Purpose: This study reports the feasibility of the symmetric fields for the usage of asymmetric fields, and checks the accuracy of treatment planning system (TPS).
Materials and methods: A GE Saturne-42 linear accelerator, which produces dual photon energies of 6 and 15 MV, and an ISIS-3D TPS were used in this study. Percent depth doses (PDDs), dose profiles, and isodose distributions for symmetric and asymmetric fields were generated in a water phantom and these data were compared to those calculated by the TPS.
Results: When the results were compared it was seen that the PDDs decreased with increasing off-axis distance and depth. The PDDs comparison demonstrated that for 15 MV, asymmetric and symmetric field PDDs can differ by as much as 4%, and the differences between the calculated by the TPS and the measured PDDs for asymmetric fields were less than 2%. It was also seen that the penumbra of symmetric and asymmetric fields were very close to each other and the difference between the calculated and the measured ones were less than 2 mm. The comparison between the measured and calculated isodose distributions for 6 and 15 MV photon beams for asymmetric field showed that a good agreement has been achieved in general, and the differences between the field edges and near the centre of the fields were found within 2 mm.
Conclusion: This study shows that the measured and calculated data using water phantom and TPS (ISIS-3D), respectively, were in good agreement. Therefore TPS can be used safely for asymmetric fields in routine practice.