Quantifying 125I placement accuracy in prostate brachytherapy using postimplant transrectal ultrasound images.

PURPOSE: The quality of a prostate brachytherapy implant depends on the accurate placement of sources. This study quantifies the misplacement of 125I sources from the intended location using intraoperative ultrasound images. METHODS AND MATERIALS: 125I sources were manually identified in the postimplant ultrasound images and compared to the preoperative 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. RESULTS: A total of 1619 125I 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. midgland vs. apex identified the greatest difference between the base and midgland by a factor of 1.29 (p < 0.0001). CONCLUSIONS: This study has identified significant misplacement of 125I 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.