In my mind I am free

14 Mar 2018 by Liangzhao

著名的物理学家和科普作家史蒂芬·霍金逝世,就在equation节这一天。因为运动神经元疾病(MND,motor neurone disease)ALS,霍金从22岁就开始被禁锢在自己的身体中,但是这并未成为他生命的巨大阻碍,他的生活非常丰富,研究、写书、讲座,参演电视节目、体验零重力,他说过,他的一生只有一个目标,那就是一个统一的宇宙理论,即宇宙从何而来,以及宇宙的未来。《霍金传》里,他的医生对他说,“因为神经元的信号无法传输给肌肉,你的肌肉会逐渐萎缩,即使像呼吸肌这样的自主神经元控制的肌肉,也会逐渐丧失功能”,“那么大脑呢?”年轻的霍金问医生,医生回答,“大脑完好无损”。这对霍金来说,是最大的好消息了。

ABC有一篇文章列举了他的很多Quotes,觉得很好,学习到很多他自己对世界的观察,列举如下。

We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the universe. That makes us something very special.

Elon musk也说过类似的话,对于一只猴子来说,我们可以把火箭推上天,将人送到月球,对宇宙有如此的深刻认识,应该算不错了。

Life would be tragic if it weren’t funny.

反过来说,一件事如果你自己不觉得有趣,就无法坚持做完,如果这个项目旷日持久,是很难坚持下去了的,取得很大成就的人,往往对所做的事情充满了热情,辛苦但不痛苦。

The great enemy of knowledge is not ignorance, it is the illusion of knowledge.

不要害怕说“我不知道”,而是要对“我知道了”保持谨慎,知识无涯,不要停下学习的脚步。

For millions of years, mankind lived just like the animals. Then something happened which unleashed the power of our imagination. We learned to talk and we learned to listen. Speech has allowed the communication of ideas, enabling human beings to work together to build the impossible. Mankind’s greatest achievements have come about by talking, and it’s greatest failures by not talking. It doesn’t have to be like this. Our greatest hopes could become reality in the future. With the technology at our disposal, the possibilities are unbounded. All we need to do is make sure we keep talking.

My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all.

I believe the simplest explanation is, there is no God. No one created the universe and no one directs our fate. This leads me to a profound realization that there probably is no heaven and no afterlife either. We have this one life to appreciate the grand design of the universe and for that, I am extremely grateful.

If aliens ever visit us, I think the outcome would be much ad when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.

对于外星人造访地球的看法,无异于美洲原住民见到哥伦布的船队。

We are in danger of destroying ourselves by our greed and stupidity. We cannot remain looking inwards at ourselves on a small and increasingly polluted and overcrowded planet.

My disabilities have not been a significant handicap in my field, which is theorectical physics. Indeed, they have helped me in a way by shielding me from leturing and administrative work that i would otherwise have been involved in. I have managed, however, only because of the large amount of help I have received from my wife, children, colleagues and students. I find that people in general are very ready to help, but you should encourage them to feel that their efforts to aid you are worthwhile by doing as well as you possibly can.

生活中无时不刻需要别人的援手,你的行为应该让别人对你的帮助变得值得。

Remember to look up at the stars and not down at your feet. Try to make sense of what you see and wonder about what makes the universe exist.Be curious. And however difficult life may seem, there is always something you can do and succeed at. It matters that you don’t just give up.

Intelligence is the ability to adapt to change.

智力是一种适应变化的能力。懂得多不意味着智力高,因为懂得多只是意味着记得东西多,计算机不是谁的硬盘大就好。只有不变的东西才需要记忆,变化的东西无法记忆,只能做分析和行动,就是适应能力——分析当前状态和应对策略,并执行。

We are all different, but we share the same human spirit.

My advice to other disabled people would be, concentrate on things your disability doesn’t prevent you doing well, and don’t regret the things it interferes with. Don’t be disabled in spirit, as well as physically.

So next time someone complains that you have made a mistake, tell him that may be a good thing. Because without imperfection, neither you nor I would exist.

BBC也列举了很多:

On Black holes:

Einstein was wrong when he said’God does not play dice’. Consideration of black holes suggests, not only that God does play dice, but that he somethimes confuses us by throwing them where they can’t be seen.

On life:

One, remember to look up at the stars and not down at your feet. Two, never give up work. Work gives you meaning and purpose and life is empty without it. Three, if you are lucky enough to find love, remember it is there and don’t throw it away.

On commercial success:

I want my books sold on airport bookstalls.

On God:

It is not necessary to invoke God to light the blue touch paper and set the universe going.

