Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry

Large area ionization chambers (LAICs) can be used to measure output factors of narrow beams. Dose area product measurements are proposed as an alternative to central-axis point dose measurements. Using such detectors requires detailed information on the uniformity of the response along the sensitive area. Eight LAICs were investigated in this study: four of type PTW-34070 (LAIC_Thick) and four of type PTW-34080 (LAIC_Thin). Measurements were performed in an x-ray unit using peak voltages of 100-200 kVp and a collimated beam of 3.1 mm (FWHM). The LAICs were moved with a step size of 5 mm to measure the chamber response at lateral positions. To account for beam positions where only a fraction of the beam impinged within the sensitive area of the LAICs, a corrected response was calculated which was the basis to calculate the relative response. The impact of a heterogeneous LAIC response, based on the obtained response maps was henceforth investigated for proton pencil beams and small field photon beams. A pronounced heterogeneity of the responses was observed in the investigated LAICs. The response of LAIC_Thick generally decreased with increasing radius, resulting in a response correction of up to 5%. This correction was more pronounced and more diverse (up to 10%) for LAIC_Thin. Considering a proton pencil beam the systematic offset for reference dosimetry was 2.4-4.1% for LAIC_Thick and  -9.5 to 9.4% for LAIC_Thin. For relative dosimetry (e.g. integral depth-dose curves) systematic response variation by 0.8-1.9% were found. For a decreasing photon field size the systematic offset for absolute dose measurements showed a 2.5-4.5% overestimation of the response for 6  ×  6 mm² field sizes for LAIC_Thick. For LAIC_Thin the response varied even over a range of 20%. This study highlights the need for chamber-dependent response maps when using LAICs for absolute and relative dosimetry with proton pencil beams or small photon beams.