Correction for influence quantities

This section summarizes the procedures to correct the raw ionization chamber reading raw f^msrmsr

MQ for influence quantities to obtain MQ^fmsr_msr using air filled reference ionization chambers¹¹.

5.4.1. Air density correction

All ionization chambers recommended for reference dosimetry in this COP are open to ambient air. The mass of air in the cavity will thus depend on atmospheric conditions (temperature and pressure). The factor k_TP to correct for these conditions is given by:

( ) cavity of the ionization chamber, and T₀ and P₀ are the reference conditions for temperature and pressure for which the calibration coefficient of the ionization chamber is valid, i.e. 20°C (or 22°C for calibrations from standards laboratories in North America) and 101.325 kPa, respectively.

5.4.2. Humidity

No correction is necessary for relative humidity if the ionization chamber is used in a range of 20% to 80% relative humidity and has a calibration coefficient valid at a relative humidity of 50%. In the unlikely case that the relative humidity is outside the range of 20–80%, a correction factor is needed [1, 159].

5.4.3. Electrometer calibration factor k_elec

When the ionization chamber and electrometer are calibrated separately, the calibration coefficient for the ionization chamber is given in units Gy/C or

11 More background and details on these corrections can be found in Refs [1, 2, 7].

a multiple (e.g. mGy/nC or cGy/nC). The calibration factor k_elec obtained for the electrometer converts the electrometer reading to charge and is expressed in units C/rdg. If the reading of the electrometer is in terms of charge, the electrometer calibration factor is dimensionless. If the ionization chamber and the electrometer are calibrated together, as one measurement assembly, no separate electrometer calibration factor has to be applied.

5.4.4. Polarity correction

The correction factor for polarity in a given radiation beam is given by:

pol=

where M₊ and M₋ are the electrometer readings obtained at positive and negative polarity, respectively and M is the electrometer reading taken at the polarity used routinely. The polarity used routinely is the same as that used during the calibration of the ionization chamber. For details on the situation where the standards laboratory has not applied this correction during calibration, refer to Ref. [1]. Given the observations discussed in Section 4, it is advised that attention be paid to long stabilization times that may be required for small volume ionization chambers. Polarity effects may also be field size dependent owing to the varying portion of the stem being irradiated, hence it is important that this effect be investigated for every ionization chamber used for small field dosimetry.

5.4.5. Recombination correction

The incomplete collection of charge in an ionization chamber cavity owing to the recombination of ions requires the use of a correction factor k_s. Two separate effects take place: (i) the recombination of ions formed by separate ionizing particle tracks, termed general (or volume) recombination, which depends on the density of ionizing particles and therefore on the dose rate; and (ii) the recombination of ions formed by a single ionizing particle track, referred to as initial recombination, which is independent of the dose rate. Both effects depend on the chamber geometry and on the applied polarizing voltage. In conventional radiotherapy beams, initial recombination is generally less than 0.2%.

In continuous radiation, i.e. ⁶⁰Co gamma rays, the two voltage method may be used and a correction factor derived using the relation:

2 M₁ being the ionization chamber reading at the normal operating voltage V₁ and V₂ being a lower voltage. This relation is based on a linear dependence of 1/M on 1/V ², which describes the effect of general recombination in continuous beams.

For clinical purposes, general recombination can be considered negligible in

60Co beams.

For pulsed beams, the recombination correction factor k_s is derived using the two voltage method [160]. This method assumes a linear dependence of 1/M on 1/V (it is advised that this assumption be verified when commissioning a new chamber) and uses the measured values of the collected charges M₁ and M₂ at the polarizing voltages V₁ and V₂, respectively, measured using the same irradiation conditions. V₁ is the normal operating voltage and V₂ a lower voltage; the ratio V₁/V₂ is ideally equal to or larger than 3. The polarity effect will change with the voltage, and M₁ and M₂ are each corrected for this effect using Eq. (37). The recombination correction factor k_s at the normal operating voltage V₁ is obtained from:

where the constants a_i are given in Table 21 for pulsed radiation [161].

For k_s < 1.03, the correction can be approximated to within 0.1% using the relation:

Note that the correction factor k_s evaluated using the two voltage method in pulsed beams corrects for both general and initial recombination. In pulsed beams, where general recombination is dominant, the recombination correction for a given chamber will scale approximately linearly with dose rate.

TABLE 21. QUADRATIC FIT COEFFICIENTS, FOR THE CALCULATION OF k_s BY THE ‘TWO VOLTAGE’

TECHNIQUE IN PULSED RADIATION, AS A FUNCTION OF THE VOLTAGE RATIO V₁/V₂ [161]

V₁/V₂ a₀ a₁ a₂

2.0 2.337 −3.636 2.299

2.5 1.474 −1.587 1.114

3.0 1.198 −0.875 0.677

3.5 1.080 −0.542 0.463

4.0 1.022 −0.363 0.341

5.0 0.975 −0.188 0.214

If it is not known if the relation between 1/M and 1/V is linear, or if there is any doubt about this, it is advised that a Jaffé plot of 1/M versus 1/V be measured.

This is especially the case for some small volume ionization chambers in which charge recombination effects may distort the saturation curve. Small volume chambers may also exhibit asymmetric saturation curves for opposing polarities (essentially a voltage dependent polarity effect). Given the observations discussed in Section 4, it is advised that attention be paid to the long stabilization times that may be required for small volume ionization chambers. For FFF beams, where dose per pulse values are substantially larger than in WFF beams, studies have shown that recombination can be treated in the same way and that the two voltage technique is accurate under the same conditions as for WFF beams [162–165].