Systematic analysis of biological and physical limitations of proton beam range verification with offline PET/CT scans
Systematic analysis of biological and physical limitations of proton beam range verification with offline PET/CT scans
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DOI：
10.1088/0031-9155/54/14/008
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发表时间：
2009-07-21
影响因子：
3.5
通讯作者：
Paganetti, H.
中科院分区：
文献类型：
--
作者：
Knopf, A.;Parodi, K.;Paganetti, H.
目前，麻省总医院（MGH）正在研究脱机正电子发射断层扫描/计算机断层扫描（PET/CT）用于质子范围验证的临床应用。通过比较患者在接受质子辐照后获得的测量活度分布与相应的蒙特卡罗（MC）模拟分布来实现验证。测量值和模拟活度分布之间的偏差可以反映从计划到交付的处理链中的错误，也可以由离线PET/CT验证方法的各种固有挑战引起。我们进行了系统分析，以评估以下方面对离线PET/CT方法的可行性和准确性的影响：(1)生物冲洗过程，(2)患者运动，(3)基于Hounsfield单元（HU）的组织分类，用于模拟活动分布，(4)肿瘤部位特异性方面。结果表明，活性分布的空间重现性在1 mm以内。然而，范围验证的可行性仅限于有限数量的位置和肿瘤部位。在质子束在软组织中停止的位置，冲刷效应导致测量范围和模拟范围之间的差异约为4毫米。在腹部盆腔肿瘤病例中，运动导致测量和模拟活动分布之间的空间偏差高达3cm。在这些后来的情况下，MC模拟的活度分布在绝对值上被限制在约35%的精度，在空间精度上被限制在约2mm，这取决于HU与辐照组织的物理和生物参数的相关性。此外，对于进一步特定的肿瘤位置，束的排列、刚性共配准和器官运动的有限精度会阻碍PET/CT范围验证的成功。所有提到的因素解释了为什么质子束范围只能在头颈部患者1-2毫米的低灌注骨结构的精度范围内得到验证，而在这种情况下，主要骨解剖结构的精确共同登记是可能的，如前所述。然而，当前方法的大多数限制是可以克服的。通过实施技术和方法的改进，如使用室内PET扫描仪，PET测量可以很快用于临床常规的质子范围验证。
The clinical use of offline positron emission tomography/computed tomography (PET/CT) scans for proton range verification is currently under investigation at the Massachusetts General Hospital (MGH). Validation is achieved by comparing measured activity distributions, acquired in patients after receiving one fraction of proton irradiation, with corresponding Monte Carlo (MC) simulated distributions. Deviations between measured and simulated activity distributions can either reflect errors during the treatment chain from planning to delivery or they can be caused by various inherent challenges of the offline PET/CT verification method. We performed a systematic analysis to assess the impact of the following aspects on the feasibility and accuracy of the offline PET/CT method: (1) biological washout processes, (2) patient motion, (3) Hounsfield unit (HU) based tissue classification for the simulation of the activity distributions and (4) tumor site specific aspects. It was found that the spatial reproducibility of the measured activity distributions is within 1 mm. However, the feasibility of range verification is restricted to a limited amount of positions and tumor sites. Washout effects introduce discrepancies between the measured and simulated ranges of about 4 mm at positions where the proton beam stops in soft tissue. Motion causes spatial deviations of up to 3 cm between measured and simulated activity distributions in abdominopelvic tumor cases. In these later cases, the MC simulated activity distributions were found to be limited to about 35% accuracy in absolute values and about 2 mm in spatial accuracy depending on the correlativity of HU into the physical and biological parameters of the irradiated tissue. Besides, for further specific tumor locations, the beam arrangement, the limited accuracy of rigid co-registration and organ movements can prevent the success of PET/CT range verification. All the addressed factors explain why the proton beam range can only be verified within an accuracy of 1-2 mm inlow-perfused bony structures of head and neck patients for which an accurate co-registration of predominant bony anatomy is possible, as shown previously. However, most of the limitations of the current approach are conquerable. By implementing technological and methodological improvements like the use of in-room PET scanners, PET measurements could soon be used to provide proton range verification in clinical routine.