Rana S, Rogers K
Arizona Center for Cancer Care, Peoria, AZ.
Med Phys. 2012 Jun;39(6Part18):3827. doi: 10.1118/1.4735618.
To evaluate accuracy of Acuros XB Advanced Dose Calculation Algorithm and Anisotropic Analytical Algorithm (AAA) with measurements in predicting doses beyond air gaps.
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 3×3 and 5×5 cm 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 3×3cm . 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.
通过测量评估Acuros XB高级剂量计算算法和各向异性解析算法（AAA）在预测气隙以外剂量方面的准确性。
在Eclipse治疗计划系统中创建了三个带有分层的虚拟体模。体模中的每一层根据水（顶部）、空气（中间）和水（底部）介质进行设定。对于位于气隙之前的5厘米水等效材料，计算了在2厘米、4厘米和6厘米气隙以外的水（底部介质）中的中心轴深度剂量。使用AAA_10.0.28和Acuros XB_10.0.28，对100厘米源皮距下的6兆伏光子束、3×3厘米和5×5厘米射野尺寸，计算到体模表面的剂量。接下来，制作了固体水和聚苯乙烯泡沫塑料来模拟虚拟体模。通过保持与剂量计算中使用的相同射野、束参数和几何形状，向体模输送100个监测单位，并使用圆柱形电离室在选定深度进行测量。将测量的中心轴深度剂量与根据Acuros XB和AAA计算的中心轴深度剂量进行比较。
对于3×3厘米射野尺寸，Acuros XB预测的剂量在测量剂量的±2%以内，但在有4厘米气隙（-2.4%）和6厘米气隙（-2.8%）的体模中，1厘米深度处除外。在所有测量深度处，Acuros XB的剂量预测都比AAA更好。在最大气隙的体模中，最小的测试射野尺寸在剂量百分比差异方面产生了最高范围（在1至5厘米深度之间）（AAA：-2.9%至-9.9%，Acuros XB：-2.8%至1.5%）。气隙内原射线衰减或侧向散射的建模不当（或两者结合）可能导致了剂量差异。
研究结果表明，当肿瘤位于患者体内小的气腔（不均匀性）以外时，Acuros XB在剂量预测中使用更准确。
