The optimal gamma passing rate thresholds of IMRT dosimetric verification in the treatment of esophageal cancer

Objective To investigate the optimal gamma passing rate of intensity-modulated radiotherapy (IMRT) dosimetric verification in the treatment of esophageal cancer using a three-dimensional dose verification system EDose™. Methods Twenty five esophageal cancer patients treated by 7-field IMRT were retrospectively reviewed. Measured dose distribution were reconstructed on CT image and evaluated by gamma analysis and DVH metrics using the EDose™ system. Plans with DVH metrics dose difference 90% under 3%/3 mm criteria were accepted. The optimal gamma passing rate for criteria of 5%/3 mm, 3%/3 mm, 2%/2 mm were investigated by drawing the receiver operating characteristic(ROC) curves and calculating the Youden Index. The sensitivity and specificity of the these optimal thresholds in the plan verification were also analyzed. Results The optimal thresholds for global gamma indices with 5%/3 mm, 3%/3 mm, 2%/2 mm were 98.66%, 94.84%, 78.56%, respectively. In the 90% common threshold, The sensitivity and specificity for common 90% threshold and optimal threshold under 3%/3 mm criteria were 0.17 vs. 0.85 and t 0.84 vs. 0.27, respectively. The sensitivity and specificity were 0.89, 0.65 and 0.23, 0.47 for optimal thresholds under 5%/3 mm and 2%/2 mm criteria, respectively. Conclusions The sensitivity of optimal threshold gamma passing rate improved significantly compared with the common threshold (90%) at 3%/3 mm criteria., The sensitivity and the specificity were more balanced at the 2%/2 mm criteria compared with those at 3%/3 mm criteria. Key words: Intensity-modulated radiotherapy; Receiver operating characteristic curve; Optimal thresholds; Dose volume histogram