Spectral fluence response and off-axis corrections for ionization chambers and silicon diodes in small photon fields

This study investigates the charged particle fluence response of two reference ionization chambers (PTW T31021 Semiflex 3D and IBA CC04) and two silicon diode detectors (PTW microSilicon and IBA Razor diode) when measuring relative off-axis profiles in medium- to small-sized photon fields. Monte Carlo (MC) simulations are employed to determine the spectral fluence distribution of charged particles at different off-axis positions in water and within the investigated detectors. The goal is to enhance our understanding of the challenges associated with the dosimetry under non-equilibrium conditions and to calculate off-axis correction factors for the detectors under investigation.

Spectral fluence correction factors p_E,glf were determined using a modified version of the EGSnrc user code egs_chamber. A standard linear accelerator model based on the Elekta Precise was used as beam source with a photon beam quality specifier TPR_20,10 of 0.671. Field sizes as small as 0.5 cm ×0.5 cm were evaluated at a water depth of 10 cm.

The analysis of spectral electron fluence in water as a function of off-axis positions across different field sizes revealed notable spectrum variations, particularly beyond the field edge and in the penumbra. These fluctuations directly impact the energy-dependent fluence response of the detectors. The assessment of p_E,glf at the field edge demonstrated that the volume-averaging effect in air-filled ionization chambers is energy-dependent, with high-energy electron fluences being more strongly disturbed than low-energy fluences. In contrast, silicon diodes reproduced the relative profile within a 2% deviation but exhibited sensitivity to low-energy fluence fluctuations, especially in the penumbra.

The findings highlight the importance of selecting appropriate detectors for relative profile measurements and the necessity of applying detector-specific off-axis correction factors to account for variations in spectral electron fluence response.