CAMRT Radiation Therapy key terms

The SI unit of absorbed radiation dose, equal to one joule of energy deposited per kilogram of material. Treatment prescriptions and dose constraints are stated in gray.

The amount of radiation energy deposited in a unit mass of tissue, expressed in gray. It predicts biological effect better than exposure or air kerma alone.

A radiation protection quantity that weights absorbed dose by a radiation-type factor reflecting biological harm. It is measured in sievert and used for occupational and public limits.

The SI unit of equivalent and effective dose, representing biological risk rather than raw energy deposition. Annual occupational limits and public limits are expressed in millisievert.

The principle of keeping radiation exposure as low as reasonably achievable, accounting for economic and social factors. It guides shielding, workload planning, and daily practice decisions.

Dividing a prescribed radiation dose into multiple smaller treatments delivered over days or weeks. It exploits repair differences between tumor and normal tissue to widen the therapeutic window.

The balance between tumor control probability and normal tissue complication probability. A favorable ratio means high tumor kill with acceptable toxicity, and it is the central aim of treatment planning.

The dose a healthy organ or structure can receive before clinically significant injury becomes likely. Planning constraints are derived from published tolerance data for each organ.

A graphical summary showing what volume of a structure receives at least a given dose. It is used to evaluate target coverage and organ-at-risk sparing in a treatment plan.

A contour on a planning image connecting points that receive the same radiation dose. It visually demonstrates dose distribution and coverage relative to targets and organs at risk.

Software that models beam geometry, dose deposition, and patient anatomy to produce a deliverable plan. It calculates dose distributions using algorithms such as superposition or Monte Carlo.

The mathematical method a planning system uses to predict dose in tissue, ranging from simple correction-based models to convolution-superposition and Monte Carlo methods. Accuracy matters most in heterogeneous tissue.

An adjustment in dose calculation that accounts for differences in density, such as lung or bone, compared with water. Ignoring it can cause clinically meaningful dose errors.

A device that accelerates electrons to high energy and directs them at a target to produce megavoltage photon beams, or directly delivers electron beams. It is the main treatment machine in modern radiotherapy.

A set of computer-controlled tungsten leaves in the accelerator head that shapes the beam aperture. It enables conformal fields, dynamic arc therapy, and intensity modulation.

A technique in which the beam fluence is varied across the field by moving collimator leaves, creating highly conformal dose distributions. It improves sparing of concave or adjacent organs at risk.

An arc-based delivery technique in which gantry rotation, dose rate, and multileaf collimator motion are modulated continuously. It produces highly conformal plans with short delivery times.

The use of imaging immediately before or during treatment to verify and correct patient position and target location. It reduces geometric uncertainty and allows smaller margins.

A volumetric imaging method using a flat-panel detector and a divergent beam mounted on the treatment unit. It provides three-dimensional verification of patient position in the treatment room.

The extra distance added around a clinical target volume to account for positioning error, organ motion, and beam uncertainty. It converts a clinical target volume into a planning target volume.

A geometric volume that adds a margin around the clinical target volume to ensure the prescribed dose actually reaches the target despite daily uncertainties. It is a planning concept, not a biological one.

The visible or palpable extent of tumor demonstrated by imaging or clinical examination. It is the starting point of the volume chain used in conformal treatment planning.

The gross tumor volume plus a margin for microscopic subclinical disease that should receive a curative dose. It represents tissue that must be treated, independent of set-up uncertainty.

Radiotherapy delivered by placing a sealed radioactive source inside or very close to the tumor. Its steep dose fall-off spares distant tissue, and dose rate depends on source activity.

A technique in which applicators are positioned in the patient first and the radioactive source is loaded afterward by remote control. It reduces staff dose compared with manual placement.

The rule that radiation intensity from a point source falls in proportion to the square of the distance. Doubling distance reduces intensity to one quarter, which underpins shielding and source handling.

The process of determining barrier thickness needed to reduce dose outside a treatment or imaging room to acceptable levels. It uses workload, use factor, occupancy, and attenuation data.

The accelerating potential or nominal energy of a treatment beam, commonly stated in megavolts for photons. Higher energy penetrates more deeply but shifts the dose maximum deeper and increases neutron production.

The ratio of dose at a given depth in tissue to the dose at the depth of maximum dose, expressed as a percentage. It describes how a beam attenuates and builds up with depth.

The shallow tissue layer where dose rises to a maximum because secondary electrons are being generated faster than they are absorbed. Skin sparing occurs because the maximum lies below the surface.

A tissue-equivalent material placed on the skin to build up dose at or near the surface. It is used when the target lies close to the skin or the surface is irregular.

An imaging method that reconstructs cross-sectional images from X-ray projections and reports tissue density in Hounsfield units. Planning systems use these data for anatomy and density correction.

A scaled number expressing tissue attenuation relative to water, with water set to zero and air near minus one thousand. It allows density information to be mapped for dose calculation.

An imaging method that uses a strong magnetic field, radiofrequency pulses, and relaxation behavior of hydrogen nuclei to produce soft-tissue contrast without ionizing radiation. It is valuable for tumor and organ delineation.

The recovery of longitudinal magnetization after a radiofrequency pulse, characterized by a time constant T1. Tissues with short T1 recover quickly and appear bright on T1-weighted images.

The decay of transverse magnetization from spin-spin interactions, characterized by a time constant T2. Tissues with long T2 lose signal slowly and appear bright on T2-weighted images.

A radioactive compound administered for diagnosis or therapy, combining a radionuclide with a carrier that targets specific tissue. Its distribution and decay determine imaging signal or therapeutic dose.

The time required for half of the radioactive atoms in a sample to decay. It determines how long a radiopharmaceutical remains active and influences imaging timing and waste handling.

A system in which a longer-lived parent nuclide decays into a shorter-lived daughter that is eluted for use. It provides on-site access to short-lived tracers such as technetium-99m.

An imaging device that detects gamma rays from an administered radionuclide using a collimator, scintillation crystal, and photomultiplier tubes. It produces planar images reflecting tracer distribution.

An imaging technique that detects pairs of annihilation photons produced when positrons interact with electrons. Coincidence detection localizes the tracer and supports quantitative metabolic imaging.