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Dosimetric Evaluation of Magnetic Resonance-Generated Synthetic CT for MR-Only Rectal Cancer Radiotherapy

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H Wang

H Wang*, K Du , J Qu , H Chandarana , I Das , NYU Langone Medical Center, New York, NY


SU-H3-GePD-J(B)-3 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Joint Imaging-Therapy ePoster Lounge - B

Purpose: The purpose of this study is to assess the dosimetric equivalence of MR-generated synthetic CT and conventional CT for treatment planning in radiotherapy of rectal cancer.

Methods: This study was conducted on eleven patients who underwent whole-body PET/MR and PET/CT examination in a prospective IRB-approved study. MR data were obtained on a 3T Siemens PET/MR hybrid scanner using a 2-point Dixon sequence. Synthetic CT (synCT) was generated from Dixon MR using a model-based method and then registered to CT using a deformable registration. Rectal tumors were artificially contoured in the synCT by a physician to define a planning tumor volume (PTV) for treatment planning comparison. Standard treatment planning directives were used to create a four-field box (4F), an oblique four-field box (O4F) and a volumetric modulated arc therapy (VMAT) plans on synCT for each PTV. The plans were recalculated on registered planCT with the same MUs as on synCT. Dose-volume metrics and gamma analysis were evaluated between synCT and CT plans.

Results: Differences in PTV Dmin, D95% and Dmax between synCT and CT plans across all patients were 0.02 ± 0.82% for 4F, -0.16 ± 0.78% for O4F and -0.31 ± 0.97% for VMAT plans. In any of the treatment techniques, no significant differences were observed in organs at risk (OAR) dose metrics including small bowel (V45Gy), bladder (V40Gy) and femoral head (V30Gy). Gamma Analysis with 2%/2mm dose difference/distance to agreement showed percentage pass rates of 98.7 ± 3.1, 98.0 ± 3.6, and 98.8 ± 2.7 for 4F, O4F and VMAT, respectively between SynCT and CT plan.

Conclusion: Planning on synCT agreed well with the dose recalculated on planning CT for conventional treatment techniques in rectal cancer radiotherapy. These results suggest the potential of MR-only radiotherapy using MR generated synCT.

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