Direct reconstruction of the Vienna applicator on MR images

Purpose: To introduce and test a direct reconstruction concept for intracavitary tandem ring applicators in MR image-based brachytherapy treatment planning. Materials and methods: Optical measurements of transparent ring-phantoms provided the geometric relation between source path and the Vienna ring applicator as visible on MRI. For the manual direct reconstruction method (PLATO), the geometry plotted on a transparency was placed on the screen and rotated to fit with visible ring holes. With the software-integrated reconstruction method (OncentraGYN), the applicator geometry was directly used when placing the visible parts of the applicator in the 3D dataset. Clinical feasibility was tested in 10 clinical insertions. Reconstruction and dose calculation were performed independently on two treatment planning systems (PLATO and OncentraGYN) using MRI alone. DVH parameters for targets and organs at risk were analysed and compared to the clinically used radiograph/MRI registration-based method. Results: The direct reconstruction concept for both methods was feasible and reduced treatment planning time. Evaluated DVH parameters for plans using direct reconstruction methods differed from clinically used plans (traditional reconstruction) in mean differences ≤0.2 Gy for plans with 7 Gy prescribed dose. Conclusion: If the relation between applicator shape visible on MRI and the source path is defined once, the reconstruction process can be performed by directly placing the applicator in the MRI dataset.