This study presents a comparison of peripheral doses (PDs) measured using an ionisation chamber with treatment planning system (TPS) data and a Monte Carlo (MC) simulation of a 6-MV photon beam. The ion chamber measurements and MC simulation produced similar results for all out-of-field distances and field sizes considered in this study. For the 0° and 90° collimation angles, the average local per cent dose differences between the MC and TPS calculations were 2.7 % (range: -2.4, +22.6) and -1.7 % (range: -12.2, +10.8), respectively. The corresponding differences between the MC calculations and the ion chamber measurements were 2.2 % (range: -2.4, 24.7) and -1.8 % (range: -17, 15.2) for all field sizes and depths, respectively. Whereas the PDs increased with field sizes, the variations with depth were negligible at large distances. The TPS calculations usually yielded higher PDs than ion chamber measurements at distances close to the field edge. In contrast, at the farther distances, the TPS results indicated lower doses than both the ion chamber and the MC data. TPS data are not sufficient for use in calculating the out-of-field doses. These results can be used to estimate non-target organ doses to patients.