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Clinical Implementation of MR-Guided Vaginal Cylinder Brachytherapy

A Owrangi

A Owrangi*, S Jolly , J Balter , Y Cao , L Young , T Zhu , J Prisciandaro , University of Michigan, Ann Arbor, MI


SU-E-T-366 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the accuracy of MR-based vaginal brachytherapy source localization using an in-house MR-visible marker versus the alignment of an applicator model to MR images.

Methods: Three consecutive patients undergoing vaginal HDR brachytherapy with a plastic cylinder were scanned with both CT and MRI (including T1- and T2-weighted images). An MR-visible source localization marker, consisting of a sealed thin catheter filled with either water (for T2 contrast) or Gd-doped water (for T1 contrast), was assembled shortly before scanning. Clinically, the applicator channel was digitized on CT with an x-ray marker. To evaluate the efficacy of MR-based applicator reconstruction, each MR image volume was aligned locally to the CT images based on the region containing the cylinder. Applicator digitization was performed on the MR images using (1) the MR visible marker and (2) alignment of an applicator surface model from Varian’s Brachytherapy Planning software to the MRI images. Resulting source positions were compared with the original CT digitization.

Results: Although the source path was visualized by the MR marker, the applicator tip proved difficult to identify due to challenges in achieving a watertight seal. This resulted in observed displacements of the catheter tip, at times >1cm. Deviations between the central source positions identified via aligning the applicator surface model to MR and using the x-ray marker on CT ranged from 0.07 – 0.19 cm and 0.07 – 0.20 cm on T1-weighted and T2-weighted images, respectively.

Conclusion: Based on the current study, aligning the applicator model to MRI provides a practical, current approach to perform MR-based brachytherapy planning. Further study is needed to produce catheters with reliably and reproducibly identifiable tips. Attempts are being made to improve catheter seals, as well as to increase the viscosity of the contrast material to decrease fluid mobility inside the catheter.

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