MO‐E‐BRB‐06: Using Generalized Equivalent Uniform Dose to Evaluate Dose Distributions in NSCLC Patients Planned for Stereotactic Body Radiotherapy

Purpose: To use generalized equivalent uniform dose (gEUD) to evaluate the dose distributions of patients with non‐small‐cell lung cancer (NSCLC) planned for stereotactic body radiotherapy (SBRT). Methods and Materials: Thirty‐eight stage I–II lung cancer patients with 41 lesions treated with SBRT were analyzed. For each patient a Pencil Beam (PB) algorithm‐based treatment plan was produced. Monte Carlo (MC)‐based treatment plans were calculated using the same number of monitor units, for each beam, as in the PB‐based plan. Spearman rank correlation coefficients between gEUD with different parameter a values and dose‐volume endpoints Dx and Vx were computed and compared. The effects of PTV diameter on dose differences between PB‐based plans and MC‐based plans were also analyzed using gEUD. Results: The Spearman rank coefficient as a function of the a value in the gEUD produced functions with unique maxima for the PTV (0.995) and normal lung tissue (0.989). For PB‐based plans, the parameter a correlated best with V5, V10, V20 and D95 for a values of 0.7, 0.9, 2.2 and −24 respectively. For the MC‐based plans, these values of a were 0.8, 1.1, 2.4, and −22, respectively. gEUD differences between MC‐ and PB‐based plans were found to be inversely proportional to the PTV diameter. These differences were found to increase with decreasing values of a, as the gEUD converges to the minimum PTV dose. Conclusion: Spearman rank analysis showed good correlation between gEUD and dose‐volume endpoints as a function of the a parameter in the gEUD model. Further investigation is needed to correlate these a values with biologically meaningful results for tumors and normal tissues in the context of lung SBRT in the clinical setting.