ARRT Radiation Therapy cheat sheet

Radiation dermatitis timeline: faint erythema at 10–15 fractions (~20–30 Gy), dry desquamation near 30–40 Gy, moist desquamation typically above 40 Gy once basal cells are depleted.

Alopecia begins 2–3 weeks after the first 2 Gy fraction; hair regrows 2–3 months after treatment ends, but scalp doses above ~45–50 Gy may cause permanent loss.

Oral mucositis appears 10–14 days after the start of radiation, peaks at 2–3 weeks, and resolves 2–4 weeks after completion unless dose exceeds mucosal tolerance.

Fatigue is the most common side effect of radiation therapy; it typically begins within the first 1–2 weeks and may persist 4–6 weeks or longer after treatment ends.

Informed consent must document diagnosis, treatment intent (curative vs palliative), technique, expected benefits, material risks, alternatives including no treatment, and the right to withdraw.

Two patient identifiers (name plus date of birth or medical record number) are verified before every simulation and treatment per National Patient Safety Goal requirements.

Weekly weights are recorded during treatment; loss greater than 5% of pre-treatment weight triggers dietitian referral and consideration of enteral supplementation.

Skin care rule: cleanse with lukewarm water and mild soap, pat dry, and avoid heat, sun, friction, adhesive tape, and deodorant on the treated field unless institutional protocol states otherwise.

NCRP Report 116 annual occupational limits: 50 mSv (5 rem) total effective dose, 150 mSv (15 rem) to the lens of the eye, 500 mSv (50 rem) to skin and extremities.

Public dose limit from a licensed facility is 1 mSv (100 mrem) per year and 0.02 mSv (2 mrem) in any single hour.

Inverse square law: I1/I2 = (d2/d1)²; doubling the distance from a point source reduces exposure to one quarter.

Half-value layer HVL = 0.693/μ, and transmitted intensity follows I = I0e^(−μx); ten half-value layers reduce intensity by a factor of about 1000.

Posting thresholds measured at 30 cm from the source: 'Caution: Radiation Area' above 0.05 mSv/h (5 mrem/h); 'High Radiation Area' above 1 mSv/h (100 mrem/h).

ALARA controls ranked as time, distance, and shielding; distance is the most practical and effective single control for staff exposure.

Linac safety interlocks: door interlock terminates beam on entry, and beam-off/beam-on indicators plus audio-visual alarms are checked daily before patient treatment.

A patient with a permanent brachytherapy implant may be released only if the calculated dose to any other person is unlikely to exceed 5 mSv (500 mrem) and written radiation-safety instructions are provided.

Volume chain: GTV (visible tumor) → CTV (GTV plus microscopic disease) → ITV (CTV plus internal motion) → PTV (CTV or ITV plus setup margin), with organs at risk contoured separately.

Van Herk margin recipe: PTV margin = 2.5Σ + 0.7σ, where Σ is the systematic error SD and σ the random error SD, giving at least 95% CTV coverage in 90% of patients.

QUANTEC central nervous system constraints: spinal cord maximum ≤50 Gy (45 Gy to full cord cross-section), brainstem ≤54 Gy, optic chiasm/nerves ≤50 Gy, lens ≤6–10 Gy.

Parotid constraint: mean dose below 26 Gy, or at least one gland below 20 Gy, reduces the incidence of grade 2 or worse xerostomia.

Thoracic constraints: lung V20 ≤30–35% and mean lung dose ≤20 Gy; heart V25 ≤10% and mean heart dose as low as reasonably achievable in breast planning.

Electron beam rule of thumb: practical range Rp ≈ E/2 cm in water and the 90% depth dose lies near E/3 to E/4 cm, so energy selection is based on target depth.

Equivalent square field size S = 2ab/(a+b); it is used to look up PDD, TMR, Sc, and Sp for rectangular fields.

ICRU-83 reporting: prescribe to the median dose (D50%), report near-maximum D2% and near-minimum D98%, and keep PTV coverage at 95% of prescription in 95% of the volume.

CT simulation practice: 2–3 mm slice thickness, reproducible immobilization, and 4DCT or respiratory gating when target motion exceeds about 1 cm.

TG-142 output constancy tolerances: ±3% daily, ±2% monthly, and ±1% annual relative to the baseline calibration.

Daily mechanical checks: lasers and optical distance indicator within ±2 mm, collimator and gantry angle indicators within ±1 degree.

Winston-Lutz test verifies coincidence of radiation field center and mechanical isocenter to within 1 mm at multiple gantry, collimator, and couch positions for SRS/SBRT programs.

SBRT delivery requires image-guided localization within 1 mm, 6D couch correction before each fraction, and motion management (compression or gating) when excursion exceeds 5 mm.

VMAT delivers dose during continuous gantry rotation by simultaneously modulating multileaf collimator positions, dose rate, and gantry speed across one or more arcs.

Half-beam blocking places the collimated field edge at the machine midline so that adjacent fields (for example cranial and spinal fields) match without divergence overlap at depth.

Total body irradiation uses extended SSD (typically 300–400 cm), AP/PA parallel-opposed fields, 5–10 cGy/min dose rate to limit pulmonary toxicity, and lung compensators; 12 Gy in 6 fractions is a common regimen.

Deep inspiration breath hold for left breast irradiation increases heart-to-chest-wall distance and lowers mean cardiac dose compared with free breathing.

Orthogonal kV or CBCT verification tolerance is typically 2–3 mm for bony anatomy and about 2 mm for soft-tissue targets before treatment.

TG-51 photon reference conditions: 10 cm depth in water, 100 cm SSD or SAD, 10 × 10 cm field, 0.6 cc Farmer chamber; Dw = M·N_Dw·P_ion·P_tp·P_elec·P_pol.

Isocentric MU calculation: MU = prescribed dose ÷ (calibrated output × TMR(d,r) × Sc(rc) × Sp(r) × wedge/tray factor × inverse-square correction).

Converting percent depth dose to TMR: TMR(d,r) = (%DD/100) × ((SSD+d)/(SSD+dmax))² × (Sp(dmax)/Sp(d)).

Gamma index combines dose difference and distance-to-agreement; 3%/3 mm with at least 95% of points passing is the standard IMRT QA threshold, and 2%/2 mm is often used for SBRT.

Isotope data: I-125 half-life 59.4 days with ~28 keV average photons; Pd-103 17 days at ~21 keV; Cs-131 9.7 days; Ir-192 73.8 days at ~380 keV average.

TG-43 brachytherapy formalism: D(r,θ) = Sk·Λ·[G(r,θ)/G(r0,θ0)]·g(r)·F(r,θ), with Λ the dose-rate constant, g the radial function, and F the anisotropy function.

In-vivo diode dosimetry action level is commonly ±5% between measured and calculated dose at the measurement point, prompting investigation when exceeded.

Chamber correction factors applied during calibration: P_ion for recombination, P_tp for temperature and pressure, P_pol for polarity, and P_elec for electrometer calibration.