Depth matters: ion recombination variations and EPOM assumptions in high energy photon beam dosimetry

To evaluate the dosimetric impact of depth-dependent ion recombination and empirical effective point of measurement (EPOM) positioning in megavoltage photon beams, with particular focus on flattening filter-free (FFF) beams. Ion recombination correction factors ([Formula: see text]) were characterised as a function of depth and field size for three ionisation chambers (Roos, SNC125, CC13) using the two-voltage method under reference conditions (SSD 100 cm and 10 cm field size) and for additional MLC-defined 5 cm and 2 cm square field sizes, on a point-by-point basis across multiple beam energies. Empirical EPOMs were derived by aligning percentage depth ionisation (PDI) curves to a reference plane-parallel chamber. The dosimetric consequences of using generic [Formula: see text] and EPOM assumptions were assessed, and scan-derived [Formula: see text] values were validated against point dose measurements. A marked depth dependence in [Formula: see text] was observed for all chambers, most notably in FFF beams. The CC13 exhibited the greatest depth-related variation, resulting in recombination-related PDD deviations of up to 1.3% at extended depths. Empirically determined EPOMs were consistently smaller than the conventional 0.6[Formula: see text] shift, with normalised values of 0.42 and 0.38 for SNC125 and CC13, respectively. Using the conventional shift would introduce a residual dose deviation of approximately - 0.5%. The combined influence of uncorrected ion recombination and the generic EPOM produced a PDD bias of 0.8% at 10 cm depth for the 10 FFF beam, which is relevant both for reference dosimetry and for depth-sensitive treatment sites. This study demonstrates that empirical, chamber-specific EPOM and [Formula: see text] correction factors improve dosimetric accuracy and PDD measurements, and consequently, reference dosimetry and TPS beam modelling for which PDD_10 cm is a key parameter. With the increasing adoption of FFF beams, reliance on generic assumptions for [Formula: see text] and EPOM introduces clinically relevant systematic deviations, approaching 1.0% at the calibration depth and becoming larger at greater depths. These corrections should be considered an essential component of linac and chamber commissioning to ensure robust reference dosimetry and accurate beam modelling. Given their measurable impact, such practices warrant inclusion in ACPSEM guidelines, in alignment with emerging best-practice frameworks and the evolving precision requirements of modern radiotherapy.