To evaluate dosimetric factors predictive for radiation-induced pneumonitis (RP) after pulmonary stereotactic body radiotherapy (SBRT).

A retrospective analysis was performed based on 59 consecutive patients treated with cone-beam CT-based image-guided SBRT for primary NSCLC (n=21) or pulmonary metastases (n=54). The majority of patients were treated with radiosurgery of 26 Gy to 80% (n=29) or three fractions of 12.5 Gy to 65% (n=40). To correct for different single fraction doses, local doses were converted to 2 Gy equivalent normalized total doses (NTDs) using α/β ratio of 3 Gy for RP. Dose-volume parameters and incidences of RP ≥ grade II SWOG were fitted using NTCP models.

Eleven patients developed RP grade II. With an average MLD of 10.3±5.6 Gy to the ipsilateral lung, a significant dose-response relationship was observed: the MLD was 12.5±4.3 Gy and 9.9±5.8 Gy for patients with and without development of RP, respectively. Additionally, volumes of the lung exposed to minimum doses between 2.5 and 50 Gy (V(2.5)-V(50)) were correlated with incidences of RP with a continuous decrease of the goodness of fit for higher doses.

The MLD and V(2.5)-V(50) of the ipsilateral lung were correlated with incidences of RP after pulmonary SBRT.