06 Dose
Exposure to dose in air
D_air = X · (W/e). 1 R ≈ 8.76 mGy in air.
Listen
Listen · English
Simulation
Exposure to dose in air — 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
X
Exposure
0.000258C/kg
D_air
Dose in air
0.00876Gy
D_air
Dose in air
8.764mGy
Explanation
What it means
Exposure X (ionisation in air) converts to dose in air by W/e = 33.97 J/C. With 1 R = 2.58×10⁻⁴ C/kg this is 8.76 mGy/R in air. The f-factor then takes you from air to tissue. 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 roentgen. Pair with the f-factor calculator for muscle or bone. Modern kV dosimetry prefers air kerma (Gy) over R. Change one input and watch the curve and the simulation follow.
Symbols
- XExposure1 R
Worked example
A typical case from the default values: X = 1 R (Exposure). Substituting into the relation gives X = 0.000258 C/kg; D_air = 0.00876 Gy; D_air = 8.764 mGy. These are teaching numbers — align them with your machine.
Typical values give
- X = 0.000258C/kg
- D_air = 0.00876Gy
- D_air = 8.764mGy
Where it comes from
The displayed formula is the working relation. D_air = X · (W/e). 1 R ≈ 8.76 mGy in air. Usual reference: Attix. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Attix
Assumptions & limits
Dry air, W/e at calibration quality. Does not include backscatter (that is BSF on ESD) or medium absorption.
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. Dry air, W/e at calibration quality. Does not include backscatter (that is BSF on ESD) or medium absorption.
Keep this
W/e = 33.97 J/C. 1 R = 2.58×10⁻⁴ C/kg.
In this specialty