SU‐E‐T‐303: Radiobiological Evaluation of the Impact of Lung Density Variation in the Effectiveness of IMRT Treatment Plans

Purpose: To radiobiologically quantify the differences in the clinical effectiveness of lung cancer IMRT plans, due to lung heterogeneity corrections. Methods: In this study, three patients were selected for each of which seven IMRT plans were generated. The first plan was produced accounting for the heterogeneity correction and it was optimized using the same dose prescription. This plan was also used as a reference for the default number of monitor units per beam. Three plans were produced with the lung densities forced to 0.25, 0.5 and 1.0, respectively while keeping the same number of monitor units per beam. Finally, three additional plans were produced using the same lung densities and reoptimizing the plans to achieve the same target coverage. The uniform dose that causes the same tumor control probability or normal tissue complication rate as the actual dose given to the patient was calculated using the biologically effective uniform dose, BEUD. Results: The treatment plans with the forced lung density and with the same number of monitor units as the heterogeneous plans had different target coverage values. The plans with the lung densities of 0.25 and 1.0 had the most and least comparable coverage to the reference plan. From the radiobiological assessment, the percentage differences of the complication‐free tumor control rate (P+) were largest for the lung density of 1.0 (up to 18%) and smallest for the lung density of 0.25 (up to 3.5%). Conclusion: The dose deviation resulting from the lack of the lung heterogeneity corrections can be quantified into a difference in clinical effectiveness using the P+ index. The variability of the average lung density increased the discrepancy between the different plans in terms of complication‐free tumor control rates. Radiobiological evaluation of the treatment plans can provide much closer association of the treatment delivered with the clinical outcome.