3D catheter reconstruction in HDR prostate brachytherapy for pre-treatment verification using a flat panel detector

Purpose High dose rate prostate brachytherapy is a widely-practiced treatment, delivering large conformal doses in relatively few treatment fractions. Inter- and intra-fraction catheter displacements have been reported. Unrecognized displacement can have a significant impact on dosimetry. Knowledge of the implant geometry at the time of treatment is important for ensuring safe and effective treatment. In this work we demonstrate a method to reconstruct the catheter positions pre-treatment, using a 'shift' imaging technique, and perform registration with the treatment plan for verification relative to the prostate. Methods Two oblique 'shift' images were acquired of a phantom containing brachytherapy catheters, representing the patient immediately pre-treatment. Using a back projection approach, the catheter paths were reconstructed in 3D and registered with the planned catheter paths. The robustness of the reconstruction and registration process was investigated as a function of phantom rotation. Catheter displacement detection was performed and compared to known applied displacements. Results Reconstruction of the implant geometry in 3D immediately prior to treatment was achieved. A mean reconstruction uncertainty of 0.8 mm was determined for all catheters with a mean registration uncertainty of 0.5 mm. A catheter displacement detection threshold of 2.2 mm was demonstrated. Catheter displacements were all detected to within 0.5 mm of the applied displacements. Conclusion This technique is robust and sensitive to assess catheter displacements throughout the implant volume. This approach provides a method to detect, in 3D, changes in catheter positions relative to the prostate. The method has sufficient sensitivity to enable clinically significant decisions immediately prior to treatment delivery.