Purpose

This study aimed to investigate the feasibility and accuracy of an analytical anisotropic algorithm calculation of the Varian Eclipse treatment planning system in the TBI treatment planning at an extended source-to-surface distance of 400 cm.

Materials and Methods

The TBI treatment was planned with the Varian Eclipse TPS using the AAA for the dose calculation. Monte Carlo calculations were performed using the EGSnrc code package. Percentage depth dose curves, central axis dose profiles and absolute doses of the 6 MV photon beams from the Varian TrueBeam linear accelerator were measured at an extended SSD of 400 cm in water, solid water and anthropomorphic phantoms.

Results

Calculated and measured PDD curves were in good agreement at the depths of 30.0 cm or less. Calculated and measured central axis dose profiles were clinically acceptable, observing the largest uncertainties close the field edges. Absolute dose measurements in water phantoms showed that the MC and AAA calculations overestimated doses by up to 2.2 % and 1.8 %, respectively. In the anthropomorphic phantom, the difference between measured and calculated mean doses in the whole body volume was 1.0 % and 6.2 % for MC and AAA, respectively. In the AAA calculation, the largest observed difference was 8.3 % in the head.

Conclusions

The Eclipse’s AAA calculation can be safely used for the TBI treatment planning at the extended SSD of 400 cm. However, in order to achieve a higher level of accuracy in the TBI planning, one must carefully validate the TPS at extended SSDs.