Room: Davidson Ballroom A
Purpose: MRI provides unparalleled soft-tissue contrast for contouring following permanent prostate brachytherapy (PPB), but seeds do not appear consistently, typically necessitating a CT scan for dose calculation. We propose a deformable registration algorithm to map implanted seeds reconstructed from x-rays to MRI in the presence of tissue deformation to eliminate the need for a CT scan for post-implant dosimetry.
Methods: Three-dimensional seed locations are reconstructed from only three x-ray images by solving a combinatorial optimization problem. MR images are pre-processed by geometric filtering to highlight signal voids produced by seeds, followed by distance map computation and Gaussian blurring to generate smooth candidate seed-only images. Reconstructed seeds are registered to these images using an affine points-to-volume registration followed by a novel deformation model. The deformation model is designed to minimize the distance between reconstructed seeds and signal voids while preserving relative seed positions in the absence of a corresponding signal void. Distances were calculated between algorithm results and seeds manually identifiable on MRI with the aid of the CT scan (25-45 seeds/patient).
Results: The proposed method was evaluated on 25 prostate cancer patients treated with PPB at our institution. Each patient had six x-rays taken immediately at the end of the implantation and T2-weighted MRI and CT taken one day after the implantation. MeanÂ±SD distance between the registered and identified seeds on MRI/CT was 1.7Â±1.1 mm after the affine registration, which improved to 1.4Â±0.9 mm following the deformable registration. The process took less than 1 min for seed reconstruction and 30 sec for registration.
Conclusion: The MRI+x-ray based algorithm eliminates the need for a CT scan for PPB post-implant dosimetry, demonstrating acceptable errors for clinical use. Ongoing work involves comparison of dose distributions in terms of dose to the prostate, rectum, and urethra.
Funding Support, Disclosures, and Conflict of Interest: A US patent (No. 8,233,686) has been filed for the fluoroscopy-based implant reconstruction algorithm.