06 Dose
Displacement correction Pdis
Effective point of measurement: photons Pdis ≈ 1 − 0.4 r / d? TG-51 uses 0.6 r_cav upstream for electrons; photons use a gradient (Pgr) instead.
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Simulation
Displacement correction Pdis — 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
z_eff
Effective depth
9.82cm
0.6 r
Shift
1.8mm
Explanation
What it means
A cylindrical cavity samples the electron fluence not at its centre but somewhere upstream, because more electrons enter from the higher-fluence side. TG-51 shifts the effective point 0.6 r_cav upstream for electrons (and uses a gradient correction Pgr at dmax for photons instead of a shift). Parallel-plate chambers are assigned Pdis = 1 (reference point at the inner surface of the front window). 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 cavity radius r (cm) and the physical depth of the centre. Read z_eff = z − 0.6 r and the shift in mm. A Farmer r = 0.3 cm → 1.8 mm upstream. Change one input and watch the curve and the simulation follow.
Symbols
- r_cavCavity radius0.3 cm
- zCentre depth10 cm
Worked example
A typical case from the default values: r_cav = 0.3 cm (Cavity radius); z = 10 cm (Centre depth). Substituting into the relation gives z_eff = 9.82 cm; 0.6 r = 1.8 mm. These are teaching numbers — align them with your machine.
Typical values give
- z_eff = 9.82cm
- 0.6 r = 1.8mm
Where it comes from
The displayed formula is the working relation. Effective point of measurement: photons Pdis ≈ 1 − 0.4 r / d? TG-51 uses 0.6 r_cav upstream for electrons; photons use a gradient (Pgr) instead. Usual reference: AAPM TG-51 / IAEA TRS-398. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AAPM TG-51 / IAEA TRS-398
Assumptions & limits
Electron beams, Farmer-like cylinders. Photon TG-51 does not use this shift (it specifies the centre at the reference depth and applies Pgr). TRS-398 uses 0.5 r for photons in some options. Always follow the protocol you are claiming.
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. Electron beams, Farmer-like cylinders. Photon TG-51 does not use this shift (it specifies the centre at the reference depth and applies Pgr). TRS-398 uses 0.5 r for photons in some options. Always follow the protocol you are claiming.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Electron beams, Farmer-like cylinders. Photon TG-51 does not use this shift (it specifies the centre at the reference depth and applies Pgr). TRS-398 uses 0.5 r for photons in some options. Always follow the protocol you are claiming.
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