Comparison of the Characteristics of Two Types of Parallel-plate Ionization Chamber Under Small-field Electron Irradiation.

Abstract

This study compared two types of parallel-plate ionization chamber to clarify the pitfalls of dosimetry in electron radiation therapy. The ion recombination correction factor and polarity effect correction factor, sensitivity, and percentage depth doses (PDDs) of PPC05 and PPC40 parallel-plate ionization chambers were compared in a small-field electron beam. The output ratios were measured for 4-20 MeV electron beams with field sizes of 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm. Furthermore, the films were placed in water and positioned in the beam with their surface perpendicular to the beam axis, and lateral profiles were obtained for each beam energy and each field. Regarding PDDs, at depths greater than the peak dose, the percentage depth dose for PPC40 was smaller than that for PPC05 in small fields and at beam energies greater than 12 MeV, which could be attributed to the lack of lateral electron equilibrium at small depths and multiple scattering events at large depths. The output ratio of PPC40 was approximately 0.025-0.038, which was lower than that of PPC05 in a 4 cm × 4 cm field. For large fields, the lateral profiles were similar, regardless of the beam energy, however, for small fields, the flatness of the lateral profile was beam energy dependent. The PPC05 chamber, which has a smaller ionization volume, is therefore more suitable than the PPC40 chamber for small-field electron dosimetry, in particular at high beam energies.