[en] Objective: To evaluate relation between hypoxia, hypoxic changes detected by ^99mTc-HL91 SPECT and the response to radiotherapy. Methods: Thirty-five patients with pathologically proven non-small cell lung cancer (NSCLC) treated by three-dimensional conformal radiotherapy were entered into the study. All patients were examined by examined by ^99mTc-HL9l SPECT imaging one or two days before radiotherapy. Twenty patients were monitored during radiotherapy(30-40 Gy) and one or two days after radiotherapy. Anterior', posterior and lateral planar images were collected 2 hours, 4 hours and 6 hours after intravenous injection approximately 740 MBq ^99mTc-HL91. Regions of interest(ROIs) were ch awn in the tumor and contralateral position and the radioactivity ratios of tumor to normal(T/N) were calculated. The response to radiotherapy was evaluated by solid tumor' s effect evaluation criterion (WHO) 3-6 months after radiotherapy. The correlations between T/N before radiotherapy, the T/N changes during radiotherapy and the response to radiotherapy were analyzed. Results: The T/N value gradually decreased before, during and after radiotherapy. The average T/N value before, during and after radiotherapy was 1.56±0.19, 1.40±0.12 and 1.29±0.13, respectively (F=10.13, P=0.010). The lower T/N ratio be- fore radiotherapy, the higher the response rate of radiotherapy (P=0.040). The larger change of T/N ratio before and after radiotherapy resulted in higher radiotherapy response, but with no statistical significance (P>0.05). Conclusions: The hypoxia status and hypoxic changes can be observed by HL91 SPECT imaging during radiotherapy which can predict the response to radiotherapy. (authors)