Dosimetric evaluation of magnetic resonance imaging-based intracavitary brachytherapy for cervical cancer.

The purpose of this study was to evaluate the dosimetric benefit of magnetic resonance imaging (MRI)-based dose adaptation in intracavitary brachytherapy (ICR) for cervical cancer. Dose-volume histograms were compared between matched conventional and optimized plans in 22 patients who were treated by MRI-based ICR. Doses to organs-at-risk (OAR) and dose covering 90% of high-risk clinical target volume (HR-CTV) were evaluated to compare OAR sparing and target coverage, respectively. The probability of RTOG rectal toxicity grade of ≥2 in the 22 patients was estimated based on the prediction model generated from previous three-dimensional CT-based ICR data. After optimization, doses to OAR showed a statistically significant decrease. The reduction percentage (reduced dose by optimization x 100/dose in the conventional plan) was higher in patients with HR-CTV >20 cc than in patients with HR-CTV >20 cc in the rectum. In patients with HR-CTV ≤ 20 cc, the mean probability of RTOG rectal toxicity grade ≥ 2 was 67.6% for the conventional plan and 47.8% for the optimized plan, based on the prediction model. In conclusion, dose adaptation by MRI-based ICR for cervical cancer resulted in significant dose reduction to the rectum, especially in patients with HR-CTV ≤ 20 cc.