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Investigation of IMRT Out-Of-Field Dose Calculation Accuracy for a Commercial Treatment Planning System

D Dellamonica

Vanderbilt University , Nashville, TN


SU-E-T-219 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Inaccuracies in out-of-field calculations could lead to underestimation of dose to organs-at-risk. This study evaluates the dose calculation accuracy of a model-based calculation algorithm at points outside the primary treatment field for an intensity modulated radiation therapy (IMRT) plan using experimental measurements.

The treatment planning system investigated is Varian Eclipse V.10 with Analytical Anisotropic Algorithm (AAA). The IMRT fields investigated are from real patient treatment plans. The doses from a dynamic (DMLC) IMRT brain plan were calculated and compared with measured doses at locations outside the primary treatment fields. Measurements were performed with a MatriXX system (2-D chamber array) placed in solid water. All fields were set vertically incident on the phantom and were 9 cm x 6 cm or smaller. The dose was normalized to the central axis for points up to 15 cm off isocenter. The comparisons were performed at depths of 2, 10, 15, and 20 cm

The measurements have shown that AAA calculations underestimate doses at points outside the primary treatment field. The underestimation occurs at 2 cm depth and decreases down to a factor of 2 as depth increases to 20 cm. In low dose (<2% of target dose) regions outside the primary fields the local dose underestimations can be >200% compared to measured doses. Relative to the plan target dose, the measured doses to points outside the field were less than 1% at shallow depths and less than 2% at greater depths.

Compared to measurements, the AAA algorithm underestimated the dose at points outside the treatment field with the greatest differences observed at shallow depths. Despite large local dose uncertainties predicted by the treatment planning system, the impact of these uncertainties is expected to be insignificant as doses at these points were less than 1-2% of the prescribed treatment dose.

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