Going the distance: validation of Acuros and AAA at an extended SSD of 400 cm

Accurate dose calculation and treatment delivery is essential for total body irradiation (TBI). In an effort to verify the accuracy of TBI dose calculation at our institution, we evaluated both the Varian Eclipse AAA and Acuros algorithms to predict dose distributions at an extended source-to-surface distance (SSD) of 400 cm. Measurements were compared to calculated values for a 6 MV beam in physical and virtual phantoms at 400 cm SSD using open beams for both 5 × 5 and 40 × 40cm2 field sizes. Inline and crossline profiles were acquired at equivalent depths of 5 cm, 10 cm, and 20 cm. Depth-dose curves were acquired using EBT2 film and an ion chamber for both field sizes. Finally, a RANDO phantom was used to simulate an actual TBI treatment. At this extended SSD, care must be taken using the planning system as there is good relative agreement between measured and calculated profiles for both algorithms, but there are deviations in terms of the absolute dose. Acuros has better agreement than AAA in the penumbra region.