Introduction An often-recurring problem about small fields dosimetry concerns the reference chamber positioning for profile and percentage depth dose measurements (PDDs). PTW proposes the following solution: a large-area plane-parallel transmission chamber, the PTW T-REF, which doesn’t perturb the beam with an appropriate use. The constructor recommends a minimum distance of 20 cm above the water surface for no perturbations in the field [1] . In this work, we tried different situations to confirm that T-REF doesn’t disturb the beam when it is placed in the fields. Analysis and comparison of percentage depth dose and dose profile curves with and without T-REF will allow us to determine if T-REF doesn’t disturb the beam. Methods The T-REF chamber has a nominal volume of 10.5 cm3, for a 40.8 mm radius. It is composed of lightweight materials (0.1 mm varnish, 0.5 mm PMMA, and 0.02 mm graphite) to limit the beam perturbations. The maximum field size that fits on the chamber area is 57.7 mm × 57.7 mm. Measurements were performed on the TrueBeam STX linear accelerator (Varian), which provide 6 MV and 10 MV photon beams, with or without flattening filter. PDDs and dose profiles were measured in a water phantom (PTW MP3) with MEPHYSTO (PTW), for 3 field sizes (1 cm × 1 cm, 2 cm × 2 cm, 3 cm × 3 cm), without T-REF reference chamber and with T-REF placed for different distances from the water surface (12 cm, 20 cm and 30 cm above the water surface). The detector used for dose measurement is the PTW MicroDiamond 60019. Results No perturbation due to the use of the T-REF in transmission could be seen in our work. The signal-to-noise-ratio is excellent and there are no significant differences between measures with and without T-REF. For PDDs, for a field size of 2 cm × 2 cm for example, the mean relative difference is equal to 0.30 % for T-REF at 12 cm, 0.38% for T-REF at 20 cm, and 0.29% for T-REF at 30 cm, for 6 MV. For 6 MV FFF, the mean relative difference is equal to 0.21% with T-REF at 12 cm, 0.20% for T-REF at 20 cm, and 0.36% for T-REF at 30 cm. For 10 MV, the mean relative difference is equal 0.39% for T-REF at 12 cm, 0.48% for T-REF at 20 cm, 0.50% for T-REF at 30 cm. For 10 MV FFF, the mean relative difference is equal to 0.15% for T-REF at 12 cm, 0.21% for T-REF at 20 cm, and 0.27% for T-REF at 30 cm. For dose profiles, mean relative differences are higher, mainly in high gradient areas (penumbra). For a field size of 2 cm × 2 cm for example, the maximal mean relative difference is equal to 0.75% for T-REF at 12 cm, for 10 MV. Conclusions Different tests allow us to conclude, for large distances to the water surface, the secondary electrons created in the T-REF chamber will be scattered out of the beam and the T-REF presence doesn’t disturb the beam. As recommended by the constructor, 20 cm between T-REF and the water surface is a suitable position for no beam perturbation.