Monte Carlo modeling of the field size dependency for thin window plane-parallel chambers in kilovoltage X-ray...

Clinical dosimetry for kV X-ray radiotherapy beams requires calibrated ionization chambers. When the clinical field size differs from the field size used for chamber calibration, in-air measurements result in a different magnitude of scatter from the detector stem or body housing affecting the sensitive volume. This results in a field size dependency based on how much of the chamber body is irradiated, which can be corrected for using the factor P_stem,air. This factor is challenging to characterize, and published data is recommended for this correction.

This study aims to use Monte Carlo (MC) modeled factors as a function of field size and beam quality to establish P_stem,air correction factor values for the PTW 23342 and 23344 chambers (PTW Freiburg GmbH, Freiburg, Germany) and validate these results with measurements.

Calculations of the P_stem,air correction factors were made using the egs_chamber user code of the EGSnrc MC package for the full suite of low energy X-ray beams which Australia's Primary Standards Dosimetry Laboratory (PSDL) use to calibrate these chamber types (20-100 kV, half value layer [HVL] 0.11-6.53 mm Al). The field size diameters ranged from 1 to 12 cm. To validate the modeling, measurements of the field size dependency were conducted on a subset of the Australian PSDL medium energy X-ray beams. The P_stem,air correction values were normalized to the results corresponding to the 5 cm field size diameter.

P_stem,air correction factors are a function of circular field size diameter and HVL. The MC modeled correction factors agreed with the measured factors, for both chamber types, within the calculated ± 1.7% (k = 2) combined uncertainty. The results in this study agreed within uncertainties with the measurements of Austerlitz et al. (2004) and the IAEA TRS398 CoP (2024) for the PTW 23342 chamber in a 100 kV beam and field size diameters from 3 to 5 cm. However, disagreement up to 3.0% was observed when comparing the results of this study with the IAEA TRS398 CoP (2024) measured data at 10 cm relative to 3 cm, for the 100 kV beam. Higher differences may be attributed to differences in the 100 kV beam qualities, and the unknown scatter contribution effect from the physical chamber holder with the measurement data. It is acknowledged that comparisons with the literature at 100 kV were made at beam qualities which exceeded the recommended upper HVL limit of 2.2 mm Al for these chambers. We consider the MC modeled data to be the more accurate characterization of the P_stem,air correction factor, since the results are not affected by additional scatter from a chamber holder. Physical measurements would not be immune from this effect and would be variable between holder designs.

MC modeled P_stem,air correction factor values for in-air measurements of kilovoltage radiotherapy beams using the PTW 23342 and 23344 ionization chambers were determined and validated with experimental measurements. This work provides field size correction factors for these ionization chambers, improving kV X-ray radiation therapy treatment accuracy with more accurate dosimetry.