Inter-application variation of dose and spatial location of D(2cm(3)) volumes of OARs during MR image based cervix brachytherapy

Purpose: Evaluation of Inter-application variation of doses and spatial location of D(2cm(3)) volumes of OARs during MR-image based cervix brachytherapy.

Materials and methods: Twenty-seven patients treated with EMBRACE protocol were analyzed. Every patient had two applications, one week apart. For each application patient had undergone MR-imaging (MR-1 and MR-2), volume delineation, reconstruction, treatment planning (plan-1 and plan-2) and dose evaluation. Both the image series were then co-registered with applicator as the reference coordinate system (Eclipse planning system v8.6.14). Inter-application dose, volume and spatial location of D(2cm(3)) variation were evaluated.

Results: The largest inter-application systematic and random dose variations were observed for sigmoid as compared to rectum and bladder. The mean (±SD) of the relative D(2cm(3)) variations were 0.6(±15.1)%, 0.9(±13.1)% and 11.9(±37.5)% for rectum, bladder and sigmoid respectively. The overlap of D(2cm(3)) volumes was more than 50% in 16(59%), 8(30%) and 3(11%) patients for rectum, bladder and sigmoid, respectively.

Conclusion: The 2cm(3) volumes between the applications/fractions are quite stable in topography for bladder and rectum, and hence the current practice of cumulative addition of D(2cm(3)) dose is expected to be valid for bladder and rectum. For sigmoid, significant topographical changes were seen, which need further validation in a larger patient population and in multi-centric settings.