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Dosimetric Uncertainty Induced by CT-MRI Fusion in Prostate Treatment Planning

X Chen

X Chen1*, J Xue1,2, J Fan1, L Chen1, C Ma1, (1) Fox Chase Cancer Center, Philadelphia, PA, (2) Cancer Center of Union Hospital, Tongji Medical College, Huazhong University of Science and Technology Wuhan, P.R. China

SU-E-J-218 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Fused CT-MRI data are increasingly used in prostate cancer radiotherapy to achieve more accurate definition of target and critical organs. However, fusion uncertainty may cause target miss or healthy tissue overdose. This study aimed to investigate the dosimetric uncertainty induced by the CT-MRI fusion in prostate treatment planning.

Methods: Patients (n=10) were treated with IMRT and image-guidance techniques including CBCT/BAT-ultrasound, gold seed markers, or Calypso for target localization. Each patient had both CT and MRI simulations, and the CT-MRI scans were performed with a <30 minute interval to minimize organ changes, except for Calypso patients who had MRI scans before radio-transponders implantation followed by CT scans one week after MRI. Paired CT-MRI images were fused primarily based on bony structure matching. The prostate, rectum and bladder were delineated both on CT and MRI, separately. Following our clinical acceptance criteria, prostate treatment plans were created using Eclipse based on MRI-defined target/organs. The dosimetric differences between CT- and MRI-defined target/organs were analyzed.

Results: Most patients showed adequate coverage for both CT- and MRI-defined targets (100%). The difference between CT- and MRI-defined targets is 2.1±4.8% for minimal dose, 0.3±0.4% for maximal dose, and 0.1±0.1% for mean dose. The maximal difference is 11.9% for minimal dose, 0.9% for maximal dose, and 0.2% for mean dose.

Conclusion: With bony structure matching, most patients showed acceptable dosimetric uncertainty induced by CT-MRI fusion. Particularly, the target is well covered by the prescribed dose. However, significant underdose may exist, for example, 11.9% minimal dose difference between CT- and MRI-defined targets. With CT-MRI fusion as a systemic error, the additional inter-fractional or intra-fractional motion may further increase the dosimetric uncertainty. Attention should be paid to the fusion uncertainty for the soft tissue target and proper PTV margins should be made to ensure target coverage, if necessary.

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