Practically achievable maximum high-risk clinical target volume doses in MRI-guided intracavitary brachytherapy for cervical cancer: a planning study.

Purpose: To explore maximum high-risk clinical target volume (HR-CTV) doses that can be practically achieved when organs at risk (OARs; bladder, rectum, and sigmoid) doses are allowed to equal current recommended thresholds in MRI-based intracavitary brachytherapy (BT) planning for cervical cancer.

Methods And Materials: Planning MRI sets were retrieved for 21 patients who received pulsed-dose-rate BT boost. Plans were generated using manual optimization (MO) by adjusting dwell positions and times to obtain the prescribed HR-CTV isodose that includes 90% of target (D90) coverage of 35 Gy while limiting OAR doses to below recommended tolerances (prescribed dose target [TGT] plans). Additional planning was performed with automatic volume optimization (VO) to evaluate target coverage relative to the MO plans. The MO and VO approaches were then applied with the objective of obtaining the highest possible HR-CTV coverage when OAR doses were allowed to equal threshold tolerance values (maximized [MAX] plans). A two-tailed paired t test was performed to determine the statistical significance of the results; significance level set at p < 0.013.

Results: MO and VO planning techniques could conform HR-CTV D90 to the prescribed dose quite similarly for TGT plans. Using the MAX approach, the HR-CTV D90 could be increased by 30% and 37% for MO and VO, respectively, without exceeding OAR thresholds. Sigmoid and often rectum were the dose-limiting structures during MAX planning.

Conclusions: Simple differences in the approach to volumetric MRI-based cervix BT treatment planning can impact HR-CTV D90. Consequently, dose escalation for MRI-guided cervix BT appears feasible in this manner should clinical circumstances warrant.