Point A vs. HR-CTV D₉₀ in MRI-based cervical brachytherapy of small and large lesions

Purpose: To evaluate the dosimetric benefits of MRI-based brachytherapy in small and large high-risk clinical target volume (HR-CTV) in cervical cancer.

Methods and materials: Twenty-eight fractions obtained from sixteen cervical cancer patients treated with MRI-based high-dose-rate brachytherapy with standard tandem and ovoid applicators were used; original fractions were optimized to HR-CTV D₉₀. Fractions were separated based on the median volume into small and large (HR-CTV <25 cm³ or >25 cm³) lesion groups. Retrospective plans prescribed to Point A were created for each fraction. D_{0.1 cc}, D_{2 cc}, and International Commission of Radiation Unit and Measurements (ICRU) points were used to compare Point A vs. HR-CTV D₉₀ plans for bladder, rectum, and sigmoid.

Results: In the small lesion group, Point A plans vs. HR-CTV D₉₀ plans had significantly higher D_{0.1 cc}, D_{2 cc}, and ICRU points for bladder, rectum, and sigmoid (p < 0.05). In the large lesion group, there was no significant difference between Point A and HR-CTV D₉₀ plans for D_{0.1 cc}, D_{2 cc}, and ICRU points to the organs at risk (OARs).

Conclusions: The dosimetric advantages to OARs offered by MRI-based brachytherapy with prescription to HR-CTV D₉₀ compared to Point A is most distinct for patients with smaller HR-CTV (<25 cm³). This study demonstrates sufficient tumor coverage with lower doses to OARs in HR-CTV D₉₀ vs. Point A plans in the small lesion group. These improvements were not seen in the large lesion group, indicating a lesser dosimetric advantage of HR-CTV D₉₀ compared to Point A planning when the cervical lesion is >25 cm³. Incorporation of interstitial needles for patients with larger HR-CTV is likely the best method to decrease dose to OARs and improve tumor coverage.