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Dosimetric Evaluation with Heterogeneity in Acuros XB Advanced Dose Calculation Algorithm and Anisotropic Analytical Algorithm (AAA)

S Rana

S Rana*, K Rogers, Arizona Center for Cancer Care, PEORIA, AZ

SU-E-T-529 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

To evaluate accuracy of Acuros XB Advanced Dose Calculation Algorithm and Anisotropic Analytical Algorithm (AAA) with measurements in predicting doses beyond air gaps.

Methods and Materials:
Three virtual phantoms with layers were created in Eclipse Treatment Planning System. Each layer in phantoms was assigned in terms of water (top), air (middle) and water (bottom) medium. Central axis depth dose in water (bottom medium) beyond 2, 4 and 6 cm air gaps were computed for 5 cm water equivalent material positioned before air gaps. Dose computation was performed for 6MV photon beam with 3x3 and 5x5 cm² field sizes at 100 cm SSD to surface of phantoms using AAA_10.0.28 and Acuros XB_10.0.28. Next, solid water and Styrofoam were manufactured to mimic virtual phantoms. By keeping identical field, beam parameters, and geometries that were used for dose computation, 100 MUs were delivered to phantoms, and measurements at selected depths were acquired with cylindrical ionization chamber. Measured central axis depth doses were compared against calculated central axis depth doses computed from Acuros XB and AAA.

Acuros XB predicted doses within ±2% of measured doses except at 1cm depth in phantoms with 4 cm air gap (-2.4%) and 6 cm air gap (-2.8%) for field size 3x3cm². Acuros XB showed better dose prediction compared to AAA at all measured depths. Smallest test field size in phantom with largest air gap produced highest range (between depths 1 and 5 cm) in percentage dose difference (AAA: -2.9% to -9.9% and Acuros XB: -2.8% to 1.5%). Improper modeling of primary beam attenuation or lateral scatter (or combination of both) within air gap may have resulted into dose discrepancies.

The findings suggest Acuros XB is more accurate to use in dose predictions when tumor is located beyond small air cavity (heterogeneity) within the patient.

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