On euthanasia:

The victim should have the right to end his life, if he wants. But I think it would be a great mistake. However bad life may seem, there is always something you can do, and succeed at. While there’s life, there is hope.

On Artificial Intelligence:

The development of full artifical intelligence could spell the end of the human race. It would take off on its own, and redesign itself at an ever increasing rate. Humans, who are limited by slow biological evolution, couldn’t compete, and would be superseded.

On fame:

The downside of my celebrity is that I cannot go anywhere in the world without beiing recognised. It is not enough for me to wear dark sunglasses and a wig. The wheelchair gives me away.

On the possibility of contace between humans and aliens:

I think it would be a disaster. The extraterrestrials would probably be far in advance of us. The history of advanced races meeting more primitive people on this planet is not very happy, and they were the same species. I think we should keep our heads low.

On Space Colonies:

I don’t think the human race will survive the next 1000 years, unless we spread into space, There are too many accidents that can be befail life on a sigle planet. But I’m an optimist. We will reach out to the stars.

On being diagnosed with motor neurone disease:

My expectations were reduced to zero when I was 21. Everything since then has been a bonus.

On death:

I have lived with the prospect of an early death for the last 49 years. I’m not afraid of death, but I’m in no hurry to die. I have so much I want to do first.

Update:

霍金2008年在TED上有一个演讲《Questioning the universe》,也值得一看:

Transcript:

There is nothing bigger or older than the universe. The questions I would like to talk about are: one, where did we come from? How did the universe come into being? Are we alone in the universe? Is there alien life out there? What is the future of the human race?

Up until the 1920s, everyone thought the universe was essentially static and unchanging in time. Then it was discoverd that the universe was expanding. Distant galaxies were moving away from us. This meant they must have been closer together in the past. If we extrapolate back, we find we must have all been on top of each other abouth 15 billion years ago. This was the Big Bang, the begining of the universe.

But was there anything before the Big Bang? If not, what created the universe? Why did the universe emerge from the Big Bang the way it did? We used to think that the theory of the universe could be divided into two parts. First, there were the laws like Maxwell’s equations and general relativity that determined the evolution of the universe, given its state over all of space at one time. And second, there was no question of the inital state of the universe.

We have made good progress on the first part, and now have the knowledge of the laws of evolution in all but the most extreme conditions. But until recently, we have had little idea about the initial conditions for the universe. However, this division into laws of evolution and initial conditions depends on time and space being separate and distinct. Under extreme conditions, general relativity and quantum theory allow time to behave like dimension of space. This removes the distinction between time and space, and means the laws of evolution can also determine the initial state. The universe can spontaneously create itself out of nothing.

Moreover, we can calculate a probability that the universe was created in different states. These predictions are in excellent agreement with observations by the WMAP satellite of the cosmic microwave background, which is an imprint of the very early universe. We think we have solved the mystery of creation. Maybe we should patent the universe and charge everyone royalties for their existence.

I now turn to the second big question: are we alone, or is there other life in the universe? We believe that life arose spontaneously on the Earth, so it must be possible for life to appear on other suitable planets, of which there seem to ba a large number in the galaxy.

But we don’t know how life first appeared. We have two pieces of observational evidence on the probability of life appearing. The first is that we have fossils of algae from 3.5 billion years ago. The Earth was formed 4.6 billion years ago and was probably too hot for about the first half billion years. So life appeared on Earth within half a billion years of it being possible, which is short comopared to the 10-billion-year lifetime of a planet of Earth type. This suggests that the probability of life appearing is reasonably high. If it was very low, one would have expected it to take most of the ten billion years available.

On the other hand, we don’t seem to have been visited by aliens. I am discounting the reports of UFOs. Why would they appear only to cranks and weirdos? If there is a government conspiracy to suppress the reports and keep for itself the scientific knowledge the aliens bring, it seems to have been a sigularly ineffective policy so far. Furthermore, despite an extensive search by SETI projects, we haven’t heard any alien civilizations at our stage of development within a radius of a few hundred light years. Issuing an insurance policy against abduction by aliens seems a pretty safe bet.

The brings me to the last of the big questions: the future of the human race. If we are the only intelligent beings in the galaxy, we should make sure we survive and continue. But we are entering an increasingly dangerous period of our history. Our population and our use of the finit resources of planet Earth are growing exponentially, along with our technical ability to change the environment for good or ill. But our genetic code still carries the selfish and aggressive instincts that were of survival advantage in the past. It will be difficult enough to avoid disaster in the next hundred years, let alone the next thousand or million.

