The accuracy of two calculation algorithms of the Varian Eclipse treatment planning system (TPS), the electron Monte Carlo algorithm (eMC) and general Gaussian pencil beam algorithm (GGPB) for calculating peripheral dose distribution of electron beams was investigated.

Peripheral dose measurements were carried out for 6, 9, 12, 15, 18 and 22 MeV electron beams using parallel plate ionisation chamber and EBT3 film in the slab phantom. Measurements were performed for 6×6, 10×10 and 25×25 cm2 cone sizes at dmax of each energy up to 20 cm beyond the field edges. The measured and TPS calculated data were compared.

The TPS underestimated the out-of-field doses. The difference between measured and calculated doses increase with the cone size. For ionisation chamber measurement, the largest deviation between calculated and measured doses is <4·29% using the eMC, but can increase up to 8·72% of the distribution using GGPB. For film measurement, the minimum gamma analysis passing rates between measured and calculated dose distributions for all field sizes and energies used in this study were 91·2 and 74·7% for eMC and GGPB, respectively.

The use of GGPB for planning large field treatments with 6 MeV could lead to inaccuracies of clinical significance.