Can MRI-only replace MRI-CT planning with a titanium tandem and ovoid applicator?

To evaluate dosimetric differences between MRI-only and MRI-CT planning with a titanium tandem and ovoid applicator to determine if all imaging and planning goals can be achieved with MRI only.

We evaluated 10 patients who underwent MRI-CT-based cervical brachytherapy with a titanium tandem and ovoid applicator. High-risk clinical target volume and organs at risk were contoured on the 3D T2 MRI, which were transferred to the co-registered CT, where the applicator was identified. Retrospectively, three planners independently delineated the applicator on the axial 3D T2 MRI while blinded to the CT. Identical dwell position times in the delivered plan were loaded. Dose-volume histogram parameters were compared to the previously delivered MRI-CT plan.

There were no significant differences in dose to D₉₀ or D₉₈ of the high-risk clinical target volume with MRI vs. MRI-CT planning. MRI vs. MRI-CT planning resulted in mean D_0.1cc bladder of 8.8 ± 3.4 Gy vs. 8.5 ± 3.2 Gy (p = 0.29) and D_2cc bladder of 6.2 ± 1.4 Gy vs. 6.0 ± 1.4 Gy (p = 0.33), respectively. Mean D_0.1cc rectum was 5.7 ± 1.2 Gy vs. 5.3 ± 1.2 Gy (p = 0.03) and D_2cc rectum 4.0 ± 0.8 Gy vs. 4.2 ± 1.0 Gy (p = 0.18), respectively. Mean D_0.1cc sigmoid was 5.2 ± 1.3 Gy vs. 5.4 ± 1.6 Gy (p = 0.23) and D_2cc sigmoid 3.9 ± 1.0 Gy vs. 4.0 ± 1.1 Gy (p = 0.18), respectively.

There were no clinically significant dosimetric differences between the MRI and MRI-CT plans. This study demonstrates that cervical brachytherapy with a titanium applicator can be planned with MRI alone, which is now our clinical standard.