The operational voltages of ionisation chambers for radiotherapy are usually in the range 100 V to 400 V. However, there is little guidance about the best voltage for each chamber type.
Manufacturers usually indicate a range of values, and there is a tendency for users to adopt the highest voltage in this range, using the logic that recombination effects will be smaller. However, in some cases the higher voltage may result in charge multiplication in the gas, and so care should be exercised when choosing the voltage.
We recommend that the user should choose the highest voltage subject to the following conditions:
- the voltage is lower than the rating of the chamber and all HV connections, as specified by the manufacturer(s)
- the voltage can be supplied by all electrometers used with the chamber
- leakage from the chamber is small (less than 0.1% of the signal, and in absolute terms less than about 10 fA)
- ion collection is within the 'ion chamber' range (charge multiplication in the gas is negligible)
- the magnitude of the recombination correction is less than 2%.
Condition (iv) and (v) can be tested by using a Jaffé plot (a plot of 1/Q versus 1/V for voltages up to the operational voltage). To produce a Jaffé plot, measure the charge Q for at least ten voltages over the range V/6 to V. Plot 1/Q versus 1/V. This graph should have a linear region for the lower voltage range (where recombination is adequately described by Boag’s theory) and slowly become non‑linear for high voltages (that is, for small values of 1/V). If the operating voltage is in this non‑linear region, it means that there are other effects going on inside the ion chamber (charge multiplication in the gas, or higher order recombination effects) which are not modelled by Boag’s theory. Use of the two-voltage method for estimating the recombination in such cases will be less accurate. To get an idea of the uncertainty, compare the recombination correction obtained by extrapolating the linear part of the curve to 1/V = 0, with that obtained using the two voltage method at the highest voltage. If they differ significantly, choose a lower voltage.
Note that there is a subjective element to this choice. By choosing a high voltage the magnitude of the recombination correction is reduced, but the uncertainty in its determination is increased.
See reference below for more information.
Polarity
Note that there is no standard way to express the polarity of the voltage and different manufacturers use different conventions. For example, to set 250 V with the central electrode positive, Table 1 indicates the displayed voltage for different models.
For plane-parallel chambers, we refer to the central electrode as the one connected to the central wire of the triaxial cable.
| Manufacturer | Model | Rear panel | Electrometer display |
|---|---|---|---|
| PTW | UNIDOS Webline | No switch | “-250 V” |
| PTW | UNIDOS E | “—” | “-250 V” |
| IBA | DOSE 1 | No switch | “250 V CEP” |
| Standard Imaging | MAX-4001 | No switch | “100% 250Vdc” |
| Keithley/Fluke | 35040 | No switch | “250 V CEP” |
| NE | 2570 | “—(NORMAL)” and “V (NORMAL)” | No display |
| NE | 2670 | TYPE A, switch in left position | “-250 V” |
Suggested voltages
We suggest that clinics use highest voltage recommended by the manufacturer (to minimise recombination corrections). This is usually around 400 V for Farmer-type chambers and 200 V for Roos chambers. In most cases there is a historical precedent and the clinic should weigh the benefits/complexities of changing the chamber voltage. If there are no other influencing factors, we suggest using the polarity that results in positive charge/current. When a chamber is sent to ARPANSA for calibration, ARPANSA will calibrate it at the nominated bias voltage and polarity.
References
[1] Ion recombination correction factors (Pion) for Varian TrueBeam high-dose-rate therapy beams, Stephen F. Kry, Richard Popple, Andrea Molineu, David S. Followill, Journal of Applied Clinical Medical Physics, Vol. 13 , No. 6 , 2012.
