Therapeutic Radiographer cheat sheet

Photon energy relates to frequency and wavelength by E = hf = hc/λ, with h = 6.626 × 10⁻³⁴ J·s and c = 3 × 10⁸ m/s; 1 eV = 1.602 × 10⁻¹⁹ J.

Inverse square law: dose rate falls with the square of distance, D₁/D₂ = (d₂/d₁)², applied to point sources in air.

Half-value layer: HVL = ln2/μ = 0.693/μ; after n half-value layers the transmitted intensity is I₀/2ⁿ.

Linear accelerator microwave sources: klystrons amplify (high peak power, low duty cycle), magnetrons oscillate (low peak power); both feed the waveguide at 2856 MHz (S-band) or 2998 MHz.

Electron gun injects electrons into the accelerating waveguide; a bending magnet (achromatic or slalom) directs the beam onto the target for photons or straight through for electrons.

Photon target is typically tungsten or tungsten-rhenium; the flattening filter and primary collimator shape the beam, and the ion chamber monitors dose rate, integrated dose and beam symmetry.

Multileaf collimator leaves project to typically 5 mm or 10 mm at isocentre; leaf transmission is around 0.5–1.5% and interleaf leakage should stay below about 2%.

Cobalt-60 teletherapy emits gamma rays at 1.17 and 1.33 MeV with a half-life of 5.26 years, giving an average energy near 1.25 MeV.

Electron depth dose: practical range Rₚ approximates E(MeV)/2 in cm in water, and surface dose rises with energy because of scatter equilibrium.

ICRP three fundamental principles: justification, optimisation (ALARA), and dose limitation for occupational and public exposure.

Occupational dose limits: 20 mSv/year averaged over 5 years (100 mSv in 5 years) with no single year above 50 mSv; eye lens 20 mSv/year; skin and extremities 500 mSv/year.

Public dose limit is 1 mSv/year; dose constraint for members of the public from a single source is typically 0.3 mSv/year.

Effective dose E = Σ w_T · H_T, where w_T is the tissue weighting factor; equivalent dose H_T = Σ w_R · D_T,R.

Radiation weighting factors: photons and electrons 1, protons 2, alpha particles 20, neutrons energy-dependent (2.5–20).

Controlled area dose rate thresholds are commonly 7.5 μSv/h and supervised areas 2.5 μSv/h above background, with local rules and warning signage required.

Time, distance and shielding: dose is proportional to time, follows the inverse square law with distance, and attenuates exponentially through shielding material.

Pregnant worker: declare pregnancy, and the dose to the fetus should not exceed 1 mSv for the remainder of the pregnancy.

Personal dosimetry: TLD or OSL badges worn at chest level, film badges for extremity rings; dosimeters are exchanged monthly or quarterly and records kept.

Informed consent requires capacity, adequate information about benefits and risks, and voluntary agreement; it is documented before the first treatment.

Common CT contrast reactions: mild (nausea, urticaria) observed; moderate (vomiting, bronchospasm) treated; severe anaphylactoid reaction needs adrenaline, oxygen and IV fluids.

Radiotherapy skin reactions follow a timeline: erythema from about week 2–3, dry desquamation, then moist desquamation in skin folds; managed with gentle washing and emollients.

Fatigue affects most patients during a course; assess with a 0–10 scale and advise activity pacing and rest.

Oral mucositis in head and neck treatment peaks around weeks 4–5; mouth care, saline or bicarbonate rinses and analgesia are core measures.

Patient positioning aids: thermoplastic masks for head and neck, Vac-Lok bags and wing boards for pelvis and thorax, and knee/ankle supports for reproducibility.

Communication: use teach-back to confirm understanding, provide written information, and involve carers with patient permission while respecting confidentiality.

Assess nutritional status and weight weekly during treatment; significant weight loss changes set-up and may require replanning.

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

Linear-quadratic model: surviving fraction S = exp(−(αD + βD²)); the α/β ratio is about 10 Gy for early effects and tumours, about 3 Gy for late effects.

