Therapeutic Radiographer flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Inverse square law

Dose rate from a point source varies inversely with the square of distance: D1/D2 = (d2/d1)². Doubling distance quarters the dose rate, which underpins distance-based protection.

Half-value layer

Thickness of a material that reduces beam intensity by half. HVL = ln2/μ = 0.693/μ, where μ is the linear attenuation coefficient. Used to specify beam quality and shielding.

Klystron versus magnetron

Both generate microwave power for linear accelerators. Klystrons amplify a low-power signal to high peak power; magnetrons oscillate directly and produce lower peak power. Both feed the waveguide.

Flattening filter function

A conical absorber in the linac head that makes the photon fluence uniform across the field. Removing it increases dose rate but requires compensation in planning, as in flattening-filter-free beams.

Cobalt-60 gamma energies

Cobalt-60 decays with a 5.26-year half-life, emitting gamma rays at 1.17 and 1.33 MeV. The average energy used in planning is about 1.25 MeV.

Electron practical range

The depth in water at which the electron depth-dose curve falls to near zero. It approximates energy in MeV divided by 2, so a 12 MeV beam reaches about 6 cm.

ALARA principle

As low as reasonably achievable: optimisation of protection by keeping doses below limits while considering economic and social factors. Applied through time, distance, and shielding.

Occupational dose limits

For radiation workers: 20 mSv per year averaged over 5 years, with no year exceeding 50 mSv. Eye lens limit is 20 mSv/year; skin and extremities 500 mSv/year.

Effective dose formula

Effective dose E equals the sum of tissue equivalent doses multiplied by tissue weighting factors: E = Σ wT·HT. It allows comparison of stochastic risk across exposure types.

Radiation weighting factors

Factors converting absorbed dose to equivalent dose: 1 for photons and electrons, 2 for protons, 20 for alpha particles, and energy-dependent values of 2.5 to 20 for neutrons.

Pregnant worker fetal dose

After a worker declares pregnancy, the dose to the fetus should not exceed 1 mSv for the remainder of the pregnancy. Working conditions may be adjusted to meet this constraint.

Controlled area threshold

An area where occupational exposure is possible and dose rates exceed 7.5 μSv/h, or where annual dose could exceed 6 mSv. Requires local rules, signage, and dose monitoring.

Informed consent elements

Valid consent requires patient capacity, sufficient information about the procedure's purpose, benefits, risks and alternatives, and voluntary agreement without coercion. Documented before treatment starts.

Radiotherapy skin reaction timeline

Erythema typically appears in week 2 to 3, followed by dry desquamation and possible moist desquamation in skin folds. Managed with gentle washing, emollients, and avoidance of friction.

Thermoplastic mask use

A moulded mask that immobilises the head and neck and reproduces set-up daily. It is indexed to the couch and marked with reference points for isocentre alignment.

Oral mucositis management

Mucositis from head and neck radiotherapy peaks around weeks 4 to 5. Care includes saline or bicarbonate rinses, oral hygiene, analgesia, and nutritional support when eating is impaired.

Teach-back communication

A method where the patient explains information in their own words, confirming understanding. It identifies gaps and supports informed consent and safe self-care during treatment.

Four Rs of radiobiology

Repair of sublethal damage, redistribution through the cell cycle, repopulation of surviving cells, and reoxygenation of hypoxic tumour cells. These processes shape fractionation response.

Linear-quadratic model

Surviving fraction S = exp(−(αD + βD²)). The α/β ratio is about 10 Gy for tumours and early effects, and about 3 Gy for late-reacting normal tissues.

Biologically effective dose

BED = nd(1 + d/(α/β)) for n fractions of dose d. It compares schedules with different fraction sizes and is converted to EQD2 by dividing by (1 + 2/(α/β)).

Oxygen enhancement ratio

Ratio of dose needed under hypoxia to dose under full oxygenation for the same effect. It is about 2.5 to 3 for photons, reflecting the radiosensitising role of oxygen.

Radiosensitivity ranking

Most radiosensitive tissues include lymphoid tissue, bone marrow, germ cells, and intestinal epithelium. Least sensitive include neurons, muscle, and mature connective tissue.

ICRU target volumes

GTV is visible tumour, CTV adds microscopic spread, and PTV adds margin for set-up and organ motion. OARs and PRVs are also defined for planning and reporting.

PTV margin formula

Margin approximately equals 2.5 times the systematic error standard deviation plus 0.7 times the random error standard deviation, accounting for both set-up and organ motion uncertainty.

DVH interpretation

A dose-volume histogram shows the fraction of a structure receiving at least a given dose. Key metrics include D95 for target coverage and V20 for lung.

Inverse planning concept

Planning starts from dose-volume objectives and iteratively optimises fluence or machine parameters to meet them. It underlies IMRT and VMAT, unlike manual forward planning.

Image guidance tolerance

Acceptable set-up correction depends on site: about 3 mm for intracranial targets and 5 mm for pelvis. Exceeding tolerance triggers repositioning and repeat imaging before treatment.

VMAT delivery characteristics

Volumetric arc therapy delivers continuous gantry rotation with dynamic multileaf collimation and variable dose rate. Treatment is typically 2 to 4 minutes per fraction.

Portal imaging purpose

Electronic portal imaging verifies field shape and position against reference images. It can also support in vivo dosimetry and transit dose measurement.

Six-degree-of-freedom couch

A treatment couch that corrects three translational and three rotational set-up errors. Rotational correction is essential for intracranial and spinal stereotactic treatments.

Daily linac QA checks

Output constancy within 3%, laser alignment, door interlock, beam-on indicator, and audiovisual monitoring are verified each treatment day before patients are treated.

TG-51 reference dosimetry

A protocol for calibrating clinical beams in terms of absorbed dose to water using an ion chamber with a calibration factor traceable to a standards laboratory.

MLC picket fence test

A test imaging adjacent leaf pairs at set positions to reveal gap or overlap errors. Leaf position accuracy should be within about 1 mm for stereotactic treatments.