Modeling radiation pneumonitis of pulmonary stereotactic body radiotherapy: The impact of a local dose-effect relationship for lung perfusion loss.

Purpose: To investigate if a local dose-effect (LDE) relationship for perfusion loss improves the NTCP model fit for SBRT induced radiation pneumonitis (RP) compared to conventional LDEs.
Methods and Materials: Multi-institutional data of 1015 patients treated with SBRT were analyzed. Dose distributions were converted to NTD with α/β = 3 Gy. The Lyman-Kutcher-Burman NTCP model was fitted to the incidence grade ≥2 RP by maximum likelihood estimation with mean lung dose (MLD), equivalent uniform doses (EUD) using three LDE functions (power-law (EUD _power ), logistic with 2 free parameters (EUD _log-free ) and logistic with fixed parameters describing local perfusion loss (EUD _Perfusion )) and volume above a threshold dose (V _x ). Models were compared with the Akaike weights (Aw) derived from the Akaike information criteria (AIC).
Results: The median time to grade ≥2 RP was 4.2 months and plateaued after 17 months at 5.4%. A strong dose-effect relationship for RP incidence was observed. The EUD _Perfusion based NTCP model had the lowest AIC. The Aw were 0.53, 0.19, 0.11, 0.11, 0.05 for the EUD _Perfusion , V _x , MLD, EUD _log-free and EUD _power LDEs respectively.
Conclusion: A LDE for perfusion loss provided modest improvement in NTCP model fit for SBRT induced radiation pneumonitis.
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