Biologically effective dose BED = nd(1 + d/(α/β)); equivalent dose in 2 Gy fractions EQD2 = BED/(1 + 2/(α/β)).

Fractionation trade-off: smaller fractions spare late-reacting tissues more than tumours because of the lower α/β of late effects.

Oxygen enhancement ratio (OER) is about 2.5–3 for photons; hypoxic cells require roughly 2.5–3 times the dose for the same effect.

Radiosensitivity: most sensitive are lymphoid and haemopoietic tissues, germ cells and intestinal epithelium; least sensitive are neurons and muscle.

Tumour staging uses the TNM system; stage influences intent (radical vs palliative) and target volumes.

Common dose-fractionation: breast 40 Gy in 15 fractions; prostate 60 Gy in 20 fractions or 74–78 Gy in 37–39 fractions; palliative bone 8 Gy single fraction or 20 Gy in 5.

ICRU 50/62 volumes: GTV (visible tumour), CTV (GTV plus microscopic spread), PTV (CTV plus set-up and organ motion margin), plus OAR and PRV.

PTV margin derives from systematic and random error: margin ≈ 2.5Σ + 0.7σ, where Σ is the standard deviation of systematic errors and σ of random errors.

Simulation CT uses flat couch top, indexed immobilisation, and reference marks aligned to the isocentre; scan slice thickness of 2–3 mm is typical for planning.

Dose calculation algorithms: pencil beam for homogeneous tissue, collapsed cone and Monte Carlo for heterogeneity correction in lung and bone.

DVH metrics: D95, D2, V20 for lung, V25 for heart, mean dose to parotids; constraints are checked before approval.

Forward planning uses manual beam weighting and wedges; inverse planning uses dose-volume objectives and iterative optimisation for IMRT/VMAT.

Electron planning selects energy from target depth, typically treating to the 90% isodose with the 80–90% level at the distal margin.

Brachytherapy planning follows the Paris system or TG-43 formalism, with dose specified to a point or volume depending on the technique.

Daily pre-treatment checks: lasers, couch position, isocentre, accessory interlocks, and correct patient identification using two identifiers.

Image guidance: kV orthogonal pairs, CBCT, or MV portal imaging; matching tolerance depends on site, commonly 3 mm for intracranial and 5 mm for pelvis.

Six-degree-of-freedom couches correct rotational error; verify shifts are applied in the correct coordinate convention before treatment.

Portal imaging verifies field shape and position; electronic portal imaging devices also support in vivo dosimetry.

VMAT delivers continuous gantry rotation with dynamic MLC and variable dose rate; delivery time is typically 2–4 minutes per fraction.

Stereotactic treatments require sub-millimetre immobilisation, intrafraction monitoring, and end-to-end verification.

Treatment interruption: record delivered monitor units and elapsed time; for prolonged interruptions, reassess plan validity and document the event.

In vivo dosimetry with diodes or EPID catches gross errors such as wrong energy, wrong accessory, or set-up error beyond tolerance.

Daily linac QA: output constancy within ±3%, laser alignment, door interlock, beam-on indicator, and audiovisual monitoring.

Monthly QA: output calibration, beam symmetry and flatness, MLC leaf position, and mechanical isocentre checks.

Annual QA: full calibration, energy constancy, dosimetry audit with an ion chamber in water, and imaging system performance.

TG-142 and TG-51 are the commonly referenced frameworks for linac QA and reference dosimetry; absorbed dose to water is the reference quantity.

Tolerance for photon output calibration is typically ±1% for the reference condition and ±2–3% for routine constancy.

MLC QA: picket fence test for leaf gap consistency, garden fence or leaf position accuracy within 1 mm for SRS.

CT simulator QA: CT number accuracy within ±5 HU, uniformity, noise, and geometric accuracy within 1 mm.

End-to-end dosimetry audits use anthropomorphic phantoms with TLD or radiochromic film; results within 3–5% are acceptable.

Record and verify systems should be tested for correct transfer of parameters, and independent MU calculation is a second check before delivery.