Purpose: To determine the extent of edge and gland position changes caused by needle insertion in patients undergoing prostate brachytherapy.
Methods and materials: Nineteen patients with T1-T3 prostate cancer were implanted with the real-time method by using a two-phase peripheral loading technique. Serial contours of the prostate at 5-mm intervals were acquired by the dose-planning system. All of the peripheral needles were then placed and spaced 5-10 mm apart by using the largest transverse ultrasound image as the reference plane. The position of the probe was relocated at the zero plane, and the difference between the preneedle and postneedle zero plane was recorded as the difference in the z axis. Axial ultrasound images were again acquired. The second set of captured images, which matched in number the first set, was contoured over the previously contoured preneedle images. Prostate gland deformation and displacement were determined by comparing the preneedle contoured image with the images captured after needle placement. Deformation was determined by calculating the differences between the edges of the gland as measured at the major axis of the gland (x and y planes). Displacement was determined by measuring the differences between the center positions of the two contoured structures. Deformation and displacement were determined on each acquired 5-mm image. Differences were compared by student's t test.
Results: The mean preneedle prostate volume was 47 ml (range, 21.5-68.7 ml), compared with 48.1 ml (range, 19.4-80.3 ml; p = 0.228) after peripheral needle placement. A median of 16 (range, 12-19) peripheral needles were placed. The median change in the base position of the prostate was 1.5 cm (range of 0 to 3.0 cm; p = 0.0034). The mean x and y deformation was 6.8 mm (median, 7.9 mm; range, 4.3-8.1 mm) and 3.6 mm (median, 3.3 mm; range, 1.0-5.5 mm), respectively. The greatest deformation for any individual slice for x was 21.6 mm and for y was 15.3 mm. The mean number of slices that were found with a >2-, 5-, and 10-mm deformation in the x axis was 7 (range, 3-10), 4 (range, 1-3), and 1 (range, 0-4), respectively. Similar deformation in the y axis was found in 6 (range, 3-10), 2.5 (range, 0-6), and 0.3 (range, 0-2) slices. The mean x and y displacement was 1.9 mm (median, 1.8 mm; range, 0.3-6.6 mm) and 2.8 mm (median, 1.9 mm; range, 2-5.8 mm). The greatest displacement for any individual slice for x was 7 mm and for y was 10 mm. The mean number of slices with a displacement >2, 5, and 10 mm in the x axis was 5 (range, 1-10), 0.8 (range, 0-5), and 0, respectively. Similar displacement in the y axis was found in 5 (range, 0-9), 1.7 (range, 0-7), and 0 slices, respectively.
Conclusions: Placing most needles in the periphery results in a minimal prostate volume increase, suggesting little need to overplan the implant when this method is used. However, significant edge and gland position changes caused by the needle insertion did occur. These changes may explain some of the difficulty in reproducing the preplan and should be taken into consideration for all types of prostate brachytherapy planning.