Water Equivalent Calorimeter for Quality Assurance in Proton Beam Therapy

现代癌症治疗在很大程度上是三种技术的结合：手术、化疗和放射治疗。放射治疗使用X射线束从许多不同的方向照射肿瘤。其效果是通过在肿瘤中沉积尽可能多的辐射剂量来杀死癌症，同时将对周围区域的辐射剂量降至最低，以节省健康组织。质子疗法是一种更精确的放射治疗形式，比传统的X射线放射治疗有显著的好处。质子失去能量--因此将其剂量储存在体内一个小得多的区域--使治疗变得更加精确：这导致了更有效的癌症治疗，癌症复发的可能性更小。这在治疗头部、颈部和中枢神经系统的深层肿瘤方面尤其重要，特别是对身体仍在发育中、特别容易受到长期辐射损害的儿童。质子治疗的优势加上设备成本的降低，导致世界各地对质子治疗的兴趣激增：现在有20多个中心，这个数字将在未来十年每3年翻一番。英国目前正在建设两个全尺寸的质子治疗中心，分别设在伦敦的大学学院医院和曼彻斯特的克里斯蒂，由英国国家医疗服务体系资助。这些治疗将为更广泛的癌症提供治疗，使更多的患者能够在离家更近的地方接受治疗。治疗这些癌症需要比传统放射治疗系统复杂得多的机械设备。质子被粒子加速器加速到合适的能量以进行处理：一旦束流离开加速器，它就必须通过一系列转向和聚焦磁铁被输送到几米外的治疗室。当质子束到达治疗室时，它必须通过龙门送到正确的位置。质子治疗机架非常巨大--超过3层楼高，重量超过100吨--必须绕着患者旋转，才能从任何角度以毫米级的精度发射光束。为了确保使用这种复杂机械的治疗是安全进行的，每天在治疗开始之前都会进行一系列的质量保证(QA)程序。这段时间的大部分时间都被用来验证质子束是否到达了正确的深度，并以几种不同的能量进行：质子在一个类似人体组织的塑料块中被计算在不同的深度。这些质子射程的QA测量需要相当长的时间来设置和调整不同的能量：整个过程可能需要一个多小时。这个项目的重点是开发一种探测器，可以比现有系统更快、更准确地测量质子射程。探测器由一层层塑料闪烁体建造而成--塑料闪烁体是为超NEMO高能物理实验开发的--看起来就像一块切成片的面包。通过闪烁体堆叠的质子在每一层中储存能量，这些能量被转化为光：通过记录来自每一层的光，可以测量质子沿其路径储存的能量的量。这样的系统提供了对组织中质子范围的直接测量，因为塑料的吸收几乎与人体组织相同。因此，对多个能量的质子射程的测量将允许在几分钟内完成完整的早晨能量QA程序，精度不到一毫米。在英国的两个中心，这将转化为每天能够额外治疗12名患者。探测器技术已经在克莱特布里奇质子治疗中心进行了成功的测试：下一步是建造一个分层探测器的原型，并证明它可以以所需的分辨率进行测量。

Modern cancer treatment is largely a combination of 3 techniques: surgery, chemotherapy and radiotherapy. Radiotherapy uses beams of X-rays to irradiate the tumour from many different directions. The effect is to kill the cancer by depositing as much radiation dose in the tumour as possible, whilst minimising the dose to the surrounding area to spare healthy tissue.Proton therapy is a more precise form of radiotherapy that provides significant benefits over conventional X-ray radiotherapy. Protons lose energy - and therefore deposit their dose - in a much smaller region within the body, making the treatment much more precise: this leads to a more effective cancer treatment with a smaller chance of the cancer recurring. This is particularly important in the treatment of deep-lying tumours in the head, neck and central nervous system, particularly for children whose bodies are still developing and are particularly vulnerable to long-term radiation damage. The advantages of proton therapy, coupled to the reduced cost of the equipment, has led to a surge in interest in proton therapy treatment worldwide: there are now over 20 centres, with this number set to double every 3 years over the next decade. The UK is currently constructing 2 full-sized proton therapy centres, to be based at University College Hospital in London and The Christie in Manchester and funded by the NHS. These will provide treatment for a much wider range of cancers, allowing more patients to be treated closer to home.Treating these cancers requires machinery that is significantly more complex than a conventional radiotherapy system. Protons are accelerated to the right energy for treatment by a particle accelerator: once the beam leaves the accelerator, it then has to be transported to the treatment rooms many metres away by a series of steering and focussing magnets. When the proton beam reaches the treatment room, it has to be delivered through a gantry to the correct place. Proton therapy gantries are enormous - more than 3 storeys tall and weighing more than a hundred tonnes - and have to rotate around the patient to deliver the beam from any angle with millimetre precision. In order to ensure that treatment with such complex machinery is carried out safely, a range of quality assurance (QA) procedures are carried out each day before treatment starts. The majority of this time is spent verifying that the proton beam travels the correct depth and is carried out for several different energies: protons are counted at different depths in a plastic block that resembles human tissue. These QA measurements of the proton range take significant time to set up and adjust for different energies: the full procedure can take over an hour.The focus of this project is to develop a detector that can make faster and more accurate measurements of the proton range than existing systems. The detector is built from layers of plastic scintillator - that have been developed for the SuperNEMO high energy physics experiment - and resembles a sliced loaf of broad. Protons passing through this scintillator stack deposit energy in each layer which is converted into light: by recording the light from each layer, the amount of energy the protons deposit along their path can be measured. Such a system provides a direct measurement of the range of protons in tissue, since the absorption of the plastic is virtually identical to human tissue. As such, a measurement of the proton range for multiple energies would allow the complete morning energy QA procedure to be carried out in a few minutes, with an accuracy of less than a millimetre. At the two UK centres, this would translate into being able to treat an extra 12 patients every single day. The detector technology has already been tested successfully at the Clatterbridge proton therapy centre: the next step is to build a prototype of the layered detector and show that it can make measurements with the required resolution.