In vivo portal dosimetry for head-and-neck VMAT and lung IMRT: Linking γ-analysis with differences in dose-volume histograms of the PTV

Purpose: To relate the results of gamma-analysis and dose-volume histogram (DVH) analysis of the PTV for detecting dose deviations with in vivo dosimetry for two treatment sites. Methods and materials: In vivo 3D dose distributions were reconstructed for 722 fractions of 200 head-and-neck (H&N) VMAT treatments and 183 fractions of 61 lung IMRT plans. The reconstructed and planned dose distributions in the PTV were compared using (a) the gamma-distribution and (b) the differences in D2, D50 and D98 between the two dose distributions. Using pre-defined tolerance levels, all fractions were classified as deviating or not deviating by both methods. The mutual agreement, the sensitivity and the specificity of the two methods were compared. Results: For lung IMRT, the classification of the fractions was nearly identical for gamma- and DVH-analyses of the PTV (94% agreement) and the sensitivity and specificity were comparable for both methods. Less agreement (80%) was found for H&N VMAT, while gamma-analysis was both less sensitive and less specific. Conclusions: DVH- and gamma-analyses perform nearly equal in finding dose deviations in the PTV for lung IMRT treatments; for H&N VMAT treatments, DVH-analysis is preferable. As a result of this study, a smooth transition to using DVH-analysis clinically for detecting in vivo dose deviations in the PTV is within reach. (C) 2014 Elsevier Ireland Ltd. All rights reserved.