Oncentra brachy

For the BT planning, an Oncentra Brachy (Elekta, Veenendaal, Netherlands) RTplan file with applicator reconstruction was provided allowing all participants to use the same applicator reconstruction and thereby facilitating a fair plan comparison. All participants except one used Oncentra Brachy in their clinic.
The following dose points were to be placed: points A, ICRU bladder, Lateral Vaginal points at 5 mm, and Recto-Vaginal Reference Point (RVRP) [19] (link), [20] (link). Three plans were required: (1) a generic “standard plan”, consisting of a set of pre-determined dwell times and positions, (2) a centre-specific “centre standard plan” (not optimized), and (3) an optimized plan according to the aims and constraints of the EMBRACE-II protocol. DVH parameters for all plans, DICOM RTdose and RTplan for the optimized plan were returned. For the second workshop the coordinates of the dose points were distributed and only an optimized plan was requested.

Two additional pre-processing steps were performed on the DICOM data after export. First, the contours of the applicator, which are usually not available in the TPS (Oncentra® Brachy – Elekta, Venendaal), were generated with an Elekta Applicator Slicer research plugin (Elekta, Venendaal) and treated as ground-truth masks. The plugin used the MRI and treatment plan with reconstructed applicator model to generate a standard radiotherapy (RT) structure file for the intracavitary applicator geometry. The resulting applicator mask was further processed with a hole filling algorithm to create continuous applicator geometries [17] .
It is important to note that, even if the plugin was commercially available, the described method to extract these ground-truth segmentations would not be feasible for an automatic workflow, as it requires prior manual placement of an applicator library model.
Second, a python script was written to read and export all available dwell positions of the applicator from the treatment plan for the purpose of evaluating registration performance. Dwell positions were exported as x,y,z-coordiantes in the patient coordinate system. Only clinically relevant dwell positions were considered for the calculation of the registration error. (Fig. 1, Appendix Table A1).Fig. 1

3D visualization of the exported applicator dwell positions in the patient coordinate system.

In this work, Oncentra Brachy (Elekta, Stockholm, Sweden) was used to create a caps design pre-plan, which consisted of two components (Figure 3A). The first component involved a manual reconstruction of the split ring channels over a length of ~35 mm. The second component required manually reconstructing IS needle virtual trajectories based on a visualization of important target structures. These virtual trajectories should extend at least ±1 cm from the split ring channels in the superior/inferior direction and intersect important target structures. Dummy dwell positions were then activated every 5 mm to complete the caps design pre-plan, which was imported into 3D Brachy as a DICOM-RT Plan file.