Moerland M A, Wijrdeman H K, Beersma R, Bakker C J, Battermann J J
Department of Radiotherapy, University Hospital Utrecht, The Netherlands.
Int J Radiat Oncol Biol Phys. 1997 Mar 1;37(4):927-33. doi: 10.1016/s0360-3016(96)00575-5.
The aim of this study is the evaluation of permanent I-125 prostate implants using radiography and magnetic resonance imaging (MRI).
Twenty-one patients underwent radiography on the simulator and MRI within 3 days after implantation of the I-125 seeds. Isocentric radiographs were used for reconstruction of the seed distribution, after which registration with the seed-induced signal voids on MRI provided the seed positions in relation to the prostate. The prostate was contoured on the transversal magnetic resonance images, and dose-volume histograms were computed to evaluate the implants. The validity of the ellipsoidal prostate volume approximation, as applied in preimplant dose calculation, was assessed by comparison of ellipsoidal volumes given by prostate width, height, and length and prostate volumes obtained by a slice-by-slice contouring method, both on postimplant MRI. Prostate volume changes due to postimplant prostate swelling were assessed from radiographs taken at 3 days and 1 month after the implantation.
The seeds were readily identified on T1-weighted spin-echo images and matched with the seed distribution reconstructed from the isocentric radiographs. The matching error, averaged over 21 patients, amounted to 1.8 +/- 0.4 mm (mean +/- standard deviation). The fractions of the prostate volumes receiving the prescribed matched peripheral dose (MPD) ranged from 32 to 71% (mean +/- standard deviation: 60 +/- 10%). Prostate volumes, obtained by the contouring method on postimplant MRI, were a factor 1.5 +/- 0.3 larger than the ellipsoidal volumes given by the prostate dimensions on postimplant MRI. Prostate volumes 3 days after the implantation were a factor 1.3 +/- 0.2 larger than the prostate volumes 1 month after the implantation. Registration of the reconstructed seed distribution and the MR images showed inaccuracies in seed placement, for example, two or more seeds clustering together or seeds outside the prostate.
Registration of the reconstructed seed distribution and the MR images enabled evaluation of target coverage, which amounted to 60 +/- 10%. The discrepancy between prescribed dose and realized dose was caused by underestimation of the preimplant prostate volume due to the ellipsoidal approximation, postimplant prostate swelling at the time of evaluation, and inaccuracies in seed placement.
本研究旨在利用X线摄影和磁共振成像（MRI）对永久性I-125前列腺植入物进行评估。
21例患者在植入I-125种子源后3天内接受了模拟定位机X线摄影及MRI检查。利用等中心X线片重建种子源分布，之后通过与MRI上种子源所致信号缺失进行配准，确定种子源相对于前列腺的位置。在横向磁共振图像上勾勒出前列腺轮廓，并计算剂量体积直方图以评估植入情况。通过比较植入后MRI上前列腺宽度、高度和长度得出的椭圆体体积与逐片勾勒法获得的前列腺体积，评估植入前剂量计算中应用的椭圆体前列腺体积近似值的有效性。根据植入后3天和1个月拍摄的X线片评估植入后前列腺肿胀导致的前列腺体积变化。
种子源在T1加权自旋回波图像上易于识别，并与从等中心X线片重建的种子源分布相匹配。21例患者的平均匹配误差为1.8±0.4 mm（平均值±标准差）。接受规定的匹配周边剂量（MPD）的前列腺体积分数范围为32%至71%（平均值±标准差：60±10%）。通过植入后MRI上的勾勒法获得的前列腺体积比植入后MRI上前列腺尺寸得出的椭圆体体积大1.5±0.3倍。植入后3天的前列腺体积比植入后1个月的前列腺体积大1.3±0.2倍。重建的种子源分布与MR图像的配准显示种子源放置存在不准确之处，例如，两个或更多种子聚集在一起或种子位于前列腺外。
重建的种子源分布与MR图像的配准能够评估靶区覆盖情况，其为60±10%。规定剂量与实际剂量之间的差异是由于椭圆体近似导致植入前前列腺体积估计不足、评估时植入后前列腺肿胀以及种子源放置不准确所致。
