06 Dose
Polarity correction Ppol
Ppol = |(M+ − M−)| / 2M_used, or the TG-51 form (M+ − M−) / 2M.
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Simulation
Polarity correction Ppol — 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
P_pol
Polarity correction
0.99751
|M+ + M−|
Asymmetry
0.1nC
Explanation
What it means
Collecting charge at +300 V versus −300 V does not give equal magnitudes: Compton current, extra-cameral charge and a true polarity effect in the cavity differ. TG-51 takes the signed difference over twice the reading at the polarity you will use. For a well-behaved Farmer at Co-60, Ppol is 1.000 ± 0.002; for plane-parallel electron chambers it can be a percent and must be measured at the user’s energy. 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 M+ and M− (same sign convention as your electrometer: signed readings) and the M you will use. If your electrometer reports absolute values, enter them as positive M+ and negative M− (or the reverse). Change one input and watch the curve and the simulation follow.
Symbols
- M+Positive-bias reading20.05 nC
- M−Negative-bias reading-19.95 nC
- MReading used20.05 nC
Worked example
A typical case from the default values: M+ = 20.05 nC (Positive-bias reading); M− = -19.95 nC (Negative-bias reading); M = 20.05 nC (Reading used). Substituting into the relation gives P_pol = 0.99751; |M+ + M−| = 0.1 nC. These are teaching numbers — align them with your machine.
Typical values give
- P_pol = 0.99751
- |M+ + M−| = 0.1nC
Where it comes from
The displayed formula is the working relation. Ppol = |(M+ − M−)| / 2M_used, or the TG-51 form (M+ − M−) / 2M. Usual reference: AAPM TG-51. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AAPM TG-51
Assumptions & limits
Does not replace a leakage / extra-cameral check. Measure at the beam quality of interest (Ppol is energy- and dose-rate dependent for some chambers). Take the same MU / time for both polarities.
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. Does not replace a leakage / extra-cameral check. Measure at the beam quality of interest (Ppol is energy- and dose-rate dependent for some chambers). Take the same MU / time for both polarities.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Does not replace a leakage / extra-cameral check. Measure at the beam quality of interest (Ppol is energy- and dose-rate dependent for some chambers). Take the same MU / time for both polarities.
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