Prostate brachytherapy postimplant dosimetry: automatic plan reconstruction of stranded implants

Plan reconstruction for permanent implant prostate brachytherapy is the process of determining the correspondence between planned and implanted seeds in postimplant analysis. Plan reconstruction informs many areas of brachytherapy quality assurance, including the verification of seed segmentation, misplacement and migration assessment, implant simulations, and the dosimetry of mixed-activity or mixed-species implants.

An algorithm has been developed for stranded implants which uses the interseed spacing constraints imposed by the suture to improve the accuracy of reconstruction. Seventy randomly selected clinical cases with a mean of 23.6 (range 18-30) needles and mean density of 2.0 (range 1.6-2.6) 2.0 (range 1.6-2.6) seeds/cm3 were automatically reconstructed and the accuracy compared to manual reconstructions performed using a custom 3D graphical interface.

Using the automatic algorithm, the mean accuracy of the assignment relative to manual reconstruction was found to be 97.7 +/- 0.5%. Fifty-two of the 70 cases (74%) were error-free; of seeds in the remaining cases, 96.7 +/- 0.3% were found to be attributed to the correct strand and 97.0 +/-0.3% were correctly connected to their neighbors. Any necessary manual correction using the interface is usually straightforward. For the clinical data set tested, neither the number of seeds or needles, average density, nor the presence of clusters was found to have an effect on reconstruction accuracy using this method.

Routine plan reconstruction of stranded implants can be performed with a high degree of accuracy to support postimplant dosimetry and quality analyses.