A framework was developed to assess the impact of respiratory motion on the dose delivered during radiotherapy using a previously validated 4D Monte Carlo-based dose reconstruction tool, 4Ddefdosxyznrc. As a surrogate for tumour motion, abdominal surface motion measurements were recorded during patient treatments using the RADPOS system. Motion traces, treatment log files and deformation vectors representing each patient's respiratory motion were used as inputs to the 4D dose reconstructions. Motion measurements were performed for 3 patients undergoing radiation therapy for non-small cell lung cancer, totalling 12 fractions. No statistically significant inter-fractional differences in 4D reconstructed dose metrics were found. A maximum difference of 2.0% in the GTV D98% was found between 4D calculations and the treatment planning system. This result is consistent with small abdominal displacements of 5.5 ± 1.4 mm, 5.4 ± 0.4 mm, and 0.40 ± 0.03 mm, respectively, observed for these 3 patients during treatments.