Purpose: Dose deposition around unplanned air cavities during MR Guided Radiotherapy (MRgRT) is influenced by the Electron Return Effect (ERE). This is clinically relevant for gas forming close to or inside Organs At Risk (OAR) that lie in the path of a single beam, e.g. intestinal track during pelvic treatment. This work aims to verify Monte Carlo calculations that predict the dosimetric effects of ERE around air cavities. For this we use GafChromic EBT3 film inside poly-methyl methacrylate (PMMA) –air- PMMA phantoms. Method: Four PMMA phantoms were produced. Three of the phantoms contained centrally located spherical air cavities (0.5, 3.5, 7.5 cm diameter), and one phantom contained no air. The phantoms were split to sandwich GafChromic EBT3 film in the centre. The phantoms were irradiated on an Elekta Unity system using a single 10x10 cm2 7 MV photon beam under the influence of a 1.5 T transverse magnetic field. The measurements were replicated using the Elekta Monaco Treatment Planning System (TPS). Gamma analysis with pass criteria 3%/3mm was used to compare the measured and calculated dose distributions. We also consider 3%/2mm, 2%/3mm and 2%/2mm pass criteria for interest.Results: The gamma analysis showed that >95 % of the points agreed between the TPS calculated and measured dose distributions, using 3%/3mm criteria. The phantom containing the largest air cavity had the lowest agreement, with most of the disagreeing points lying inside the air cavity (dose to air region).Conclusion: The dose effects due to ERE around spherical air cavities are being calculated in the TPS with sufficient accuracy for clinical use.