The contribution of the cable to the polarity effect in ionization chamber dosimetry.

BACKGROUND: The polarity effect refers to the difference in the magnitude of the electrical signal produced by an ionization chamber at opposite bias voltages. There are multiple causes, one of which is the small electrical current produced in parts of the ionization chamber, including the electrical connections, produced directly by radiation. PURPOSE: To show that cable irradiation can be the main source of anomalously large polarity effects in certain cases when small volume chambers are used in large megavoltage radiation fields. To present an alternative to the use of a multiplicative polarity correction. METHODS: The polarity correction for a small volume (0.02 cm3) ionization chamber was measured at 1.5 cm depth in a variety of field sizes in a 36 cm x 32 cm x 30 cm water tank for 6 MV photon and 6 MeV electron beams. Depth-ionization measurements were then made with the 25 cm x 25 cm field. Readings were taken at opposite bias voltages at each depth before moving to the next depth. RESULTS: The polarity effect increased with field-size in both cases. In the photon case the polarity correction increased from 0.5% to 2.0% over the depth range where charged particle equilibrium exists. In the electron case the polarity correction increased to very large values (∼400%) where the signal approached zero. The bias-independent component was relatively constant at all depths. Measurements of the response of the cable without the chamber confirmed that it responds to scattered radiation. CONCLUSIONS: In our measurements the relatively large polarity corrections are due to a small bias-independent contribution which arises in the cable. In these cases, the large polarity correction is a result of the definition of the correction as a relative factor, and not the result of anomalous behaviour in the ionization volume.