TA13-Recent Advances in Radiotherapy Treatment Techniques
By David Huang, PhD, DABR
Chief Physicist
Memorial Sloan-Kettering Cancer Center @ Mercy Medical Center
It is well accepted that local tumor control and normal tissue complications have sigmoidally shaped dose-response curves. The success of radiotherapy is highly depends on the radiosensitivity of the particular tumor being treated relative to that of the surrounding normal tissues. For tumor sites where the tumor control curve is less steep than the normal tissue complication curve, the high doses required for tumor cure may cause unacceptable normal tissue complications. The goal of radiotherapy is to sufficiently separate the dose-response curves of local tumor control and normal tissues complication, and also the total volume of normal tissue irradiated.
During the past two decades, advances in radiological imaging and computer technology have significantly enhanced our ability to achieve this goal through the development of three-dimensional image-based conformal therapy (3DCRT). Intensity-modulated radiation therapy (IMRT) is an especially advanced method of 3DCRT that utilizes sophisticated computer controlled radiation beam delivery to improve the conformality of the dose distribution to the shape of the tumor. This is achieved by varying beam intensities within each beam portal, as opposed to uniform beam intensities as in conventional 3DCRT. IMRT usually also incorporates computerized inverse treatment plan (ITP) optimization as opposed to the manual optimization techniques of conventional 3DCRT. Both 3DCRT and IMRT utilize sophisticated strategies for patient immobilization and positioning, image-guided treatment planning, and computer enhanced treatment verification. In the heart of these techniques is advanced computer technology and 3D patient imaging with CT, MR and/or PET.
In the radiosurgery community, they are also benefited from the advances in radiological imaging and computer technology and robotic technology. The Linac based radiosurgery moves from cone-collimator system to mini-MLC system which makes the prescribed dose more conformal to the target volume while sparing more surrounding normal tissues. For Gamma Knife, the newer APS (Automatic Positioning System) make it possible to use smaller helmet sizes with larger number of shots. This approach does improve the dose conformality with reasonable treatment time. Besides, the marriage of Robotic technology and imaging technology produces a unique machine, Cyberknife, for Sterotactic Radiosurgery/Radiotherapy field. To some extents, Cyberknife is a “true” Image-guided-radiotherapy (IGRT) machine.
Brachytherapy has advantages to place the radiation source directly to the tumor and to have shorter treatment course as compared to the external beam therapy. In addition to imaging improvement such as 3D ultrasound image, the introduction of robotic technology may improve the precise execution and optimization of entry ports for the needles. It makes seed implant procedure for prostate cancer more accuracy and more acceptable. And, hopefully, it will improve the outcome of treatment as well.
In my talks, besides the clinic application of IMRT techniques for various sites, I will address the advances in radiotherapy treatment techniques in the fields of Sterotactic Radiosurgery/Radiotherapy and prostate seed implant procedure. I would also like to cover the newer treatment unit for radiotherapy, such as Cyberknife and Tomotherapy, in my talk.
Indeed, IMRT and other advanced treatment techniques in radiotherapy are great techniques for radiotherapy; however, we should not neglect the importance of comprehensive QA procedure for these new techniques. Without a comprehensive QA program, from image acquisition, treatment plan design, treatment delivery, to outcome follow-up, patient may not be benefited from these techniques. Besides, there are some limitations for these techniques which one should realize when implementing these advanced techniques in clinic. Furthermore, to make these techniques better for the cancer patients, future developments/improvements are needed for them. The developments/improvements include, but not limit to, target delineation, organ motion, plan biological evaluation model etc.
TA13－放疗技术进展
Chief Physicist
Memorial Sloan-Kettering Cancer Center @ Mercy Medical Center
已经有证据表明肿瘤局部控制和正常组织并发症的剂量反应曲线为S形。肿瘤的放射治疗能否成功很大程度上取决于治疗肿瘤和周围正常组织的放射敏感性。如果肿瘤局部控制的剂量反应曲线没有正常组织并发症的剂量反应曲线陡，肿瘤治愈所需要的高剂量可能会引起不能接受的正常组织并发症。放射治疗的目的是将肿瘤局部控制和正常组织并发症的剂量反应曲线很好地分开，同时也将照射正常组织的总体积分开。
在过去的20多年里，影像学和计算机技术的发展使我们有能力通过开展基于影像的三维适形放射治疗（3DCRT）来达到这一目的。调强放射治疗（IMRT）是一种更先进的三维适形放射治疗（3DCRT），它通过使用计算机控制的先进治疗方式来改善剂量分布和肿瘤形状的适合度。它通过改变每个射野内的射束强度来实现，而不是象常规三维适形放射治疗每个射野内使用均匀的射束强度。与常规三维适形放射治疗中使用手动优化技术不同，调强放射治疗通常使用计算机化的逆向治疗计划（ITP）优化。三维适形放射治疗和调强放射治疗都使用先进的病人固定、定位技术、图像引导的治疗计划和计算机化的治疗验证。这些技术的核心是计算机技术和CT、MR和/或PET等三维成像技术。
立体定向放射外科技术也得益于影像学、计算机技术和机器人技术的发展。基于直线加速器的放射外科不使用锥形准直器系统，而是使用微型多叶准直器来使处方剂量与靶区更加适形，同时保护周围正常组织。对于伽玛刀，新的自动定位系统（APS）可以使头盔更小，实施更多的弧形照射。这种方法确实在合理的时间内改善了剂量分布的适形度。另外，实现机器人技术和影像技术完美结合的赛博刀可以用于立体定向放射外科或立体定向放射治疗。在某种程度上说，赛博刀是真正意义上的图像引导的放射治疗机。
近距离治疗的优点是可以将放射源直接放到肿瘤内或肿瘤附近，并且其治疗时间比外照射的治疗时间短。除了三维超声等三维影像技术的改善外，通过使用机器人技术可以改善插植精度和优化插植方式。这样可以使前列腺癌的粒子植入更加准确，更能够被医务人员和病人所接受，并且很可能可以改善治疗效果。
在本报告中，除了讲述几种肿瘤调强放射治疗的临床应用外，还将提及立体定向放射外科或立体定向放射治疗、前列腺粒子植入等方面的放射治疗技术进展。另外，本报告还将涉及关于新的放射治疗机（比如赛博刀和断层治疗机）方面的内容。
的确，调强放射治疗和其他先进技术是放射治疗中重大的技术进展，但对于这些新技术，我们不能忽略综合质量保证的重要性。只有实施了综合的质量保证（从图像采集、治疗计划设计、治疗实施到病人随访），病人才能从这些技术中获益。另外在临床使用这些先进技术时要注意这些技术的缺点和限制。而且，为了使这些技术更好地为肿瘤病人服务，需要对其进行进一步的开发和完善。其中包括（但不限于）靶区勾画、器官运动和计划评估的生物学模型等。