Impact of dosimetric differences between CT and MRI derived target volumes for external beam cervical cancer radiotherapy

Objectives: The use of MRI is becoming more prevalent in cervical cancer external beam radiotherapy (RT). The aim of this study was to investigate the impact of dosi-metric differences between CT and MRI-derived target volumes for cervical cancer external beam RT. Methods: An automated planning technique for volumetric modulated arc therapy was developed. Two auto-mated planning plans were generated for 18 cervical cancer patients where planning target volumes (PTV5) were generated based on CT or MRI data alone. Dose metrics for planning target volumes and organs at risk (OARs) were compared to analyse any differences based on imaging modality. Results: All treatment plans were clinically acceptable, Bladder doses (V40) were lower in MRI-based plans (p = 0.04, 53.6 +/- 17.2 % vs 60.3 +/- 13,1 % for MRI vs CT, respectively). The maximum dose for left iliac crest showed lower doses in CT-based plans (p = 0.02, 47,8 +/- 0.7Gy vs 47.4 +/- 0.4 Gy MRI vs CT, respectively). No significant differences were seen for other OARs. Conclusions: The dosimetric differences of CT- and MRIbased contouring variability for this study was small. CT remains the standard imaging modality for volume delineation for these patients. Advances in knowledge: This is the first study to evaluate the dosimetric implications of imaging modality on target and OAR doses in cervical cancer external beam RT.