Polarity effect in commercial ionization chambers used in photon beams with small fields.

Ionization chambers are the instruments of choice for use in photon dosimetry. Ionization chambers together with radiographic films represent the best detectors for measurement of dose distribution for a quality assurance (QA) program in intensity-modulated radiotheraphy (IMRT). The polarity effect was investigated for seven different commercially available ionization chambers. This study concentrated on measuring the dependence of the polarity effect at various energies, and for various field size, ionization chamber, and electrometer combinations. Of the seven chambers, CC01, PTW23323, and PTW31006 had the largest polarity effect for small field sizes. The materials of the central electrode of these three chambers were steel or air-equivalent plastic C-552. The magnitude of the polarity effect was shown to be strongly dependent on the material of the collecting electrode. This polarity effect dependence was observed for the ionization chambers and small field sizes studied.