Sci-Sat AM: Brachy - 08: MRI-guided planning and maximum achievable HR-CTV doses in cervix brachytherapy.

PURPOSE

To present an institutional experience with MRI-based intracavitary brachytherapy planning for cervix cancer treatments using the EMBRACE protocol and to evaluate maximum HR-CTV doses that can be achieved when OAR (bladder, rectum, and sigmoid) doses are allowed to equal GECESTRO recommended thresholds.

METHOD

Dose metrics from treatment plans for 20 patients created using MR images (for contouring HR-CTV and OARs) fused with CT images (for applicator reconstruction) are presented. Starting with a standard Manchester loading, plans were manually optimized (MO) by adjusting dwell positions and times to obtain the desired HR-CTV D90 target coverage of 35 Gy while limiting OAR doses to below recommended tolerances. In addition, retrospective planning was done using: (i) volume optimization (VO) to compare differences with MO in obtaining the desired target coverage; and (ii) MO and VO techniques to get the highest possible HR-CTV coverage by allowing OAR doses to equal tolerance values. The latter plans are referred to as MAX plans.

RESULTS AND CONCLUSIONS

3D MRI-guided treatment planning for cervix brachytherapy was shown to improve dose-volume coverage of the target and OARs. MO could conform HR-CTV D90 to the prescribed dose similar to the VO technique. Sigmoid was often the dose limiting structure. With respect to the prescribed HR-CTV D90 dose of 35 Gy, MAX plans could increase the prescribed dose by about 22% and 30% for MO and VO plans, respectively, without exceeding OAR thresholds. Consequently, dose escalation for MRI-guided cervix brachytherapy appears feasible should clinical circumstances warrant.