CMD cheat sheet

Absorbed dose: 1 Gy = 1 J/kg = 100 cGy = 100 rad; 1 cGy = 1 rad.

Equivalent dose: H (Sv) = D (Gy) × wR, where wR = 1 for photons and electrons, ~20 for alpha particles.

Effective dose: E = Σ wT × HT, with wT summing to 1.0 across all tissues (ICRP 103).

Inverse-square law: dose rate falls as 1/d²; doubling SSD from 80 to 160 cm reduces photon fluence to one quarter.

Percent depth dose depends on energy, field size, SSD, and depth; PDD increases with field size and SSD and with lower energy at shallow depths.

Mayneord F factor converts PDD between SSDs: F = ((SSD2 + dmax)/(SSD2 + d))² × ((SSD1 + d)/(SSD1 + dmax))².

Tissue-air ratio (TAR) and tissue-phantom ratio (TPR) are independent of SSD; TMR = TPR at dmax reference depth.

Scatter-air ratio (SAR) is zero for a 0×0 field, so TAR(d, r) = TAR(d, 0) + SAR(d, r).

Backscatter factor (BSF) = Dmax / Dmax(0×0 field); peak scatter factor equals BSF when the chamber is at dmax with full buildup.

Brachytherapy dose rate: D(r) = Λ × (GK/GL) × g(r) × F(r,θ) × S_K, per TG-43 formalism.

Dose-rate constant Λ has units cGy·h⁻¹·U⁻¹, where 1 U = 1 cGy·cm²·h⁻¹ (air kerma strength).

Radial dose function g(r) describes attenuation and scatter along the transverse axis, normalized to g(r0) = 1.

2D anisotropy function F(r,θ) accounts for capsule and source self-attenuation off the transverse axis.

Permanent implant: total dose ≈ initial dose rate × effective half-life / (0.693) when decay is the only dose-limiting factor.

Iodine-125 half-life 59.4 days, average energy ~28 keV; Pd-103 half-life 17 days, average energy ~21 keV.

Ir-192 HDR half-life ~73.8 days, average energy ~380 keV; Co-60 ~5.27 y with 1.17 and 1.33 MeV photons.

Cs-131 half-life ~9.7 days, average energy ~30 keV; used for permanent implants with shorter treatment time than I-125.

Linear accelerator photon output is calibrated so that 1 MU delivers 1 cGy at dmax in water at the calibration geometry (typically 10×10 cm, 100 cm SSD or SAD).

Monitor unit equation (SAD setup): MU = Dose / (PDD × Sc × Sp × inverse-square × tray × wedge × other factors).

Collimator scatter factor Sc is measured in air with a buildup cap; phantom scatter Sp is derived from total scatter factor Scp / Sc.

Square-field equivalent: for rectangular field a×b, side = 2ab/(a+b) (area/perimeter method).

Wedge factor is the ratio of doses with and without the wedge at the same depth and MU; it is depth and field-size dependent.

Electron output increases with applicator size because of increased scatter; surface dose rises with lower energy and larger fields.

Electron practical range R90 ≈ E(MeV)/4 cm in water; therapeutic depth is roughly E/3 to E/4 cm.

Electron PDD falls steeply after R90; at R50 the dose is 50%, and bremsstrahlung tail is typically 2-5% of maximum.

IMRT QA: per TG-218, the gamma passing rate criterion is 95% with 3%/2 mm and a 10% dose threshold, with a 2% action limit on absolute dose.

Gamma index combines dose difference and distance-to-agreement: γ = min √((ΔD/Dcrit)² + (Δr/DTA)²), pass if γ ≤ 1.

VMAT arcs should be verified with the same gantry speed and dose rate as treatment; dynamic QA is required for each arc.

Patient-specific QA measurement devices include 2D detector arrays, 3D diode/ion-chamber arrays, radiochromic film, and log-file analysis.

Setup margins: van Herk formula M = 2.5Σ + 0.7σ gives the margin covering 90% of patients with 95% minimum dose.

Stereotactic treatments require ≤1 mm targeting accuracy, so CBCT or ExacTrac verification and 6-DOF couch correction are standard.

TBI: extended SSD (typically 300-400 cm) with bolus and compensators; midline dose is specified, and lung heterogeneity correction is required.

TSEBT total skin electron: 6-9 MeV at extended SSD with six dual-field positions; surface dose is verified with TLD or film dosimetry.

ALARA: dose to staff and public kept as low as reasonably achievable, with annual limits of 50 mSv (occupational) and 1 mSv (public).

Time, distance, and shielding are the three exposure controls; doubling distance reduces exposure to one quarter of the original.

Half-value layer: transmitted intensity I = I0 × e^(−μx), and HVL = 0.693/μ for a monoenergetic beam.

Primary barrier thickness uses NCRP Report 151 workload W, use factor U, and occupancy T: B = P × d² / (W × U × T).

Secondary barrier combines leakage (0.001 of primary) and patient-scattered radiation contributions.

CTDIvol = (1/nT) ∫ D(z) dz over the scan length; DLP = CTDIvol × scan length (mGy·cm).

Effective dose from CT can be estimated as DLP × k, where k is region-specific (~0.015 mSv/mGy·cm for abdomen).

Personnel dosimeters: TLD badges read monthly; ring badges are worn at the hand for fluoroscopy and brachytherapy procedures.