06 Dose
Exposure to air dose
D_air = X (W/e) = 0.00876 Gy/R under charged-particle equilibrium.
Listen
Listen · English
Simulation
Exposure to air dose — 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
D_air
Air dose
0.00876Gy
D_air
Air dose
8.7643mGy
Explanation
What it means
Under CPE the energy spent creating ion pairs in air is exactly the absorbed dose in air: multiply exposure (C/kg) by W/e. 1 R = 2.58×10⁻⁴ C/kg × 33.97 J/C = 8.76 mGy. Collision kerma equals this dose under CPE; without CPE (build-up region, MV in air without a cap) kerma and dose part company. 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 exposure in R. Read D_air in Gy and mGy, and the equivalent charge per kg. Same numbers as the exposure calculator in Foundations, grouped here with the other kerma/dose tools. 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 D_air = 0.00876 Gy; D_air = 8.7643 mGy. These are teaching numbers — align them with your machine.
Typical values give
- D_air = 0.00876Gy
- D_air = 8.7643mGy
Where it comes from
The displayed formula is the working relation. D_air = X (W/e) = 0.00876 Gy/R under charged-particle equilibrium. Usual reference: ICRU 85 / TG-51. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: ICRU 85 / TG-51
Assumptions & limits
Dry air, Co-60 / diagnostic quality W/e. Humidity (~0.3%) and recombination are separate correction factors. Not a tissue dose — apply the f-factor or μen ratio next.
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, Co-60 / diagnostic quality W/e. Humidity (~0.3%) and recombination are separate correction factors. Not a tissue dose — apply the f-factor or μen ratio next.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Dry air, Co-60 / diagnostic quality W/e. Humidity (~0.3%) and recombination are separate correction factors. Not a tissue dose — apply the f-factor or μen ratio next.
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