06 Dose
Absorbed dose — TG-51
Water dose from a fully corrected chamber reading and kQ.
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Simulation
Absorbed dose — TG-51 — 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
Dose to water
0.6054Gy
D_w
Dose to water
60.539cGy
Explanation
What it means
AAPM TG-51: dose to water in a high-energy photon or electron beam is the fully corrected chamber reading times the Co-60 absorbed-dose calibration coefficient times the quality conversion k_Q. It replaced TG-21 (air-kerma) in North American clinics. 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
M is raw charge (or reading). Apply k_TP, P_ion, P_pol, k_elec. N_D,w^{Co} from an ADCL. k_Q from %dd(10)x (photons) or R50 (electrons) using TG-51 tables. Result is Gy per reading at the reference point. Change one input and watch the curve and the simulation follow.
Symbols
- MReading12.5 nC
- k_TPTemp/pressure correction1.01
- k_ionRecombination correction1.003
- k_elecElectrometer correction1
- k_polPolarity correction1
- N_D,wCalibration coefficient0.048 Gy/nC
- k_QBeam quality factor0.996
Worked example
A typical case from the default values: M = 12.5 nC (Reading); k_TP = 1.01 (Temp/pressure correction); k_ion = 1.003 (Recombination correction); k_elec = 1 (Electrometer correction); k_pol = 1 (Polarity correction); N_D,w = 0.048 Gy/nC (Calibration coefficient); k_Q = 0.996 (Beam quality factor). Substituting into the relation gives D_w = 0.6054 Gy; D_w = 60.539 cGy. These are teaching numbers — align them with your machine.
Typical values give
- D_w = 0.6054Gy
- D_w = 60.539cGy
Where it comes from
The displayed formula is the working relation. Water dose from a fully corrected chamber reading and kQ. Usual reference: AAPM TG-51. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AAPM TG-51
Assumptions & limits
Reference conditions (10×10, SSD or SAD as specified, depth 10 cm photons). k_Q is chamber-model specific. Does not include gradient / effective-point shifts for electrons (those are extra in TG-51).
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. Reference conditions (10×10, SSD or SAD as specified, depth 10 cm photons). k_Q is chamber-model specific. Does not include gradient / effective-point shifts for electrons (those are extra in TG-51).
Keep this
M is the raw reading. Enter each correction. P_TP = (273.2+T)/295.2 × 101.33/P.
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