.

Surface registration

An optical imaging device can be used to acquire a picture of the surface of the body during the planning process. When the patient is set up for treatment delivery, this image can be laid onto the skin and any discrepancies of position corrected.

T1/T2 weighted images

Signal intensities in MR images that relate specific tissue characteristics.

Target volume

The volume of tissue at which treatment is aimed.

Therapeutic ratio

The ratio of the maximally tolerated dose of radiation to the minimally curative or effective dose.


The procedure used to obtain a set of slice images of the patient.


Radiotherapy is delivered using thin slices of radiation rotated around the patient.

There are two types of delivery method:

  • serial tomotherapy, where the slices align along the horizontal plane of the patient
  • rotational tomotherapy, where the radiation is delivered as a continual helix, by moving the patient horizontally through the treatment beam.
Total body irradiation (TBI)

Total body irradiation (TBI) is a form of radiotherapy used for patients about to undergo a bone marrow or stem cell transplant to destroy any undetectable cancer cells. Fractions of radiation are given to the whole body to destroy the cells of the bone marrow.

Treatment planning

The process whereby the therapeutic strategy of the clinical oncologist is realised as a set of treatment instructions together with a physical description of the distribution of the prescribed dose in the patient.

Treatment planning system (TPS)

The hardware and software used for simulating the irradiation geometry to be used for patient treatment and for calculating the distribution of dose within the patient.

Software tools use 3D patient data from CT and other imaging modalities to visualise volumes of interest. The main function is to design the optimum dose distribution with the patient in three dimensions. It can network with the linear accelerator and CT scanner and with facilities for designing shielding blocks and compensators.

Ultrasound imaging

Visualisation of internal body structures by transmitting and receiving sound waves.

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