CAMRT Radiation Therapy flashcards

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

Half-value layer

The thickness of a specified material that reduces the intensity of a photon beam to half its original value; numerically equal to 0.693 divided by the linear attenuation coefficient.

Compton scatter

Inelastic collision in which a photon transfers part of its energy to an outer-shell electron and is deflected; dominant in soft tissue at diagnostic energies and the main source of scattered dose.

Hounsfield unit

A normalized CT attenuation value scaled so water equals 0 HU and air equals about minus 1000 HU, computed as 1000 times the difference from water divided by water attenuation.

Inverse-square law

The intensity of radiation from a point source is inversely proportional to the square of the distance, so tripling distance reduces intensity to one ninth of its original value.

Grid ratio

The ratio of grid strip height to the distance between strips; higher ratios remove more scattered radiation but demand accurate centering and increase patient dose.

Photoelectric effect

A photon is completely absorbed by an inner-shell electron, which is ejected; probability rises steeply with atomic number and falls rapidly with increasing photon energy.

Photostimulable phosphor plate

A computed radiography detector that stores a latent image in electron traps and releases visible light when scanned by a laser, with intensity proportional to absorbed x-ray dose.

Occupational dose limit

For radiation workers, an effective dose of 20 mSv per year averaged over five years, with no single year exceeding 50 mSv; the public limit is 1 mSv per year.

Larmor equation

The precessional frequency of a nucleus equals the gyromagnetic ratio multiplied by magnetic field strength; for hydrogen, 42.58 MHz per tesla, giving about 64 MHz at 1.5 T.

T1 relaxation

Recovery of longitudinal magnetization after a radiofrequency pulse as spins exchange energy with their surroundings; tissues with short T1 appear bright on T1-weighted images.

T2 star

Transverse relaxation that includes both true spin-spin decay and dephasing from static field inhomogeneity; it is always shorter than true T2 and is used in gradient-echo imaging.

Repetition time

The interval between successive radiofrequency excitation pulses; short values emphasize T1 contrast, while long values reduce T1 weighting and permit full longitudinal recovery.

Gadolinium chelate

A paramagnetic contrast agent that shortens T1 of nearby water protons; it is cleared renally, so severe renal impairment raises the risk of nephrogenic systemic fibrosis.

Specific absorption rate

Radiofrequency energy deposited per unit mass of tissue, measured in watts per kilogram; it increases with field strength and flip angle and constrains fast high-energy sequences.

Chemical shift

The small difference in precessional frequency between fat and water protons, expressed in parts per million; it increases with field strength and produces fat-water misregistration artifacts.

Gradient coils

Coils that superimpose small linear magnetic field gradients across the bore, enabling slice selection, frequency encoding, and phase encoding of spatial position in the image.

Technetium-99m

A nuclear medicine radionuclide with a 6.02-hour half-life and a 140 keV gamma emission, eluted from a molybdenum-99 generator and used in most planar and SPECT imaging.

Radioactive decay law

Activity decreases exponentially over time according to the half-life; after n half-lives, the remaining activity equals the initial activity divided by 2 raised to n.

Gamma camera collimator

A lead septal grid placed before the detector that only admits gamma rays traveling roughly parallel to the holes, thereby projecting the source distribution onto the crystal.

FDG-PET

Imaging with fluorine-18 fluorodeoxyglucose, which is taken up like glucose and trapped after phosphorylation; uptake reflects regional glycolytic rate, especially in malignant tissue.

Coincidence detection

Simultaneous registration of two 511 keV annihilation photons traveling in opposite directions, defining a line of response along which the positron emission occurred.

Standardized uptake value

A semiquantitative PET metric equal to measured tissue activity concentration divided by injected dose normalized to body weight, with decay correction applied.

Effective half-life

The combined clearance of a radiopharmaceutical by physical decay and biological excretion, calculated so its reciprocal equals the sum of the physical and biological reciprocals.

Iodine-131 therapy

A beta- and gamma-emitting radionuclide concentrated by thyroid tissue; its short-range beta particles deliver therapeutic dose for hyperthyroidism and thyroid carcinoma ablation.

Gray

The SI unit of absorbed radiation dose, equal to one joule per kilogram of material; it replaces the older rad, where one gray equals one hundred rad.

Linear-quadratic model

A cell survival model in which surviving fraction equals e to the negative of alpha times dose plus beta times dose squared; the alpha/beta ratio describes tissue fractionation sensitivity.

EQD2

The equivalent dose delivered in 2-Gy fractions, calculated as total dose times (alpha/beta plus fraction dose) divided by (alpha/beta plus 2), allowing comparison of schedules.

Bragg peak

The sharp maximum in energy deposition near the end of a charged particle track; protons deposit most dose at a controllable depth with negligible dose beyond it.

Percentage depth dose

The absorbed dose at a given depth expressed as a percentage of the maximum dose on the central axis; it increases with field size and decreases with greater depth.

Therapeutic ratio

The separation between the tumor control probability curve and the normal tissue complication probability curve; a favorable ratio means tumor control is achieved at acceptable toxicity.

Brachytherapy falloff

Dose rate around an implanted sealed source decreases approximately with the inverse square of distance, producing highly localized high dose near the source and rapid sparing farther away.

Four Rs of fractionation

Repair of sublethal damage, redistribution of cells through the cycle, repopulation of surviving tumor cells, and reoxygenation of hypoxic regions; these explain fractionation benefits.