2489 - Impact of Lung Dose on Pulmonary Function Decline after Stereotactic Body Radiation Therapy for Lung Cancer

Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) is an important treatment option for small lung cancers. Conflicting findings have been reported regarding the effects of SBRT on pulmonary function tests (PFTs). We conducted a longitudinal analysis of PFT results in patients following lung SBRT to explore the association between lung dose parameters and changes in PFT.

Materials/Methods: Patients who had PFT performed before and after SBRT were enrolled. Patients were excluded if they had undergone pulmonary resection or initiated or altered treatment for COPD between the pre- and post-SBRT PFT. Post-SBRT PFTs were classified into early (within 6 months), middle (6-12 months), late (12-24 months), and very late (24 months or more) periods based on the time elapsed since SBRT. These results were compared with baseline PFTs (Wilcoxon signed-rank test). We evaluated the following pulmonary function test parameters: VC, ERV, IRV, TV, IC, FVC, FEV1, and FEV1/FVC ratio. Correlations between the rates of change of these PFT parameters and various lung dose parameters were evaluated (Spearman rank correlation coefficient). The evaluated lung dose parameters were mean lung dose, D33.3%, V5, V10, V20, V30, V60, V90, and V120 (cc) (all converted to equivalent dose in 2 Gy fractions). For lung dose parameters that correlated significantly with changes in respiratory function, cutoff values were determined using ROC analysis. Using the determined cutoff values, we evaluated the predictability of lung function deterioration of RTOG PFT toxicity scale Grade =1 (=10%).

Results: A total of 61 patients (total PFTs: 150) were enrolled. Compared with baseline, no significant changes were observed in PFTs in the early, middle, or late periods. In the very late period, we observed significant decreases of 5% in VC, 7% in FVC, and 7% in FEV1 (p = 0.014, 0.003, and 0.011, respectively). There was a significant negative correlation between the change rate of VC and D33.3%, FVC and D33.3%, and FEV1 and V20 (Table). The cutoff values predicting Grade =1 decline in VC, FVC, and FEV1 were 0.58 Gy (D33.3%), 0.55 Gy (D33.3%), and 121.1 cc (V20), respectively. When the 2 groups were divided by these cutoff values, the groups above the cutoff values had significantly greater reductions in VC, FVC, and FEC1 than the groups below the cutoff values.

Conclusion: Our study demonstrated a significant correlation between the decrease in pulmonary function (VC, FVC, and FEV1) and dosimetric parameters (D33.3% and V20) in lung cancer patients treated with SBRT over 24 months.