06 Dose
IAEA TRS-398 absorbed dose
D_w = M N_{D,w} k_Q. The IAEA twin of TG-51; k_Q is tabulated versus TPR20,10.
Listen
Listen · English
Simulation
IAEA TRS-398 absorbed 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_w
Absorbed dose to water
0.993Gy
D_w
Absorbed dose to water
99.3cGy
Explanation
What it means
TRS-398 is the internationally used absorbed-dose-to-water protocol. The chamber carries N_{D,w} at Co-60 from a PSDL/SSDL; you correct the reading M for pT, recombination, polarity, electrometer and apply k_Q for the user’s beam quality. Photon quality is TPR20,10 (not %dd(10)x of TG-51). Numerically D_w agrees with TG-51 to ~0.5–1% when both are done carefully. 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 fully corrected M (nC), N_{D,w} (Gy/nC) and k_Q. A Farmer N_{D,w} ≈ 0.05 Gy/nC, k_Q(6 MV) ≈ 0.993, M = 20 nC → D_w = 0.993 Gy. Change one input and watch the curve and the simulation follow.
Symbols
- MCorrected reading20 nC
- N_{D,w}Calibration coefficient0.05 Gy/nC
- k_QBeam-quality factor0.993
Worked example
A typical case from the default values: M = 20 nC (Corrected reading); N_{D,w} = 0.05 Gy/nC (Calibration coefficient); k_Q = 0.993 (Beam-quality factor). Substituting into the relation gives D_w = 0.993 Gy; D_w = 99.3 cGy. These are teaching numbers — align them with your machine.
Typical values give
- D_w = 0.993Gy
- D_w = 99.3cGy
Where it comes from
The displayed formula is the working relation. D_w = M N_{D,w} k_Q. The IAEA twin of TG-51; k_Q is tabulated versus TPR20,10. Usual reference: IAEA TRS-398. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: IAEA TRS-398
Assumptions & limits
M must already include kTP, ksat (Pion), kpol, kelec. k_Q from the TRS-398 table for your chamber type and TPR20,10 — this calculator does not interpolate the table. Cross-calibrated chambers (electrons) use a different k_Q path.
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. M must already include kTP, ksat (Pion), kpol, kelec. k_Q from the TRS-398 table for your chamber type and TPR20,10 — this calculator does not interpolate the table. Cross-calibrated chambers (electrons) use a different k_Q path.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. M must already include kTP, ksat (Pion), kpol, kelec. k_Q from the TRS-398 table for your chamber type and TPR20,10 — this calculator does not interpolate the table. Cross-calibrated chambers (electrons) use a different k_Q path.
In this specialty