PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s):Stereotactic Body Radiation Therapy (SBRT) is an established technique used to treat early-stage Non-Small Cell Lung Cancer (NSCLC). Currently, photon treatment plans are calculated using convolution-superposition algorithms (CSAs) such as the Anisotropic Analytical Algorithm (AAA). The latest generation of algorithms look to solve the Linear Boltzmann Transport Equation (LBTE) in one of two ways: analytically or indirectly through statistics (Monte Carlo). CSAs are more commonly used but suffer from inaccuracies in heterogeneous media. Acuros External Beam (AXB) directly accounts for heterogeneities by solving the LBTE, providing a more accurate dose calculation than AAA. We evaluated tissue dose differences between the AAA and AXB generated plans.
Materials/Methods:We identified 15 cases of Stage I-II NSCLC treated with curative intent SBRT at our institution from 2015-2016 who were planned in AAA to receive 50 Gy in 5 fractions using a treatment planning system. The patients’ delivered plans were retrospectively recalculated using AXB. The dose volume histogram was generated for structures commonly contoured in lung SBRT based on RTOG 0813; structures included gross tumor volume (GTV) and planning target volume (PTV) and the organs at risk (OARS), specifically the spinal cord, skin, heart, great vessels, chest wall, brachial plexus, total lung, and bronchi. Both peripheral and central tumors were included. Dose to 95% of the PTV volume was the primary endpoint. The Wilcoxon signed-rank test tested variance between the dose distribution generated by AXB and by AAA.
Results:PTV 95% dose differed significantly between AAA and AXB generated plans (p=0.0027).The mean, min, and max PTV 95% dose generated by AAA were 4966 cGy (SD=77.260 cGy), 4722 cGy, and 5002 cGy, respectively, whereas, the AXB generated plan yielded values of 4890 cGy (STD=118.4 cGy), 4589 cGy, and 5065 cGy, respectively. The mean GTV dose generated by AAA was higher overall (mean=5619 cGy) when compared to GTV dose calculated by AXB (mean = 5360 cGy); however, this difference was not significant in our cohort. On average, the PTV 95% dose was 70.60 cGy (SD =91.68) higher for AAA plans compared to plans re-calculated with AXB. The D0.03 cc for the esophagus, spinal cord, skin, heart, great vessels, and bronchi were on average 13.57 cGy (p=0.017), 7.293 cGy (p=0.0004), 5.488 cGy [NS], and 10.854 cGy [NS] lower when re-calculated with AXB, respectively. The D0.03 cc for the chest wall and brachial plexus were on average 46.51 cGy (p=0.04) and 8.043 cGy [NS] higher when re-calculated with AXB.
Conclusion:We found a statistically difference in dose distribution for the PTV 95% dose for AAA plans re-calculated with AXB. Overall, AXB re-calculation of the PTV 95% dose resulted in a lower dose distribution to the target when compared to the original plan, which was calculated in AAA. Given that AXB has shown to more accurately calculate dose-distribution. Our data suggests that AAA model overestimates the PTV 95% dose in lung SBRT.
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