Purpose It has been proposed that beam modulation and opposing beam configurations can cancel effects of the Electron Return Effect (ERE) during MR‐guided Radiotherapy (MRgRT). However this may not always be the case for unplanned gas cavities outside of the target in the pelvic region. We evaluate dosimetric effects, including effects in the rectal wall, due to unplanned spherical air cavities during MRgRT.MethodsNine virtual cuboid water phantoms containing spherical air cavities (0.5 to 7.5 cm diameter) and a reference phantom without air were created. Monte Carlo dose calculations of 7 MV photons under the influence of a 1.5 T transverse magnetic field were produced using Monaco 5.19.02 Treatment Planning System (TPS) (Elekta AB, Stockholm, Sweden). Cavities in the path of a single and multiple beam plans were considered.Dose distributions of phantoms with and without air cavities were compared (ΔD%) using a spherical coordinate system originating in the centre of the cavity. Effects in the rectal wall were quantified by comparing Dose Volume Histogram (DVH) parameters for solid and gaseous filling from simulated rectal wall structures.ResultsMax(ΔD%) of ~70% and 20% were observed around large cavities in the path of a single and multiple beam plans respectively. ~45 cm3 of phantom surrounding the largest cavity in a single beam received dose changes of >10%. Dmean in the rectal wall was unchanged when comparing gaseous and solid filling in the path of a single beam; however D1cc and Dmax increased by up to ~45% and ~63% respectively.ConclusionUnplanned gas cavities in the path of a single beam during pelvic MRgRT with a 1.5 T transverse magnetic field cause dose changes which may impact toxicity in the rectal wall, depending on local dose and fractionation. Effects are reduced but not eliminated with a five‐beam plan.