CMD key terms

The amount of ionizing radiation energy deposited per unit mass of material, measured in gray (1 Gy = 1 joule per kilogram). It is the primary quantity used for prescribing and reporting radiotherapy dose.

The SI unit of absorbed radiation dose, equal to one joule of energy deposited per kilogram of matter. Clinical radiotherapy prescriptions typically range from a few gray per fraction to 60-70 Gy total.

A single dose value that, if delivered uniformly across a volume, would produce the same biological effect as the actual non-uniform dose distribution. It summarizes heterogeneous dose into one comparable number.

A graph or table showing how much of a defined structure's volume receives at least a given dose. Cumulative DVHs are standard tools for evaluating plan quality and comparing competing treatment plans.

The minimum dose received by 95 percent of a target volume. It is a common coverage metric used to confirm that nearly all of a planning target volume receives the prescribed dose.

The percentage of a structure's volume receiving at least 20 Gy. For lung, V20 is widely used to estimate the risk of radiation pneumonitis after thoracic radiotherapy.

The average dose delivered across both lungs, calculated from the dose distribution. Along with V20, it is a key predictor of symptomatic radiation pneumonitis risk.

A modeled estimate of the likelihood that a specified normal organ will suffer a defined complication at a given dose distribution. It supports plan comparison and selection decisions.

A modeled estimate of the probability of eradicating a tumor with a given dose distribution. It is used in radiobiological plan evaluation alongside normal tissue complication probability.

Dividing a total radiation dose into multiple smaller treatments, typically given once daily. It exploits differences in repair between tumors and normal tissues and is described by the linear-quadratic model.

A radiobiological model in which cell survival after dose D is described by the expression exp(-alphaD - betaD squared). It underpins fractionation calculations and biologically effective dose comparisons.

The dose at which linear and quadratic cell-killing components of the linear-quadratic model are equal. Low ratios (about 3 Gy) indicate late-responding tissues; high ratios (about 10 Gy) indicate early-responding tumors.

A measure of the biological effect of a dose schedule that accounts for dose per fraction and tissue alpha-beta ratio. It allows comparison of different fractionation schemes on a common scale.

The written specification of the target volume, total dose, dose per fraction, and fractionation schedule ordered by the radiation oncologist. It is the reference against which the plan is evaluated.

A stated limit on dose or volume for a structure, such as a maximum point dose or a volume receiving above a threshold. Constraints guide optimization and plan acceptability decisions.

The depth range just below the skin where dose from a megavoltage photon beam rises to a maximum before attenuating. Its depth depends on beam energy and field size.

The maximum absorbed dose along the central axis of a beam, occurring at the depth where build-up and attenuation balance. For high-energy photons it typically lies several millimeters to centimeters below the skin.

The ratio of absorbed dose at a given depth to the dose at the depth of maximum dose, expressed as a percentage along the central axis. It depends on energy, field size, and source-to-surface distance.

The ratio of dose at a point in a phantom to the dose at the same point in air, for a given field size. It is used in isocentric calculations and is related to percentage depth dose.

The principle that radiation intensity from a point source decreases in proportion to the square of the distance from the source. Dosimetrists use it to correct dose when distance changes.

The distance from the radiation source to the machine isocenter, typically 100 cm on modern linear accelerators. Isocentric treatment techniques are set up and calculated using this distance.

The distance from the radiation source to the patient's skin surface along the central axis. It is used in percentage depth dose calculations for non-isocentric setups.

The ratio of dose at a point in a phantom to dose at the same point from primary radiation only, reflecting contributions from scattered photons. It increases with field size and decreases with depth.

The point in space about which the gantry, collimator, and couch rotate, where their axes intersect. Treatments are commonly planned and delivered relative to this point.

The rotational position of the linear accelerator's gantry around the patient, measured in degrees from a defined reference. It determines the beam entry direction for each field or arc.

A display showing the patient anatomy as seen from the radiation source along the beam direction. It helps dosimetrists shape fields and avoid critical structures.

A device with independently movable tungsten leaves that shape the radiation field to match target projections. It enables conformal fields, dynamic delivery, and intensity modulation.

A delivery technique in which beam fluence is modulated across the field to create highly conformal dose distributions. It uses inverse planning and multileaf collimator motion.

An arc-based technique that varies dose rate, gantry speed, and multileaf collimator position during rotation. It produces conformal dose in one or more arcs with short delivery times.

An optimization approach in which desired dose objectives and constraints are specified, and the planning system calculates fluence that best meets them. It contrasts with forward planning by trial and error.

The mathematical process of adjusting beam parameters or fluence to minimize deviation from planning objectives. It produces the deliverable plan that is then evaluated against clinical goals.

A numeric ratio describing how well the prescription isodose volume matches the target volume. Common definitions compare the treated volume or prescription volume with the planning target volume.

A metric quantifying how uniform dose is within a target volume, often from the difference between maximum and minimum or prescription dose. Lower values indicate more uniform coverage.

A geometric volume that adds margins around the clinical target volume to account for setup error, organ motion, and other uncertainties. It ensures the prescribed dose actually covers the target.

The volume containing the gross tumor plus any suspected microscopic spread that must receive a specified dose. It is expanded to form the planning target volume.

The visible or palpable extent of tumor identified on imaging or examination. It is the innermost target volume from which clinical and planning target volumes are derived.

The process of aligning images from different modalities or time points into a common coordinate system. It supports target delineation, fusion of planning scans, and evaluation of delivered dose.

An imaging system that acquires volumetric CT images using a flat-panel detector mounted on the treatment machine. It verifies patient position and anatomy immediately before treatment.

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

The process of positioning and immobilizing a patient so that the treatment geometry matches the planned geometry. Accuracy is verified with imaging and documented at each treatment.