Session Item

Freiburg Flap (FF) is the most commonly used applicator in HDR surface brachytherapy. Treatment planning is performed on CT, as opposed to MRI that provides superior soft tissue contrast. An optimized MRI sequence - Pointwise Encoding Time Reduction with Radial Acquisition (PETRA) – presented in this study demonstrated a potential to make an MR-only treatment planning feasible. PETRA was used to digitize FF and create an MR-only treatment plan and compared to CT.

A FF (Elekta, Stockholm, Sweden) consisting of 12 catheters (24 spherical beads per catheter, 10 mm diameter) was sandwiched between 2 flat plexiglass  plates (30 x 20 x 1.7 cm³). An optimized coronal 3D PETRA sequence (TR\TE\TI 3.3\0.07\100 ms, FOV 306x306 mm², isotropic 0.8 mm-resolution, flip angle 4°, BW 407 Hz/px) was used to acquire MR images on a 3T SIEMENS Vida stimulator with a UltraFlex Large 18 and Spine 32 coil. The phantom was subsequently scanned on a helical CT (115 mA, 120 kV, 1.25 mm slice thickness). Applicators were localized and plans were created to the target defined as volume at 3 mm depth in the Oncentra Brachy (Elekta Brachytherapy, Netherlands) Treatment Planning System (TPS) for both CT and MRI. The TPS dwell points were calculated with step size of 10 mm from projected tip position in the TPS. MR images were registered onto CT images via rigid registration. Catheter difference (distance between the corresponding dwell positions between CT and MR-based digitization) were compared for each spatial dimension and for 3D. Relative percent dose from CT or MR-only treatment planning at a point 8, 10, 15, 20, and 25 mm beneath each dwell point (1380 points in total) were compared.

The projected catheter trajectory between CT and MR showed good agreement (Hausdorff distance of 0.99 ± 0.36 mm for dwell points, Fig 1a). The catheter differences between the MR and CT-generated trajectories along each spatial dimension for all dwell points were 0.19 ± 0.42, 0.28 ± 0.11, 0.27 ± 0.66 mm for x-, y-, and z-dimension, respectively (Fig 1b-d). The catheter difference in 3D was 0.82 ± 0.37 mm (Fig 1e). The difference in the relative dose between CT and MR-only plans (Fig 2a) were not statistically significant (paired t-test, α = 0.05). Differences in the mean relative dose at each depth were all < 0.04% (Fig 2c).

Fig. 1 a) Projected catheter trajectory in CT and MR, b-e) mean catheter differences in x, y, z, and 3D for individual catheters.

Fig. 2 a) Treatment plans generated with CT and MR, b) phantom setup, c) mean (± SD) relative dose at each depth.

The MR-only digitization of FF using PETRA sequence was shown to achieve < 1 mm accuracy compared to the CT. The optimized PETRA sequence created an MR-only plan that provides comparable dose profile to the plan generated via CT-based approach.