06 Dose
Roentgen-to-dose f-factor
f_med = 0.876 (μen/ρ)_med / (μen/ρ)_air rad/R. In SI, D = X (W/e) (μen/ρ ratio).
Listen
Listen · English
Simulation
Roentgen-to-dose f-factor — 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
f
f-factor
0.961rad/R
D
Medium dose
0.961rad
D
Medium dose
0.00961Gy
Explanation
What it means
The historical f-factor converts exposure in roentgen to absorbed dose in a medium: f = 0.876 (μen/ρ)_med/(μen/ρ)_air rad/R. For muscle near 100–150 keV, f ≈ 0.94; for bone it is much higher at photoelectric energies. 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
Enter X and the two μen/ρ values (XCOM). In SI, D_med = X(C/kg)×(W/e)×ratio, which is what 0.876 rad/R already encodes for air. Change one input and watch the curve and the simulation follow.
Symbols
- XExposure1 R
- (μen/ρ)_medMedium μen/ρ0.0294 cm²/g
- (μen/ρ)_airAir μen/ρ0.0268 cm²/g
Worked example
A typical case from the default values: X = 1 R (Exposure); (μen/ρ)_med = 0.0294 cm²/g (Medium μen/ρ); (μen/ρ)_air = 0.0268 cm²/g (Air μen/ρ). Substituting into the relation gives f = 0.961 rad/R; D = 0.961 rad; D = 0.00961 Gy. These are teaching numbers — align them with your machine.
Typical values give
- f = 0.961rad/R
- D = 0.961rad
- D = 0.00961Gy
Where it comes from
The displayed formula is the working relation. f_med = 0.876 (μen/ρ)_med / (μen/ρ)_air rad/R. In SI, D = X (W/e) (μen/ρ ratio). Usual reference: Attix / Johns & Cunningham. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Attix / Johns & Cunningham
Assumptions & limits
Charged-particle equilibrium in the medium. f is energy-dependent — using a single number across a kV spectrum is an approximation (use spectrum-weighted μen).
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. Charged-particle equilibrium in the medium. f is energy-dependent — using a single number across a kV spectrum is an approximation (use spectrum-weighted μen).
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Charged-particle equilibrium in the medium. f is energy-dependent — using a single number across a kV spectrum is an approximation (use spectrum-weighted μen).
In this specialty