Evaluation of the AAA Treatment Planning Algorithm for SBRT Lung Treatment: Comparison with Monte Carlo and Homogeneous Pencil Beam Dose Calculations

Purpose: To evaluate the dose calculation accuracy of the Varian Eclipse anisotropic analytical algorithm ( AAA) for stereotactic body radiation therapy (SBRT), and to investigate the dosimetric consequences of not applying tissue heterogeneity correction on complex SBRT lung plans. Materials and Methods: Nine cases of non-small-cell lung cancer (NSCLC) that were previously treated with SBRT at our center were selected for this study. Following Radiation Therapy Oncology Group 0236, the original plans were calculated using pencil beam without heterogeneity correction (PBNC). For this study, these plans were recalculated by applying tissue heterogeneity correction with the AAA algorithm and with the Monte Carlo (MC) method, keeping the number of monitor units the same as the original plans. Two kinds of plan comparison were made. First, the AAA calculations were compared with MC. Second, the treatment plans that were calculated with AAA were compared with the original PBNC calculations. The following dose-volume parameters were used for the comparison: V-100%; V-90%; the maximum, the minimum, and the mean planning target volume (PTV) doses (D-max, D-min, and D-mean, respectively); V-20Gy, V-15Gy, V-10Gy, V-5Gy; Dmean for the lung; and Dmax for the critical organs. Results: Comparable results were obtained for AAA and MC calculations: except for Dmax, Dmin, and Dmean, the differences in the patient-average values of all of the PTV dose parameters were less than 2%. The largest average difference was observed for Dmin (3.8 +/- 5.4%). Average differences in all the lung dose parameters were under 0.2%, and average differences in normal tissue Dmax were under 0.3 Gy, except for the skin dose. There were appreciable differences in the PTV and normal tissue dose-volume parameters when comparing AAA and PBNC calculations. Except for V-100% and V-90%, PBNC calculations on average underestimated the dose to the PTV. The largest discrepancy was in the PTV maximum dose, with a patient-averaged difference of 11.1 +/- 4.6%. Conclusions: Based on our MC investigation, we conclude that the Eclipse AAA algorithm is sufficiently accurate for dose calculations of lung SBRT plans involving small 6-MV photon fields. Our results also demonstrate that, although dose calculations at the periphery of the PTV showed good agreement when comparing PBNC with both AAA and MC calculations, there is a potential to significantly underestimate the dose inside the PTV and doses to critical structures if tissue heterogeneity correction is not applied to lung SBRT plans.