PURPOSE: To identify factors that are predictive of satisfactory acute and long-term pulmonary tolerance of definitive irradiation and, conversely, factors that are predictive of excessive impairment of pulmonary functions. To determine if there is any correlation between early elevation of biochemical markers obtained in blood of irradiated patients and subsequent pulmonary abnormalities as detected by clinical findings, pulmonary function tests, and/or radiographic findings of pneumonitis/fibrosis. MATERIALS AND METHODS: This was a multi-institutional prospective trial sponsored by the Radiation Therapy Oncology Group. Eligible patients had surgically unresectable or medically inoperable stage II or III non-small cell lung cancer. Pretreatment evaluation included baseline dyspnea index (BDI) and pulmonary function tests (PFT). Radiation therapy consisted of once-daily treatment with 2 Gy to a total of 60 to 66 Gy. A quantitative nuclear medicine perfusion study was correlated to the radiation therapy portals to assess the proportion of lung irradiated. Blood for serum markers (surfactant apoprotein, procollagen type III, interleukin [IL]-1, interleukin-6, and tumor necrosis factor-α) was drawn prior to the beginning of radiation therapy and then weekly during treatment (at 10, 20, 30, 40, 50, and 60 Gy). Post-treatment follow-up included PFT every 3 months for 1 year and then annually. The BDI was reevaluated at the same intervals. RESULTS: There were 127 analyzable patients. Squamous cell carcinoma was the predominant histology and 93% of the patients had AJCC stage III disease. The median survival time is 10.9 months with 43% of patients living 1 year and 10% living 3 years. Grade ≥2 acute lung toxicity was seen in 18% of patients; patients least likely to develop lung toxicity are those with undetectable levels of IL-6 at 10 Gy and diffusing capacity of the lung for carbon monoxide percent (DLCO%) >54. Patients most likely to develop acute toxicity are those with elevated IL-6 and age >60 years. Grade ≥2 late lung toxicity was seen in 30% of patients. Karnofsky performance status was the only pretreatment factor predictive of late lung toxicity. The proportion of lung within the irradiated field, BDI indices, physician-assessed baseline dyspnea, and baseline PFT were not predictive of pulmonary toxicity. Using grade ≥2 toxicity as an event, age >60 years, gender, and a surfactant level <797 at 20 Gy were predictive of late lung toxicity. CONCLUSIONS: Elevated levels of serum IL-6 after 10 Gy of lung irradiation appear to predict grade ≥2 acute lung toxicity, and high serum levels of surfactant apoproteins at 20 Gy correlated with grade ≥2 late pulmonary toxicity. These findings need to be confirmed but could be useful in a model to predict risk of pulmonary injury with high doses of radiation. For future studies, it is necessary to evaluate serum markers at multiple time-points during treatment, and quality control is critical during the collection, storage, and analysis of these serum markers. © 2007 Lippincott Williams & Wilkins, Inc.