Therapeutic Radiographer key terms

The amount of radiation energy deposited per unit mass of tissue, measured in gray (Gy). One gray equals one joule per kilogram and describes the physical dose delivered, not its biological effect.

The SI unit of absorbed radiation dose, equal to one joule per kilogram of material. Therapeutic courses are typically prescribed in fractions of 1.8 to 3 Gy.

A radiation protection quantity that weights absorbed dose by a radiation-type factor to reflect biological harm, expressed in sieverts. Used for occupational and public dose limits, not for treatment prescribing.

The principle that radiation exposure should be kept as low as reasonably achievable, accounting for economic and social factors. It drives shielding, workload planning, and routine staff protection practice.

A designated region where radiation doses may exceed specified thresholds and where access is restricted, monitored, and governed by local rules. Staff entering require appropriate training and dose surveillance.

A wearable device, such as a thermoluminescent or optically stimulated badge, that records cumulative occupational radiation dose. Results are reviewed against dose limits to confirm safe working practice.

The principle that radiation intensity from a point source falls with the square of distance. Doubling distance reduces intensity to one quarter, underpinning safe distance practice and source handling.

A machine that accelerates electrons along a waveguide and directs them onto a target to produce high-energy photon or electron beams for external beam treatment.

A set of computer-controlled tungsten leaves in the accelerator head that shapes the radiation field to match the target volume, enabling conformal and intensity-modulated delivery.

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

The shallow depth range just below the skin surface where dose rises to its maximum as secondary electrons reach equilibrium. It affects skin sparing and is why bolus may be applied.

A tissue-equivalent material placed on the skin surface to build up dose in superficial layers, compensating for missing tissue or bringing the maximum dose closer to the surface.

A treatment planning method in which the desired dose distribution and constraints are specified, and software optimises beamlet or arc weights to meet them. It underlies intensity-modulated and volumetric arc techniques.

A delivery technique using non-uniform beam intensities, produced by dynamic multileaf collimator motion, to create concave dose distributions that spare nearby organs at risk.

A rotational delivery technique in which the gantry moves around the patient while leaf positions and dose rate vary continuously, producing highly conformal dose in a single or few arcs.

A graph showing the fraction of a structure's volume receiving at least a given dose. It is used to evaluate target coverage and organ-at-risk constraints during plan review.

Normal tissues and structures near the target whose radiation tolerance may limit the prescribed dose. They are outlined and constrained during planning to reduce toxicity.

The visible or palpable extent of tumour demonstrated by imaging or examination. It is the starting volume from which clinical and planning target volumes are derived.

The gross tumour volume plus a margin for suspected microscopic spread that must receive the prescribed dose. It is a clinical, not a geometric, concept.

A geometric volume that adds margins around the clinical target volume to account for organ motion, setup variation, and delineation uncertainty, ensuring the prescribed dose reaches the target.

Equipment such as thermoplastic masks, vacbags, or cradles that restricts patient movement and improves setup reproducibility between fractions, supporting tight planning margins.

The use of imaging, such as cone-beam CT, ultrasound, or planar kilovoltage, immediately before or during treatment to verify and correct patient position relative to the planned position.

Acquisition of treatment beam images using an electronic detector opposite the source to verify field placement and patient position during or before delivery.

Dividing a prescribed total dose into multiple smaller treatments, usually delivered daily, to exploit differences in repair between tumour and normal tissues and reduce late toxicity.

A radiobiological parameter describing a tissue's sensitivity to fraction size. Low values indicate strong fractionation sensitivity, typical of late-responding normal tissues and some tumours such as prostate.

A mathematical model describing cell survival after radiation as a function of dose, with linear and quadratic components. It underpins fractionation calculations and biologically effective dose comparisons.

The ratio of dose needed to produce a given effect under hypoxia compared with full oxygenation. It quantifies how oxygen increases radiosensitivity and explains the rationale for hypoxic modification.

The degree to which cells or tissues respond to radiation, influenced by cell type, proliferation rate, oxygenation, and repair capacity. It varies widely between tumours and normal tissues.

The difference between the actual patient position during treatment and the intended reference position. It is quantified and managed through immobilisation, imaging verification, and margin design.

Planned and systematic activities that ensure equipment, processes, and treatments meet defined standards. It includes calibration, testing schedules, and documentation to maintain safe and accurate delivery.

The specific measurements and tests performed to verify that equipment and processes meet specified tolerances, such as output constancy or leaf position accuracy.

A structured evaluation performed after equipment installation to confirm it meets manufacturer specifications and contractual requirements before clinical use and commissioning begin.

The process of measuring and documenting machine and planning system performance to establish the data and models used clinically, including beam data collection and validation of calculations.

A classification system describing the extent of cancer by primary tumour size and invasion, regional lymph node involvement, and distant spread. It guides treatment intent and technique selection.

Delivery of chemotherapy at the same time as a course of radiotherapy, used to radiosensitise tumour cells and improve local control in selected cancers. It increases acute toxicity.

Side effects occurring during or shortly after radiotherapy, typically in rapidly dividing tissues such as skin and mucosa. They are usually reversible but may require treatment interruption.

Effects appearing months to years after treatment, often in slowly dividing tissues such as lung, kidney, or spinal cord. They are generally irreversible and limit the dose that can be given.

The process of explaining a proposed treatment, its benefits, risks, and alternatives so the patient can make a voluntary decision. It must be documented before radiotherapy begins.

The process of placing and aligning the patient reproducibly on the treatment couch using reference marks, lasers, and immobilisation, matching the position used at planning CT.