(Saturday, 4/4/2020) [Mountain Time (GMT-6)]
Purpose: The dose calculation algorithms in Varian’s Eclipse treatment planning system differ for lung SBRT cases. The AAA algorithm can overestimate dose to small lesions within lung tissue. The AcurosXB (AXB) algorithm demonstrates greater accuracy, but clinical implementation of AXB is challenging due to significant differences in resulting dosimetry between AXB and AAA. Moreover, dosimetric goals for lung SBRT have historically been based on superposition/convolution algorithms such as AAA. This study retrospectively compares AXB and AAA plans across 33 lung SBRT patients to identify factors that contribute to AXB-AAA dose differences, and determine if these factors can predict patients where large dose differences are likely before treatment planning has begun.
Methods: 33 patients were planned with AAA for lung SBRT treatments and the dose was recalculated with AXB using fixed monitor units (MU). The D(95%) coverage difference between AXB and AAA plans was evaluated and compared against the following factors to determine statistical significance: number of fractions, ITV volume, PTV volume, ratio of ITV/PTV volumes, PTV mean density, bilateral lung mean density, MU, and MU/cGy.
Results: The average D(95%) coverage difference between AXB and AAA was 9.9% ± 9.0%. 39% (13/33) of cases showed >10% coverage difference. The largest difference was 34.8%. Statistical significance (p<0.05) was found for three factors: ITV/PTV volume ratio (p=0.0027), PTV mean density (p<0.0001), and bilateral lung mean density (p=0.0033). 11 of the 13 cases with >10% coverage difference had both ITV/PTV ratio <0.3 and PTV mean density <0.31 g/cc.
Conclusion: Using a combined criteria of ITV/PTV volume ratio <0.3 and PTV mean density <0.31 g/cc may predict cases where there will be a significant difference in dose coverage between AAA and AXB calculations. This evaluation can be performed immediately following physician contouring of PTV volumes to help guide treatment planning.
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