Purpose: The objective of this study was to develop and
validate an open-source module for MRI to transrectal ultrasound (TRUS)
registration to support tumor-targeted prostate brachytherapy.

Methods and Materials: In this study, fifteen patients with prostate
cancer lesions visible on multiparametric MRI were selected for the
validation. T2-weighted images with 1 mm isotropic voxel size and
diffusion weighted images were acquired on a 1.5T Siemens. Three
dimensional (3D) TRUS images with 0.5 mm slice thickness were acquired.
The investigated registration module was incorporated in the open-source
3D Slicer platform and can compute rigid and deformable transformations.
An extension of 3D Slicer, SlicerRT, allows import and export to DICOM-RT
formats. For validation, similarity indices, prostate volumes and
centroid positions were determined in addition to registration errors for
common 3D points identified by an experienced radiation oncologist.

Results: The average time to compute the registration was 35±3 seconds.
For the deformable and rigid registration respectively, Dice similarity
coefficients were 0.93±0.01 and 0.87±0.05 while the 95% Hausdorff
distance was 2.2±0.3 and 4.2±1.0 mm. MRI volumes obtained after the rigid
and deformable registration were not statistically different (p>0.05)
from reference TRUS volumes. For the deformable and rigid registration
respectively, 3D distance error between centroid positions were 0.4±0.1
and 2.1±1.0 mm while registration errors between common points were
2.3±1.1 and 3.5±3.2 mm. Deformable registration was found significantly
better (p<0.05) than rigid registration in all parameters.

Conclusions: An open-source MRI to TRUS registration platform was
validated for integration in the brachytherapy workflow.