Document Type : Original Research


1 Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran

2 Department of Radiation Oncology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran



Background: Magnetic Resonance Imaging (MRI) has become a complementary imaging method for the treatment planning process due to the limitations of Computed Tomography (CT) imaging. 
Objective: This study aimed to assess the effect of co-registered MRI and CT (MRI/CT)-based target delineation on the dose to the target, small bowel, bladder, and femoral heads during Helical Tomotherapy (HT).
Material and Methods: In this cross-sectional prospective study, MRI in a prone position were obtained for 12 patients with rectal cancer at one-day intervals with simulation CT. Following the co-registration process with the deformable 
algorithm, target volumes are defined. Gross Tumor Volume (GTV), Clinical Target Volume (CTV), and Planning Target Volume (PTV) were delineated for each CT and MRI/CT. 
Results: GTV, CTV, and PTV mean values were significantly higher in the CT-based target delineation method than those in the MRI/CT-based method. In MRI/CT-based plans, the mean HI value was significantly lower, and the mean Conformity Index (CI) value was significantly higher than that in CT-based plans. In a small bowl, the most of dosimetric parameters (Dmax, Dmean, D50%, D50%, V40%, and V45%) were significantly higher for the CT-based plans. In the bladder, all dosimetric parameters, except V30%, were statistically higher in CT-based plans.  
Conclusion: Co-registered MRI/CT-based treatment planning can produce 
better dose coverage for the target and reduce the delivered dose to the Organs at Risk (OARs) when compared to CT-based planning.