Dosimetric study reveals field- and filter-dependent variations in 6 MV photon tissue-maximum ratios, indicating that beam ratios require dedicated modeling outside reference geometries.
Accurate characterization of the tissue–maximum ratio (TMR) is important for beam modelling and dosimetric verification, particularly for flattening-filter-free (FFF) beams and non-reference fields. This study characterized TMR for 6 MV flattened (FF) and FFF photon beams from an Elekta Versa HD across square fields from to . Central-axis water-phantom measurements using PTW microDiamond and Semiflex detectors were compared with GATE/Geant4 Monte Carlo simulations. For the reference field, simulated percentage depth-dose values agreed with measurements within 1 percentage point between 10 and 30 cm depth. TMR increased systematically with field size beyond the buildup region. At 10 cm depth, measured TMR increased from 0.6887 to 0.8215 for FF and from 0.6829 to 0.8130 for FFF. At 20 cm, the corresponding ranges were 0.4432-0.6016 and 0.4366-0.5830. Monte Carlo-measurement differences remained within 0.011 TMR units. The ratio also increased with field size, confirming that it is not field-size invariant outside reference geometry.
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mouden et al. (2026) studied this question.
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