Free study resource
Free Dosimetry Cheat Sheet (PDF)
The quick-reference dosimetry sheet our students keep next to the console: beam parameters, isodose concepts, brachytherapy isotope data, and the calculation formulas that come up on every plan. Here's a preview — the full printable PDF covers it all, plus OAR constraints, prescription tables, and organ tolerance limits.
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Beam parameters
Typical megavoltage buildup depths and depth-dose values for a 10×10 cm field.
| Energy | dmax depth | PDD at 10 cm |
|---|---|---|
| 6 MV | ~1.5 cm | ~67% |
| 10 MV | ~2.5 cm | ~74% |
| 15 MV | ~3.0 cm | ~77% |
| 18 MV | ~3.5 cm | ~80% |
Isodose concepts
- Isodose curves connect points of equal dose; dose builds up to dmax, then falls with depth.
- Beam edges bow outward from divergence and scatter, so penumbra widens with depth.
- A wedge tilts the isodose lines toward its thin end — the thick end absorbs more dose.
- On plan review, the 95% isodose line should cover the PTV and the 105% line should stay inside the target.
- Multi-beam plans stack isodose distributions; hot spots form where beams intersect.
Brachytherapy isotope table
Average energies and half-lives for the sources you see in clinic and on exams.
| Source | Avg energy | Half-life |
|---|---|---|
| Co-60 | 1.25 MeV | 5.27 y |
| Cs-137 | 0.662 MeV | 30.0 y |
| Ir-192 | ~0.38 MeV | 73.8 d |
| I-125 | 28 keV | 59.4 d |
| Pd-103 | 21 keV | 17.0 d |
| Ra-223 | 5.0–7.5 MeV (α) | 11.4 d |
Calculation formulas
A sample of the formulas inside the full sheet.
Inverse square law
I₂ = I₁ · (d₁/d₂)²
Dose rate vs distance from a point source.
Equivalent square
eq = 4 · A / P
Field size for dosimetry-table lookups.
Monitor units (SAD)
MU = D / (K · Sc · Sp · TMR · TF)
MU for an isocentric (SAD) setup.
Monitor units (SSD)
MU = D / (K · Sc · Sp · PDD · TF · f_ISL)
MU for a fixed SSD setup; f_ISL = (SSD/SSD+d)².
Inverse-square correction
f_ISL = (SAD₁ / SAD₂)²
Correct dose-per-MU when source distance changes.
Wedge transmission
TW = D_wedge / D_open
Wedge factor / transmission to apply.
Linear-quadratic survival
S = exp[−(α·D + β·D²)]
Cell survival vs single fraction dose D.
Biologically effective dose
BED = n·d · (1 + d / (α/β))
Biological effect for n fractions of size d.
EQD2
EQD2 = D · (d + α/β) / (2 + α/β)
Convert a regimen to 2-Gy equivalent dose.
Half-life
T½ = ln(2) / λ
Relates decay constant to half-life.
The full study suite pairs this cheat sheet with timed mock exams, adaptive quizzes, and mastery tracking across every dosimetry and planning topic — built by a working therapist and instructor.