00 Foundations
Dose from photon fluence
D = Φ E (μen/ρ) with the MeV-to-joule conversion.
Listen
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Simulation
Dose from photon fluence — Change the numbers; the scene follows.
Where it works
Linear accelerator
Isocenter
At isocenter, on the central axis through the patient (or a phantom in the same place).
Open this machineFormula
Variables
Results
D
Dose / collision kerma
0.04967Gy
D
Dose
49.6675mGy
Explanation
What it means
Collision kerma and absorbed dose (under CPE) equal energy fluence times μen/ρ. Writing energy fluence as ΦE and converting MeV/g to J/kg gives D(Gy) = Φ E (μen/ρ) × 1.602×10⁻¹⁰. This is the bridge from Monte Carlo fluence tallies to gray. This is a working relation in Radiation physics.
Where it is used
Clinically it sits on the Linear accelerator — Isocenter. At isocenter, on the central axis through the patient (or a phantom in the same place). Radiation physics lives at the x-ray target, the linac head, and inside the patient: how a photon is born, how it scatters, and how it dies. Use these relations before you trust a spectrum, a wall, or a kV-versus-MV contrast argument.
Linear accelerator · Open this machineHow to use it
Enter fluence, photon energy, and μen/ρ (XCOM). Typical water μen/ρ: ~0.03 cm²/g at 100 keV, ~0.03 at 1 MeV (slowly varying in the Compton plateau). Change one input and watch the curve and the simulation follow.
Symbols
- ΦFluence1.0000e+10 cm⁻²
- EPhoton energy1 MeV
- μen/ρMass energy absorption0.031 cm²/g
Worked example
A typical case from the default values: Φ = 1.0000e+10 cm⁻² (Fluence); E = 1 MeV (Photon energy); μen/ρ = 0.031 cm²/g (Mass energy absorption). Substituting into the relation gives D = 0.04967 Gy; D = 49.6675 mGy. These are teaching numbers — align them with your machine.
Typical values give
- D = 0.04967Gy
- D = 49.6675mGy
Where it comes from
The displayed formula is the working relation. D = Φ E (μen/ρ) with the MeV-to-joule conversion. Usual reference: Attix / ICRU. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Attix / ICRU
Assumptions & limits
Charged-particle equilibrium assumed so D ≈ K_col. Secondary electron escape, radiative fraction (g), and polyenergetic spectra need spectrum-weighted μen/ρ.
Pitfalls
Do not mix free-electron Compton kinematics with photoelectric-dominated kV imaging. Check keV versus MeV, and never treat a spectrum as one photon. Charged-particle equilibrium assumed so D ≈ K_col. Secondary electron escape, radiative fraction (g), and polyenergetic spectra need spectrum-weighted μen/ρ.
Keep this
Φ in cm⁻², E in MeV, μen/ρ in cm²/g. Factor 1.602×10⁻¹⁰ converts MeV/g to Gy.
In this specialty