CAMRT Radiation Therapy cheat sheet

Beam attenuation follows I = I0·e^(−μx), where μ is the linear attenuation coefficient and x is path length; the half-value layer equals 0.693/μ.

Photoelectric absorption dominates at low photon energy and scales roughly with Z³/E³, which is why high-Z contrast agents (iodine Z=53, barium Z=56) improve soft-tissue contrast.

Compton scatter dominates at diagnostic energies in soft tissue and is nearly independent of atomic number; it degrades contrast and is the main source of occupational dose.

CT numbers follow Hounsfield units: HU = 1000·(μ_tissue − μ_water)/μ_water; water is 0 HU and air is approximately −1000 HU.

Inverse-square law: intensity falls with the square of distance, so doubling source-to-detector distance reduces intensity to one quarter.

Grid ratio equals grid height divided by interspace width; higher ratio improves scatter cleanup but raises patient dose and requires tighter centering.

The photostimulable phosphor plate in computed radiography stores a latent image in electron traps; a scanning laser releases stored energy as blue luminescence proportional to dose.

ALARA dose limits for occupational exposure are 20 mSv/year averaged over 5 years (100 mSv per 5 years) with a 50 mSv single-year cap; public exposure is 1 mSv/year.

The Larmor equation states precession frequency f = γB, where γ for hydrogen is 42.58 MHz/T; at 1.5 T the proton frequency is about 63.9 MHz.

T1 relaxation is longitudinal recovery (spin-lattice); T2 is transverse decay (spin-spin); T2* adds static field inhomogeneity and is always shorter than T2.

TR controls T1 weighting: short TR with short TE gives T1-weighted images; long TR with long TE gives T2-weighted images; long TR with short TE gives proton density.

Gadolinium chelates shorten T1 of nearby protons; they are extracellular agents cleared renally, and risk of nephrogenic systemic fibrosis rises with severe renal impairment.

The specific absorption rate depends on field strength squared and flip angle squared; it limits how fast high-energy radiofrequency pulses can be applied.

Chemical shift in parts per million equals the frequency difference in Hz divided by the Larmor frequency in MHz; fat-water shift grows with field strength.

Gradient coils encode spatial position by superimposing linear field gradients; slice selection uses a gradient with a matched radiofrequency bandwidth.

The safety zone IV (above 3 T, or a patient with a pacemaker) requires explicit MR-conditional device labeling before scanning; ferromagnetic projectiles remain the leading hazard.

Technetium-99m has a 6.02-hour half-life, 140 keV gamma emission, and is eluted from a molybdenum-99 generator; parent Mo-99 has a 66-hour half-life.

Radioactive decay follows A = A0·e^(−0.693t/T½); after n half-lives the remaining activity is A0/2ⁿ.

The gamma camera uses a collimator to define direction, a sodium iodide crystal to convert gamma photons to light, and photomultiplier tubes to produce position signals.

FDG-PET images glucose metabolism; FDG is trapped after phosphorylation by hexokinase and is not further metabolized, so uptake reflects glycolytic rate.

The PET photopeak is 511 keV from positron annihilation; coincidence detection of two back-to-back photons localizes the event along a line of response.

Standardized uptake value equals measured activity concentration divided by injected dose per body weight, corrected for decay.

Effective half-life combines physical and biological clearance: 1/T_eff = 1/T_phys + 1/T_bio.

Iodine-131 emits beta particles (tissue range about 1-2 mm) plus gamma; it is used for thyroid ablation and therapy, while I-123 is preferred for imaging.

Absorbed dose is measured in gray (1 Gy = 1 J/kg); the older unit rad equals 0.01 Gy, and 1 Sv equals 100 rem for equivalent dose.

The linear-quadratic model gives surviving fraction S = e^(−αD−βD²); the α/β ratio is about 10 Gy for early-responding tumors and about 3 Gy for late tissues.

Equivalent dose in 2-Gy fractions uses EQD2 = D·(α/β + d)/(α/β + 2); biologically effective dose uses BED = D·(1 + d/(α/β)).

The Bragg peak of protons deposits maximum energy at a defined depth, with near-zero exit dose beyond the peak; spread-out Bragg peaks cover tumor volumes.

In external beam therapy, percentage depth dose increases with field size and decreases with depth and with higher beam energy for the buildup region.

Tumor control probability and normal tissue complication probability curves are sigmoid; the therapeutic ratio is the separation between them.

Brachytherapy dose falls off as the inverse square of distance from the source, so dose rate at distance d is proportional to 1/d².

Fractionation exploits the four Rs: repair of sublethal damage, redistribution through the cell cycle, repopulation, and reoxygenation.