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

Abstract Purpose/Objective(s) 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. Materials/Methods 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. Results There were no significant differences in dose to D 90 or D 98 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. Conclusion 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.