Out only chance of long-term survival is not to remain inward-looking on planet Earth, but to spread out into space. The answers to these big questions show that we have made remarkable progress in the last hundred years. But if we want to continue beyond the next hundred years, our future is in space. That is why I am in favor of manned – or should I say, personned space flight.

All of my life I have sought to understand the universe and find answers to these questions. I have been very lucky that my disability has not been a serious handicap. Indeed, it has probably given me more time than most people to pursue the quest for knowledge. The ultimate goal is a complete theory of the universe, and we are making good progress. Thank you for listening.


罗伯特·M·波西格

12 May 2017 by Liangzhao

4月24日的时候,写那本《禅与摩托车维修艺术》的罗伯特·M·波西格(Robert M. Prisig)去世了。书翻看过很多次,这本书虽然讨论的是哲学,却不无太多抽象得让人眩晕,这是此书的一大特点,他在书中穿插讲述了自己和儿子克里斯(Chris)横跨半个美国,骑着摩托车旅行的故事,因此看起来不会累。

zen

波西格早年经历过精神崩溃,受到了多次的电击治疗,我想,他也许是通过这次旅行和这本书,尝试着告别过去,“斐德罗”取自古希腊的人物名,但实则指的是先前的自己,这个斐德罗,在尝试统一古典和科学,主观和客观二元论的过程中发疯掉了,所以他就以第三者的视角去观察他。从这次旅行出发,通过摩托车这一具体的事物,给读者讲述一下自己对科技与古典,理性与感性的看法。

作者首先抛出了这样的问题,为什么科技给人带来一种疏离感?“这种态度不难理解,经过大城市的工业区时,你会看到整片所谓的科技区。门前围了高高的铁丝网,大门紧锁,告示牌上写着‘禁止跨越’。在一片污浊的空气之后,你看到的是奇形怪状而又肉楼的金属物和砖块,也不知用途为何。”, “它特殊的形状、外观还有神秘感,一切都在叫你‘滚开’”。

整个书都在围绕“良质”这个概念讲述,这个中文翻译过来的词,乍看起来很抽象,其实对应于英文的“Quality”一词,即“品质、特质、质量”,生活中肯定有这样的体验,一些人做的一些事总是有一些“美感”,这些人在做事情的时候,有很多的特质:比如专注、沉稳,那么做出来的工作显得富有质感,十分漂亮。如果你感受过这一点,那么至少能理解了这个词所传达的含义。

而且,这个词一个有趣的特征是:不要给它下定义。一旦下了定义,具体描述“良质”是什么,就会显得非常僵硬,作者说,它只可意会,不可言说。波西格并没有做具体描述,而是写了一本书,用“良质”承载,这也许他最初的用意,也是此书自70年代以来,畅销不衰的原因。

书中很多篇幅描述了旅行中所见到的风景,其中以爬山为主,却不仅仅是在描述爬山的过程:

实体的山往往能象征人们灵性长进的路。就好像那些在我们身后的山谷里的人们,大部分人望着灵性的高峰,但是医生从来不曾攀上过,只是听听别人的经验就已经很满足,而自己不愿意花费任何心血。有一些人则是靠着有经验的想到,他们知道最安全的路,因而能够很顺利地到达他们的目的地。但是还有另外一批人,不但没有经验,而且不太相信别人的经验,想要走出自己的路。其中很少有人能成功,但是总有一些靠着自己的意志、运气还有上天的恩典而做到了。

又说道:

爬山必须尽可能地少费力,不要存有任何妄想,而要以自身的状况决定速度。如果你已经觉得很不耐烦,那就加快速度,如果有点气喘就慢下来,要在这两者之前保持平衡。当你的思想不再集中在眼前的行动上,每爬一步不是为了爬上山顶,你会发现,这里有一片锯齿状的叶子;这块岩石有点松动;从这里山顶上的雪不太容易看见,即使愈来愈接近山顶。这些都是你应该注意的事。如果你只是为了爬到山顶,这种目标是很肤浅的,维持山的活力是靠周遭的环境,而不单单只是山顶而已。

还有:

任何想要以己为荣的目标,结局都非常悲惨。现在我们就开始付出代价了。如果你想通过爬上山顶来证明你有多么伟大,你几乎不可能成功。即使你做到了,那也是一种很虚幻的胜利。为了维持这种成功的形象,你必须在其他方面一再地证明自己,而内心则常常恐惧别人可能会发现这种形象是虚幻的,所以这么做是错的。

作者试图告诫,虽然目标很重要(决定了你的方向),但是你行动的过程也非常重要,当目标明确后,你所要做的就是专注于此时此地,如书中所言:

