ABR Radiation Oncology key terms

The average energy a charged particle deposits per unit path length as it travels through tissue, expressed in keV per micrometer. High-LET radiation such as alpha particles or carbon ions produces dense ionization tracks and more complex DNA damage.

The ratio of a reference photon dose to the dose of a test radiation needed to produce the same biological effect. It rises as LET increases and is used to convert physical dose into biologically equivalent dose for particle therapy.

The ratio of dose required to achieve a given effect under hypoxia to the dose required under normal oxygenation. Values near 3 for low-LET radiation fall toward 1 as LET increases, reflecting reduced oxygen dependence of dense-track damage.

A parameter of the linear-quadratic model describing the dose at which linear and quadratic cell-killing components contribute equally. Low values indicate a strong dose-per-fraction sensitivity, typical of prostate cancer and late-responding normal tissues.

A mathematical description of cell survival after irradiation in which surviving fraction equals the exponential of negative (alpha times dose plus beta times dose squared). It underpins fractionation calculations and isodose-equivalent comparisons in clinical practice.

A quantity that converts a fractionated schedule into an equivalent dose for a reference fraction size using the alpha/beta ratio. It allows comparison of regimens with different dose per fraction and total dose.

Increased cell survival observed when cells are held in suboptimal growth conditions after irradiation, allowing repair of damage that would otherwise be expressed. It is one mechanism by which surviving fraction can change after the radiation exposure ends.

Rejoining of radiation-induced lesions that individually are not lethal, demonstrated by the split-dose recovery effect. It explains why dividing a dose into fractions spares tissues with repair capacity.

A plot of surviving fraction against radiation dose, usually on a logarithmic scale, used to derive radiosensitivity parameters. Its shoulder at low dose reflects repair capacity and its slope at high dose reflects intrinsic radiosensitivity.

The reassortment of cells within the cell cycle between dose fractions, moving surviving cells from radioresistant phases into more sensitive ones. It is one of the classic four R's of fractionation.

Proliferation of surviving clonogens during a course of radiotherapy, which can offset cell kill if overall treatment time is prolonged. Accelerated repopulation is a rationale for avoiding treatment gaps in rapidly dividing tumors.

Regions of a tumor with oxygen tension low enough to reduce radiation-induced free-radical fixation of DNA damage. Hypoxic cells are relatively radioresistant and are a target of hypoxia-directed strategies and high-LET therapy.

The process by which previously hypoxic tumor cells become better oxygenated after fractionated irradiation as oxygenated cells are killed and tumor vasculature improves. It increases the effectiveness of subsequent fractions.

Biological responses such as DNA damage or cell death observed in cells that were not directly irradiated but received signals from irradiated neighbors. It is relevant to low-dose exposures and to understanding non-targeted effects of radiation.

A delayed, persistent increase in mutation and chromosome aberration rates in the progeny of irradiated cells many generations after exposure. It is a form of non-targeted radiation effect relevant to radiation protection and carcinogenesis.

Describes the penetrating ability of a photon beam, commonly characterized by percentage depth dose or tissue-phantom ratio at a stated depth and field size. Higher beam energy shifts the depth of maximum dose deeper and reduces surface dose.

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

The ratio of dose at a given depth to dose at the depth of maximum dose in a phantom, measured with a fixed source-to-detector distance. It is used in isocentric dose calculations because it is independent of source-to-surface distance.

The shallow depth range in a megavoltage photon beam where dose rises from the surface to the depth of maximum dose as secondary electrons reach equilibrium. Its thickness increases with beam energy and skin sparing depends on it.

A condition in which the number of electrons entering a small volume equals the number leaving it, so absorbed dose approximates collision kerma. It is required for accurate dose measurement and is approached beyond the build-up region.

A gas-filled detector that collects charge produced by ionizing radiation to measure exposure or absorbed dose. Cylindrical chambers are used for reference dosimetry, while parallel-plate chambers suit surface and electron measurements.

A task group report specifying reference dosimetry of megavoltage photon and electron beams using an ionization chamber calibrated in terms of absorbed dose to water. It defines beam quality indices and correction factors for clinical reference calibration.

The ratio of dose with full scattering conditions to dose from primary radiation alone at the same point. It quantifies the contribution of scattered photons and depends on field size, depth, and beam energy.

A patient-specific quality assurance metric that compares measured and calculated dose distributions by combining dose-difference and distance-to-agreement criteria into a single pass or fail value. Lower gamma values indicate closer agreement between planned and delivered dose.

A cumulative or differential plot showing the volume of a structure receiving at least a given dose. It summarizes a three-dimensional dose distribution and is used to evaluate target coverage and organ-at-risk constraints.

A metric comparing the volume receiving a prescription dose with the target volume, often expressed as a ratio of intersection to union. Values near one indicate that the high-dose region closely matches the intended target.

A delivery technique in which beam fluence is modulated across many small fields to produce steep dose gradients and concave dose distributions around targets. It requires inverse planning and rigorous patient-specific quality assurance.

An arc-based form of intensity modulation in which gantry rotation, dose rate, and multileaf collimator positions vary continuously during treatment. It delivers conformal dose efficiently but demands accurate motion and delivery verification.

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

A geometric volume that adds a margin to the clinical target volume to account for setup variation and internal organ motion. It ensures the prescribed dose is likely to cover the clinical target throughout treatment.

The volume of tissue that contains the demonstrated tumor plus any subclinical microscopic disease that must be treated. It is defined by the physician before geometric margins for motion and setup are added.

The visible or palpable extent of tumor as determined by imaging and clinical examination. It is the starting point of the ICRU volume chain and excludes subclinical spread.

The system defining prescription dose and reporting for an implanted source, historically based on milligram-hours or, in modern practice, on dose to a target volume such as the prostate or cervix high-risk region. It links source strength and geometry to clinical effect.

The principle that radiation intensity from a point source falls in proportion to the square of the distance from it. It governs external-beam output changes with distance and dose falloff around brachytherapy sources.

Remote afterloading delivery of a highly active source at dose rates above roughly 12 Gy per hour, usually in a few large fractions. It offers outpatient treatment but requires strict source and applicator verification.

Pre-treatment measurement and analysis verifying that a planned modulated dose distribution can be delivered accurately for an individual patient. It typically uses phantom measurements and gamma analysis before treatment begins.

Dividing a total radiation dose into multiple smaller treatments, usually one per day, to exploit repair, redistribution, reoxygenation, and repopulation differences between tumors and normal tissues. It improves the therapeutic ratio.