PURPOSE: In radiation therapy, NTCP is very important indicator of selecting the optimal treatment plan. In our study, we tried to find out usefullness of NTCP in lung cancer by comparng the incidence of radiation pneumonitis with NTCP. METHODS AND MATERIALS: From August 1993 to December 1994, thirty six patients with locally advanced non-small cell lung cancer were treated by concurrent chemoradiation therapy. Total dose of radiation therapy was 6480cGy (120cGy, bid) and chemotherapeutic agents were mitomycin C, vinblastine, cisplatin (2 cycles, 4 weeks interval). We evaluated the development of radiation pneumonitis by CT scan, chest x-ray and clinical symptoms. We used grading system of South Western Oncology Group (SWOG) for radiation pneumonitis. Dose Volume Histograms (DVH) were analyzed for ipsilateral and whole lung. Non uniform DVH was translated to uniform DVH by effective volume method. With these data, we calculated NTCP for ipsilateral and whole lung. Finally we compared the clinical results to NTCP. RESULTS: Eight of thrity six patients developed radiation pneumonitis. Of these 8 patients, 6 had grade I severity and 2 had grade II. The average NTCP value of the patients who showed radiation pneumonitis was significantly higher than that of the patients without pneumonitis (66% vs. 26.4%). But the results of pulmonary function test was not correlated with NTCP. CONCLUSION: NTCP of lung is very good indicator for selecting rival treatment planning in lung cancer. According to the results of NTCP, it may be possible to adjust target volume and optimize target dose. In the near future, we are going to analyze the effect of hyperfractionation and concurrent chemotherapy in addition to NTCP.