[en] Purpose/objective: To further investigate the utility of plasma TGFβ levels as a marker for identifying during treatment those patients at risk for the development of radiation pneumonitis. Materials and methods: The study group consists of 35 consecutive patients who have received thoracic irradiation (RT) with curative intent for lung cancer (60-66 Gy/6-6.5 wks or 73.6 Gy/4 wks; n=30), Hodgkins' disease (35 Gy/4-4.5 wks; n=4), or thymoma (55 Gy/6 wks; n=1). No patient had prior chemotherapy. Plasma samples were obtained before, weekly during, and at each follow-up after radiation therapy. TGFβ was extracted from plasma using an acid/ethanol method. An enzyme linked immunosorbant assay was used to quantify plasma TGFβ levels. All patients have been followed for at least 6 months, unless pneumonitis developed at an earlier date. Plasma TGFβ levels were analyzed without knowledge of the clinical status of each patient. Pneumonitis was defined clinically by NCI common toxicity criteria. Results: 11 patients developed pneumonitis (lung cancer-10, Hodgkins' disease-0, thymoma-1); the remaining 24 did not. The mean plasma TGFβ at each week of RT normalized to baseline was plotted as a function of time (Fig 1). The error bars represent the standard error of the mean. Those patients who developed pneumonitis had plasma TGFβ levels that were above baseline by the completion of RT. In contrast, the plasma TGFβ levels were at or below baseline in patients who did not develop symptomatic lung injury. Conclusion: Plasma TGFβ levels are useful to monitor the risk of pneumonitis in patients receiving thoracic RT. Patients whose plasma TGFβ level remains at or below baseline late in treatment are at low risk of developing pneumonitis. These patients might be candidates for dose escalation