Estimation of the cable effect in megavoltage photon beam by measurement and Monte Carlo simulation

Ionization chambers are widely used for dosimetry with megavoltage photon beams. Several properties of ionization chambers, including the cable effect, polarity effect, and ion recombination loss, are described in standard dosimetry protocols. The cable effect is categorized as the leakage current and Compton current, and careful consideration of these factors has been described not only in reference dosimetry but also in large fields. However, the mechanism of Compton current in the cable has not been investigated thoroughly. The cable effect of ionization chambers in 6 MV X-ray beam was evaluated by measurement, and the mechanism of Compton current was investigated by Monte Carlo simulation.

Four PTW ionization chambers (TM30013, TM31010, TM31014, and TM31016) with the same type of mounted cable, but different ionization volumes, were used to measure output factor (OPF) and cable effect measurement. The OPF was measured to observe any variation resulting from the cable effect. The cable effect was evaluated separately for the leakage current and Compton current, and its charge per absorbed dose to water per cable length was estimated by a newly proposed method. The behavior of electrons and positrons in the core wire was analyzed and the Compton current for the photon beam was estimated by Monte Carlo simulation.

In OPF measurement, the difference in the electrometer readings by polarity became obvious for the mini- or microchamber and its difference tended to be larger for a chamber with a smaller ionization volume. For the cable effect measurement, it was determined that the contribution of the leakage current to the cable effect was ignorable, while the Compton current was dominant. The charge due to the Compton current per absorbed dose to water per cable length was estimated to be 0.36 ± 0.03 pC Gy^-1  cm^-1 for PTW ionization chambers. As a result, the contribution of the Compton current to the electrometer readings was estimated to be 0.002% cm^-1 for the Farmer-type, 0.011% cm^-1 for the scanning, and 0.088% cm^-1 for microchambers, respectively. By the simulation, it was determined that the Compton current for MV x-ray could be explained by not only recoil electrons due to Compton scattering but also positron due to pair production. The Compton current estimated by the difference in outflowing and inflowing charge was 0.45 pC Gy^-1  cm^-1 and was comparable with the measured value.

The cable effect, which includes the leakage current and Compton current, was quantitatively estimated for several chambers from measurements, and the mechanism of Compton current was investigated by Monte Carlo simulation. It was determined that the Compton current is a dominant component of the cable effect and its charge is consistently positive and nearly the same, irrespective of the ionization chamber volume. The contribution of Compton current to the electrometer readings was estimated for chambers. The mechanism of Compton current was analyzed and it was confirmed that the Compton current can be estimated from the difference in outflowing and inflowing charge to and from the core wire.