Teh Bin S, McGary John E, Dong Lei, Mai Wei-Yuan, Carpenter L Steve, Lu Hsin H, Chiu J Kam, Woo Shiao Y, Grant Walter H, Butler E Brian
Department of Radiology/Section of Radiation Oncology, Baylor College of Medicine, The Methodist Hospital, Houston, Texas, USA.
Cancer J. 2002 Nov-Dec;8(6):476-83. doi: 10.1097/00130404-200211000-00012.
The purpose of this study was to investigate the role of a rectal balloon for prostate immobilization and rectal toxicity reduction in patients receiving dose-escalated intensity-modulated radiotherapy for prostate cancer.
Patients with localized prostate cancer who were undergoing intensity-modulated radiotherapy were treated in a prone position, immobilized with a customized Vac-Lok bag (MED-TEC, Orange City, IA). A rectal balloon with 100 cc of air was used to immobilize the prostate. The prostate displacements were measured using computed tomography (CT)-CT fusion on 10 patients who received radioactive seed implant before intensity-modulated radiotherapy. They were scanned twice weekly during 5 weeks of intensity-modulated radiotherapy, and breathing studies were also performed. Rectal toxicity was evaluated by use of Radiation Therapy Oncology Group scoring in 100 patients. They were treated to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). Dose-volume histogram of the rectum was assessed. A film phantom was constructed to simulate the 4-cm diameter air cavity that was created by the rectal balloon. Kodak XV2 films (Rochester NY) were used to measure and compare dose distribution with and without the air cavity. A fraction of 1.25 Gy was delivered to the phantom at isocenter with 15-MV photons by use of the NOMOS Peacock system and the MIMiC treatment delivery system (Sewickley, PA).
The anterior-posterior and lateral prostate displacements were minimal, on the order of measurement uncertainty (approximately 1 mm). The standard deviation of superior-inferior displacement was 1.78 mm. Breathing studies showed no organ displacement during normal breathing when the rectal balloon was in place. The rectal toxicity profile was very favorable: 83% (83/100) patients had no rectal complaint, and 11% and 6% had grade 1 and 2 toxicity, respectively. Dose-volume histogram analysis revealed that in all of the patients, no more than 25% of the rectum received 70 Gy or greater. As visualized by film dosimetry, the dose at air-tissue interface was approximately 15% lower than that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, the differential was approximately 8% and 5%, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal, with or without the air cavity.
The use of a rectal balloon during intensity-modulated radiotherapy significantly reduces prostate motion. Prostate immobilization thus allows a safer and smaller planning target volume margin. It has also helped spare the anterior rectal wall (by its dosimetric effects) and reduced the rectal volume that received high-dose radiation (by rectal wall distension). All these factors may have further contributed to the decreased rectal toxicity achieved by intensity-modulated radiotherapy, despite dose escalation and higher-than-conventional fraction size.
本研究旨在探讨直肠球囊在接受剂量递增调强放疗的前列腺癌患者中对前列腺固定及降低直肠毒性的作用。
接受调强放疗的局限性前列腺癌患者采用俯卧位,使用定制的Vac-Lok袋(MED-TEC,奥兰治城,爱荷华州)进行固定。使用充有100 cc空气的直肠球囊固定前列腺。对10例在调强放疗前接受放射性粒子植入的患者,利用计算机断层扫描(CT)-CT融合技术测量前列腺位移。在调强放疗的5周期间,每周对他们进行两次扫描,并进行呼吸研究。采用放射肿瘤学组评分对100例患者的直肠毒性进行评估。他们接受的平均剂量为76 Gy,分35次给予(每次2.17 Gy)。评估直肠的剂量体积直方图。构建一个薄膜体模来模拟由直肠球囊形成的直径4 cm的空气腔。使用柯达XV2胶片(纽约罗切斯特)测量并比较有无空气腔时的剂量分布。使用NOMOS Peacock系统和MIMiC治疗投送系统(宾夕法尼亚州西威克利),在等中心以15-MV光子将1.25 Gy的剂量给予体模。
前列腺的前后向和侧向位移极小,在测量不确定度范围内(约1 mm)。上下向位移的标准差为1.78 mm。呼吸研究表明,当直肠球囊就位时,正常呼吸过程中没有器官位移。直肠毒性情况非常良好:83%(83/100)的患者无直肠不适,11%和6%的患者分别有1级和2级毒性。剂量体积直方图分析显示,所有患者中,不超过25%的直肠接受了70 Gy或更高剂量。通过胶片剂量测定法可见,空气-组织界面处的剂量比没有空气腔时低约15%。剂量迅速增加,因此在1 mm和2 mm深度处,差异分别约为8%和5%。有无空气腔时,前列腺后壁深度处的剂量覆盖基本相同。
在调强放疗期间使用直肠球囊可显著减少前列腺运动。因此,前列腺固定可使计划靶体积边缘更安全且更小。它还通过其剂量学效应保护了直肠前壁,并通过直肠壁扩张减少了接受高剂量辐射的直肠体积。尽管剂量增加且分次剂量高于传统剂量,但所有这些因素可能进一步导致调强放疗实现的直肠毒性降低。