Intra-operative guidance in Transrectal Ultrasound (TRUS) guided prostate brachytherapy requires localization of inserted radioactive seeds relative to the prostate Seeds were reconstructed using a typical C-arm, and exported to a commercial brachytherapy system for dosimetry analysis Technical obstacles for 3D reconstruction on a non-isocentric C-arm included pose-dependent C-arm calibration; distortion correction; pose estimation of C-arm images; seed reconstruction;, C-arm to TRUS registration In precision-machined hard phantoms with 40-100 seeds, we correctly reconstructed 99 8% seeds with a mean 3D accuracy of 0 68 mm In soft tissue phantoms with 45-87 seeds, clinically realistic 15 degrees C-arm motion, we correctly reconstructed 100% seeds with an accuracy of 1 3 mm The reconstructed 3D seed positions were then registered to the prostate segmented from TRUS In a Phase-1 clinical trial, so far on 4 patients with 66-84 seeds, we achieved intra-operative monitoring of seed distribution, dosimetry We optimized the 100% prescribed iso-dose contour by inserting an average of 3 75 additional seeds, making intra-operative dosimetry possible on a typical C-arm, at negligible additional cost to the existing clinical installation