06 Dose
Kerma from energy fluence
K = Ψ (μtr/ρ). Collision kerma uses μen/ρ = μtr/ρ (1−g).
Listen
Listen · English
Simulation
Kerma from energy fluence — Change the numbers; the scene follows.
Where it works
Water phantom
Ion chamber
In the water tank under the linac, at the ion chamber — reference dosimetry happens here.
Open this machineFormula
Variables
Results
K
Kerma
0.00015Gy
K_col
Collision kerma
0.00015Gy
K
Kerma
0.15mGy
Explanation
What it means
Kerma is kinetic energy released per unit mass: K = Ψ (μtr/ρ). Collision kerma K_col = Ψ (μen/ρ) = K(1−g) excludes radiative losses (bremsstrahlung). Under CPE, absorbed dose equals collision kerma. This is a working relation in Dosimetry.
Where it is used
Clinically it sits on the Water phantom — Ion chamber. In the water tank under the linac, at the ion chamber — reference dosimetry happens here. Dosimetry is the chamber in water under the linac, or the well counter in the hot lab: TG-51, TRS-398, kerma, and recombination. These numbers are the calibration the rest of the department borrows.
Water phantom · Open this machineHow to use it
Ψ in J/m²; μtr/ρ in cm²/g is converted internally (×0.1 → m²/kg). g is a few 0.001 at Co-60 in water and rises with energy and Z. Change one input and watch the curve and the simulation follow.
Symbols
- ΨEnergy fluence0.05 J/m²
- μtr/ρMass energy-transfer0.03 cm²/g
- gRadiative fraction0.003
Worked example
A typical case from the default values: Ψ = 0.05 J/m² (Energy fluence); μtr/ρ = 0.03 cm²/g (Mass energy-transfer); g = 0.003 (Radiative fraction). Substituting into the relation gives K = 0.00015 Gy; K_col = 0.00015 Gy; K = 0.15 mGy. These are teaching numbers — align them with your machine.
Typical values give
- K = 0.00015Gy
- K_col = 0.00015Gy
- K = 0.15mGy
Where it comes from
The displayed formula is the working relation. K = Ψ (μtr/ρ). Collision kerma uses μen/ρ = μtr/ρ (1−g). Usual reference: Attix / ICRU 85. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Attix / ICRU 85
Assumptions & limits
CPE assumed for D ≈ K_col. At MV in electronic disequilibrium (build-up, lung interfaces) dose ≠ kerma.
Pitfalls
kQ is for that chamber and that beam quality — not a neighbour's value. Polarity and recombination are measured, not copied. A ⁶⁰Co N_D,w is not an MV calibration until kQ is applied. CPE assumed for D ≈ K_col. At MV in electronic disequilibrium (build-up, lung interfaces) dose ≠ kerma.
Keep this
Ψ in J/m²; μtr/ρ in cm²/g = 0.1 m²/kg. Then K in Gy.
In this specialty