06 Dose
CEMA (converted energy per mass)
C = Φ (S_col/ρ). Charged-particle analogue of kerma; equals dose under δ-ray equilibrium.
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Simulation
CEMA (converted energy per mass) — 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
C
CEMA
3.2044Gy
C
CEMA
320.44cGy
Explanation
What it means
Kerma is for uncharged particles (photons, neutrons): energy transferred to charged particles per mass. CEMA is the same idea for the charged particles themselves: fluence × mass collisional stopping power. Under δ-ray equilibrium, D = C. The distinction matters in a small cavity (Spencer–Attix Δ-cutoff) and at an interface where equilibrium fails. 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 fluence Φ (cm⁻²) and S_col/ρ (MeV·cm²/g). 1 MeV·cm²/g × 1 cm⁻² = 1.602×10⁻¹⁰ Gy. A 1 MeV electron fluence of 10¹⁰ cm⁻² at S/ρ ≈ 2 MeV·cm²/g → C ≈ 3.2 Gy. Change one input and watch the curve and the simulation follow.
Symbols
- ΦFluence1.0000e+10 cm⁻²
- S_col/ρMass collisional stopping power2 MeV·cm²/g
Worked example
A typical case from the default values: Φ = 1.0000e+10 cm⁻² (Fluence); S_col/ρ = 2 MeV·cm²/g (Mass collisional stopping power). Substituting into the relation gives C = 3.2044 Gy; C = 320.44 cGy. These are teaching numbers — align them with your machine.
Typical values give
- C = 3.2044Gy
- C = 320.44cGy
Where it comes from
The displayed formula is the working relation. C = Φ (S_col/ρ). Charged-particle analogue of kerma; equals dose under δ-ray equilibrium. Usual reference: ICRU 85. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: ICRU 85
Assumptions & limits
Unrestricted collisional stopping power (radiative is excluded: that energy leaves as bremsstrahlung, analogous to (1−g) in kerma). Monoenergetic fluence, no angular distribution (use planar fluence × (S/ρ)/μ if you start from a beam).
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. Unrestricted collisional stopping power (radiative is excluded: that energy leaves as bremsstrahlung, analogous to (1−g) in kerma). Monoenergetic fluence, no angular distribution (use planar fluence × (S/ρ)/μ if you start from a beam).
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Unrestricted collisional stopping power (radiative is excluded: that energy leaves as bremsstrahlung, analogous to (1−g) in kerma). Monoenergetic fluence, no angular distribution (use planar fluence × (S/ρ)/μ if you start from a beam).
In this specialty