evaluating the effect of co-registered diagnostic mr images based ct simulation on target volume delineation and dose distribution for tomotherapy of rectal cancer

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.