The effect of asymmetric collimation on the dose distribution and verification of treatment planning system for asymmetric fields.
OBJECTIVE
This study reports the feasibility of the symmetric fields for the usage of asymmetric fields, and checks the accuracy of treatment planning system (TPS).


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.


CONCLUSIONS
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.
Demir
B
B
Department of Radiation Oncology, Cerrahpa?a Medical Faculty, University of Istanbul, Istanbul, Turkey.
Bilge
H
H
Günhan
B
B
Koca
A
A
Karaçam
S
S