自我的爬山者就像一支失调的乐器,他的步伐不是太快就是太慢,他也可能失去欣赏树梢上的美丽阳光的机会。在他步履蹒跚的时候却不休息,仍然继续前进。有的时候,刚刚才观察过前面的情况,他又会再看一遍。所以他对周围环境的反应不是太快就是太慢。他谈论的话题永远是别的事和别的地方。他的人虽然在这里,他的心却不在这里。因为他拒绝活在此时此地,他想要赶快爬到山顶,但是一旦爬上去之后仍然不快乐,因为山顶立刻就变成“此地”。

关心你手头的工作实在太重要了。作者说道,“有一次我在他们家等着一起上路,我注意到水龙头在滴水,我记得上次就已经滴了,事实上已经滴了很久。我提醒他这件事,约翰告诉我,他换过新的皮圈但还是滴水,他说了这些就不再提了,也就是说事情到此为止。如果你试过修理水龙头,但是情况依旧,那就表示你命中注定有一个会滴水的水龙头。”

我并不想仓促行事,因为仓促本身就是20世纪最要不得的态度,当你做某件事的时候,一旦想要求快,就表示你再也不关心它,而想去做别的事。所以我想慢慢来,用我找到被剪断了的销子的态度,有了这种态度才能发现原因,这样才能仔细而且透彻地进行这件事,除此之外,别无他法。

除了专注力,作者十分强调“平静的内心”,指出只有保持内心的宁静,才能更加容易看到“良质”,急于求成会一叶蔽目,看不到事物的全貌。他说“知识分子最难了解这种良质,因为他们反应过快,立刻将一切化成思考的形式。而最容易看见良质的是儿童以及未受过教育的人,还有丧失文化的人”, 要达到内心的平静,并不容易,本书所提到的,就是一种收获平静的方式。

newyorker

现代人的冥想,来自Newyorker

事实上,要心平气和并不简单。我进一步解释说,那是整个事情的灵魂,保养的良好与否就取决于你是否有这种态度。我们所谓机器运转是否正常正式心平气和的具体表现。最后考验的往往是你的定力。如果你把持不住,在你维修机器的时候,很可能就会把你个人的问题导入机器之中。


质子治疗基础(二)

07 Apr 2017 by Liangzhao

(内容摘录并改编自Harald Paganetti的《Proton Beam Therapy》。)

目前的技术挑战

建设规模

质子束的产生需要使用粒子加速器——回旋加速器(Cyclotron)或者同步加速器(Synchrotron),两者都有利有弊。回旋加速器体积较小,但是引出的能量只有一种,必须在射束路径上放置吸收箔(Absorber)以改变能量。回旋加速器产生的射束要窄很多,而且能量之间可以在亚秒内切换,从另一方面看,它是脉冲型射束,在应用一些调制技术时又缺乏灵活性。

因为设备体积的关系,目前质子加速器安装在治疗室外,一台质子加速器可以配置多个治疗室1,因为用于引导和偏转射束的磁铁系统也要占用一定的空间。而光子加速器我们知道,一个机房足够容纳所有的设备。

如果磁场系统想要偏转高达250MeV的质子束,那么机架直径可达数米,如果机房要求机架360度旋转2,成本会更高。质子加速器要想替代光子直线加速器成为放射治疗的标准配置,至少要在机房建面和成本上大大降低——然而短期看,这两者仍然是制约质子加速器最主要的因素。

鉴于此,目前科研团队正致力于打造更加小巧,可以安装一个治疗室的质子设备。利用超导可以减少加速器和机架的尺寸,目前一些回旋加速器直径可以在2m之内,并且可以挂在在机架上(即便如此,这比常规加速器房间尺寸还是要大)。小的同步加速器直径可以在6m内,如果不要求机架360度旋转,那么目前我们可以把质子加速器安装到相当于光子治疗室两倍左右大小的房间中,与此同时,它的成本也在不断降低。

射线质相关因素

利用射束扫描技术,窄束(Beamlet)可以实现高度的剂量适形,然而实际应用时这些窄束几乎不可能是单能的,且吸收箔可以扩大能量的岐离,也就是布拉格峰宽度变大。能量的切换(通过iso-energy layer)也需要一定的时间。窄束的宽度还受到传送系统设计的影响。典型的窄束宽度(Sigma)分别在12mm(对于低能射束)以及3mm的水平(对高能射束)。

宽度越小,意味着调制能力越高,然而带来了治疗时间(扫描时间)的延长,从剂量学上看,窄束的鲁棒性差,在实施时(尤其对于器官移动的病例)会带来更高的不确定性。对于病人的位移,高剂量率治疗是一种理想的手段,这对科室的QA水平有更高的要求。

