Muren Ludvig Paul, Smaaland Rune, Dahl Olav
Department of Oncology and Medical Physics, Haukeland University Hospital, N-5021 Bergen, Norway.
Radiother Oncol. 2003 Dec;69(3):291-304. doi: 10.1016/s0167-8140(03)00246-9.
A major challenge in conformal radiotherapy of bladder cancer is to determine adequate treatment margins. For this purpose, we therefore quantified the internal motion of the urinary bladder as well as the external patient set-up variation during a course of fractionated radiotherapy. In the light of the recently introduced ICRU-62 concept, the planning organ at risk volume, we also studied the internal motion of nearby organs at risk, the rectum and intestine.
Weekly CT scans and electronic portal images (EPIs) were sampled from 20 patients during radical, conformal bladder irradiation (60-64 Gy/2 Gy in five fractions weekly). The planning scans were acquired with 70 ml of bladder contrast instilled, and patients were instructed to void before the treatment/repeat scanning sessions. Internal motion of the bladder, rectum and intestine was measured by 3-D image matching of the repeat scans to the patients' planning scans. Internal margins (CTV-to-ITV) were determined using both a direct empirical approach and an analytically derived margin recipe. The external patient set-up variability was determined by 2-D matching of front and lateral EPIs to corresponding digitally reconstructed radiographs.
A total of 149 CT scans (20 for planning, 129 during the treatment course) and 133 sets of EPIs were analysed. Bladder volumes were smaller during treatment than in the planning situation in 85% of the repeat scans. Nevertheless, we found the repeat scan bladder volumes to extend outside the planning scan bladder contours in 89% of the scans, on average with 9% of the volume (range: 0-47%). Eight patients (40%) had at least one repeat scan (25 scans in total) where displacements >15 mm were observed at one or more sides of the bladder. CTV-to-ITV margins of 10 mm inferior, 20 mm superior, 11 mm left, 8 mm right, 20 mm anterior and 14 mm posterior were required to simultaneously encompass all bladder deflections except for the largest outward deflection in all directions in 84% of the patients. Including patient set-up variation (CTV-to-PTV), we found that an additional safety margin of 2-6 mm had to be added in the various directions. The rectum expanded outside the planning contours in all repeat scans, on average with 24% of the volume (range: 2-69%). The volume of intestine found close to the bladder were significantly and negatively correlated to the bladder volume in almost half of the patients.
This study documented both a large internal motion of the bladder and a substantial patient set-up variation. Our current treatment margins have been adjusted according to the findings of this study. Considerable variation in position and volume of the rectum and intestine was also documented.
膀胱癌适形放疗中的一个主要挑战是确定合适的治疗边界。因此，为了实现这一目的，我们对分次放疗过程中膀胱的内部运动以及患者的外部摆位变化进行了量化。鉴于最近引入的国际辐射单位与测量委员会（ICRU）-62概念，即计划危及器官体积，我们还研究了附近危及器官（直肠和肠道）的内部运动。
从20例接受根治性适形膀胱照射（每周5次，每次2 Gy，总量60 - 64 Gy）的患者中采集每周的CT扫描图像和电子射野图像（EPI）。计划扫描时向膀胱内注入70 ml造影剂，并且指导患者在治疗/重复扫描前排尿。通过将重复扫描图像与患者的计划扫描图像进行三维图像匹配来测量膀胱、直肠和肠道的内部运动。使用直接经验法和分析得出的边界公式来确定内部边界（临床靶区CTV到内部靶区ITV）。通过将前后和侧位EPI与相应的数字重建放射图像进行二维匹配来确定患者的外部摆位变异性。
共分析了149次CT扫描（20次用于计划，129次在治疗过程中）和133组EPI。在85%的重复扫描中，治疗期间的膀胱体积小于计划扫描时的体积。然而，我们发现89%的扫描中重复扫描的膀胱体积超出了计划扫描的膀胱轮廓，平均超出9%（范围：0 - 47%）。8例患者（40%）至少有一次重复扫描（总共25次扫描），在膀胱的一侧或多侧观察到位移>15 mm。对于84%的患者，需要在下方10 mm、上方20 mm、左侧11 mm、右侧8 mm、前方20 mm和后方14 mm的CTV到ITV边界，以便同时涵盖除所有方向上最大向外位移之外的所有膀胱偏移。包括患者摆位变化（CTV到计划靶区PTV），我们发现在各个方向上必须额外增加2 - 6 mm的安全边界。在所有重复扫描中，直肠体积超出计划轮廓，平均超出24%（范围：2 - 69%）。在近一半的患者中，发现靠近膀胱的肠道体积与膀胱体积呈显著负相关。
本研究记录了膀胱的大幅内部运动和患者显著的摆位变化。我们目前的治疗边界已根据本研究结果进行了调整。还记录了直肠和肠道在位置和体积上的相当大的变化。
