Quantifying Iodine-125 Placement Accuracy In Prostate Brachytherapy Using Post-Implant Transrectal Ultrasound Images

PURPOSE: The quality of a prostate brachytherapy implant depends on the accurate placement of sources. This study quantifies the misplacement of iodine-125 sources from the intended location using intra-operative ultrasound images. METHOD AND MATERIALS Iodine-125 sources were manually identified in the post-implant ultrasound images and compared to the pre-operative plan. Due to the subjective nature of the identifying sources, only sources identified with high confidence were included in the analysis. Misplacements from the original intended coordinate were measured along the X, Y and Z-axes and were stratified between overall misplacements and regions of the prostate gland. RESULT 1619 iodine-125 sources using 357 strands were implanted in 15 patients’ prostate glands, with 1197 (74%) confidently identified for misplacement analysis. The overall mean displacement was 0.49 cm and in the X, Y and Z direction was 0.13, 0.15 and 0.38 cm respectively. Greater source misplacement occurred in the anterior part of the prostate gland than the posterior part of the prostate gland by a factor 1.33 (p<0.0001). Comparing sources in the lateral vs. medial regions of the prostate, no statistically significant differences on source misplacement were observed. Comparing misplacement in the base vs. mid-gland vs. apex identified the greatest difference between the base and mid-gland by a factor of 1.29 (p<0.0001). CONCLUSIONS This study has identified significant misplacement of iodine-125 sources from their intended locations with the greatest error misplacement occurring in the Z direction. Source misplacement tends to occur more commonly in the anterior gland and in the base of the prostate.