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), logistic with 2 free parameters (EUD) and logistic with fixed parameters describing local perfusion loss (EUD)) and volume above a threshold dose (V). 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 based NTCP model had the lowest AIC. The Aw were 0.53, 0.19, 0.11, 0.11, 0.05 for the EUD, V, MLD, EUD and EUD LDEs respectively.

Conclusion: A LDE for perfusion loss provided modest improvement in NTCP model fit for SBRT induced radiation pneumonitis.