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Purpose: To evaluate whether single nucleotide polymorphisms (SNPs) in the transforming growth factor-β1 (TGFβ1) gene are associated with radiation sensitivity using an objective radiologic endpoint. Methods and Materials: Preradiation therapy and serial postradiation therapy single photon emission computed tomography (SPECT) lung perfusion scans were obtained in patients undergoing treatment for lung cancer. Serial blood samples were obtained to measure circulating levels of TGFβ1. Changes in regional perfusion were related to regional radiation dose yielding a patient-specific dose-response curve, reflecting the patient's inherent sensitivity to radiation therapy. Six TGFβ1 SNPs (-988, -800, -509, 869, 941, and 1655) were assessed using high-resolution melting assays and DNA sequencing. The association between genotype and slope of the dose-response curve, and genotype and TGFβ1 ratio (4-week/preradiation therapy), was analyzed using the Kruskal-Wallis test. Results: 39 white patients with preradiation therapy and ≥ 6-month postradiation therapy SPECT scans and blood samples were identified. Increasing slope of the dose-response curve was associated with the C(-509)T SNP (p = 0.035), but not the other analyzed SNPs. This SNP was also associated with higher TGFβ1 ratios. Conclusions: This study suggests that a polymorphism within the promoter of the TGFβ1 gene is associated with increased radiation sensitivity (defined objectively by dose-dependent changes in SPECT lung perfusion). [ABSTRACT FROM AUTHOR]