Zelefsky M J, Cohen G, Zakian K L, Dyke J, Koutcher J A, Hricak H, Schwartz L, Zaider M
Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Cancer J. 2000 Jul-Aug;6(4):249-55.
Recent studies have demonstrated that magnetic resonance spectroscopic imaging (MRSI) of the prostate may effectively distinguish between regions of cancer and normal prostatic epithelium. This diagnostic imaging tool takes advantage of the increased choline and creatine versus citrate ratio found in malignant, compared with normal, prostate tissue. The purpose of this report is to present our initial experience integrating MRSI data into an intraoperative computer-based optimization planning system for prostate cancer patients who underwent permanent interstitial I 125 implantation. The goal of this approach was to achieve dose escalation to intraprostatic tumor deposits on the basis of MRSI findings without exceeding the tolerance of adjacent normal tissue structures.
MRSI was obtained before surgery for four consecutive patients with clinically localized prostate cancer. The ratios of choline and citrate for the prostate were analyzed, and regions in which malignant cells were suspected to be present were identified. These ratios were calculated on a spatial grid overlying the axial MRSI of the prostate. MRSI coordinates containing these suspicious regions were registered to the intraoperative ultrasound images. A computer-based treatment planning system, which relied on a genetic algorithm, was used to determine the optimal seed distribution necessary to achieve maximal target volume coverage with the prescription dose and to maintain urethra and rectal doses within tolerance ranges. The treatment planning system was specifically designed to escalate the dose to MRS-positive voxels while at the same time achieving preferential sparing of surrounding normal tissues. Patients underwent transperineal interstitial implantation with I 125 by use of this intraoperatively generated plan. Postimplant computed tomographic scans were performed on the same day of the procedure in all cases, and dosimetric guidelines of the American Brachytherapy Society were used to assess implant quality.
Based on the postimplant computed tomographic evaluation, the intraoperative optimization treatment planning program was able to achieve a minimum dose of 139% to 192% of the 144-Gy prescription dose to the MRS-positive voxels. The percentage of the prostate volume receiving 100% of the prescription dose ranged from 92% to 97%, and the dose delivered to 90% of the target for the target volume ranged from 96% to 124%. Despite the dose escalation achieved for the positive voxels, the urethral and rectal doses were maintained within tolerance ranges. The average and maximal rectal doses ranged from 28% to 43% and 69% to 115% of the prescription dose, respectively. The average and maximal urethral doses ranged from 66% to 144% and 118% to 166% of the prescription dose, respectively.
Using this brachytherapy optimization system, we could demonstrate the feasibility of MRS-optimized dose distributions for I 125 permanent prostate implants. This approach may have an impact on the ability to select regions within the prostate to safely employ dose escalation for patients treated with permanent interstitial implantation and to improve outcome for patients with organ-confined prostatic cancers.
近期研究表明，前列腺磁共振波谱成像（MRSI）可有效区分癌灶区域与正常前列腺上皮。与正常前列腺组织相比，该诊断成像工具利用了恶性前列腺组织中胆碱和肌酸与枸橼酸盐比例升高这一特点。本报告旨在介绍我们将MRSI数据整合到术中基于计算机的优化计划系统中的初步经验，该系统用于接受永久性间质I 125植入的前列腺癌患者。此方法的目标是根据MRSI结果实现前列腺内肿瘤灶的剂量递增，同时不超过相邻正常组织结构的耐受量。
对4例临床局限性前列腺癌患者在手术前进行MRSI检查。分析前列腺的胆碱与枸橼酸盐比例，确定疑似存在恶性细胞的区域。这些比例在覆盖前列腺轴向MRSI的空间网格上计算得出。将包含这些可疑区域的MRSI坐标与术中超声图像配准。使用基于遗传算法的计算机治疗计划系统，确定实现最大靶区体积覆盖并将尿道和直肠剂量维持在耐受范围内所需的最佳籽源分布。该治疗计划系统专门设计用于增加对MRS阳性体素的剂量，同时优先保护周围正常组织。患者通过使用术中生成的计划接受经会阴I 125间质植入。所有病例均在手术当天进行植入后计算机断层扫描，并采用美国近距离放射治疗学会的剂量学指南评估植入质量。
根据植入后计算机断层扫描评估，术中优化治疗计划程序能够使MRS阳性体素接受的剂量达到144 Gy处方剂量的139%至192%。接受100%处方剂量的前列腺体积百分比范围为92%至97%，靶区体积中90%的靶区接受的剂量范围为96%至124%。尽管对阳性体素实现了剂量递增，但尿道和直肠剂量仍维持在耐受范围内。直肠平均剂量和最大剂量分别为处方剂量的28%至43%和69%至115%。尿道平均剂量和最大剂量分别为处方剂量的66%至144%和118%至166%。
使用这种近距离放射治疗优化系统，我们能够证明MRS优化剂量分布用于I 125永久性前列腺植入的可行性。这种方法可能会影响为接受永久性间质植入的患者选择前列腺内安全进行剂量递增的区域的能力，并改善器官局限性前列腺癌患者的治疗效果。
