Objective: To evaluate the differences between target and normal tissue delineation between magnetic resonance imaging (MRI) and computed tomography (CT) in cervical cancer patients and to explore the differences in dosimetry after brachytherapy planning.

Methods: High-dose-rate brachytherapy was performed on 11 patients. Planning CT and MRI were performed with tandem and ring in place. The radiation oncologist contoured the rectum, the bladder, the sigmoid, and the high-risk clinical target volume (HR-CTV) on CT and MRI. The values compared between CT and MRI included D90 and D100 to HR-CTV; coronal, sagittal, and axial measurements of HR-CTV; and minimum dose to most irradiated 0.1-, 0.5-, 1.0-, and 2.0-cm³volumes for the organs at risk (OAR). Doses were converted to the equivalent dose in 2 Gy by applying the linear quadratic model. Volume optimization was also performed, and the above parameters were evaluated.

Results: Magnetic resonance imaging showed a significantly greater HR-CTV length in the sagittal plane (P = 0.006), with CT showing a greater length in the coronal plane (P = 0.004). The equivalent dose in 2 Gy to 2.0 cm³ for the bladder was greater on CT than MRI (P = 0.041). The remainder of the dose volume histogram values for the OAR were similar between CT and MRI. With volume optimization, no significant differences were seen between HR-CTV dose parameters or doses to OAR.

Conclusions: The CT- and MRI-based brachytherapy tissue delineation seems adequate for evaluation of OAR and target tissues, although the shapes of HR-CTV and OAR do differ. When adopting volume-based prescription, these differences may lead to altered target dosing. The clinical impact of these differences seems to be small and may demonstrate that planning with CT, if combined with one MRI, may be sufficient.