物理学因素

剂量学

放射治疗中,我们要求处方剂量与实际的照射剂量在2.5%以内。其中的物理学因素主要是在TPS计算方面。光子束计划中为加快计算速度常用解析算法(Analytical Algorithms),这在质子束计划计算中有很大缺陷,粒子野中陡峭的剂量梯度在解析算法中都近似化,尤其是描述骨头和软组织交界处质子的散射反应时。目前,蒙卡算法可以得到最接近的结果,被认为是金标准。

质子计划要求更低的不确定性,是因为质子束在组织中射程短,在不均匀的人体中散射影响大,在做计划时,充分考虑这些因素十分关键。

射程不确定性

前面说过,质子加速器通过调整能量来改变射程,也就是改变布拉格峰的位置,准确的调整十分重要,原因十分明显,射程过短造成靶区欠量(要比光子束更为严重,下面解释),射程过长有造成正常组织受到极高的剂量照射。在光子计划中,我们一般在各个方向(前-后、左-右、上-下)做等量的外扩即可,在质子计划中,我们就要在射束方向的外扩距离做专门的考虑(在肿瘤靶区远离体表的方向增加一定的距离),与常规的PTV概念是不同的。射程的不确定性是粒子束治疗的独有特点,如果要保证处方剂量得到足够的靶区覆盖率,一般要外扩(在射束方向的远端)3.5%的处方深度外加几个毫米,这意味着,正常组织将会受到额外剂量的照射3。对于IMPT,我们可以使用多个方向的射野合成所需的剂量分布(每个射野产生的剂量分布不是均匀的),这样一来可以将这种不确定性均分到各个射野中,以增加计划的鲁棒性,这被称之为鲁棒性优化(Robust Optimization),这一技术已经整合到很多商用的TPS中。

和光子束类似,质子束也是在水中进行标定,实际治疗中人体组织的不均匀性也会带来射程的变化,这一点质子要比光子计划更加严重(下图),CT扫描所得到的三维图像,是模拟“光子”在组织中相对于水(CT值)的衰减程度,在质子计划中,通常不是将CT值(HU)转换成电子密度,而是相对于水的阻止本领(因为光子和质子在组织中的物理过程不同)。

PT

病人在治疗期间的位移(Intra-fraction位移)也会造成实际射程和计划的不同,常规IMRT可以将治疗期间的随机误差平均到20~30个分次中,但是随着分次数的减少(低分割的SRT/SBRT),摆位和治疗期间的监测显得非常重要,因此质子治疗中的图像引导及自适应计划制作的频率高。

器官的运动(Inter-fraction)也给质子治疗带来很大的挑战,例如,同一个位置,高密度的肿瘤组织不时地会被低密度的肺组织(约为水的三分之一)取代,造成肺组织剂量过高,除了不同密度组织叠加的问题,胸廓(Rib-cage)运动引起水等效深度的显著变化,这些效应必须在计划时仔细考量。

PT

体内射程验证(Range Verification in vivo)

目前EPID越来越多地被用于光子束病人治疗期间实时地剂量验证,原因在于光子的穿透能力强,可以穿过人体到达EPID的探测板,通过EPID上得到的剂量分布,我们就可以逆向推出实际人体中的剂量分布情况。

因为质子束几乎全部在人体内部被阻挡,所以不能通过这种方法来解决。目前不少的方法是基于探测人体中核反应产生的次级辐射,如伽玛射线4。这些方法是一个热门研究领域,目前也有很多亟待解决的问题5,所以尚未被应用于临床。

PT

生物学因素

组织剂量限值

我们假设一定的剂量会产生一定的组织反应。然而计算的剂量是沉积在CT上每个像素点上的,而真正的生物反应是在亚细胞的层面上——从氧自由基的产生到DNA双键的断裂,到修复通路的丧失,到这些反应的累积致器官毒性,有着复杂的步骤,而这些问题,我们才刚有所认知,剂量反应关系对不同的病人完全不同,基因层面的个体化治疗研究还在“婴儿期”,所以,目前我们所施予的剂量依赖的是“基于经验模型”的物理学概念,而不是直接根据TCP和NTCP优化治疗方案。

而这些“经验模型”也仅是来自于三维适形时代(非IMRT),三维适形计划给予靶区很均匀的剂量,所以我们现在很多的评估便是基于平均剂量,通过IMRT或IMPT产生的不均匀剂量难以有效评估6

相对生物效应

当使用不同的射线质时,即使是相同的剂量,产生的临床反应也可能不同(缘于微观环境中剂量沉积的模式不同),我们用相对生物效应(RBE,Relative Biological Effectiveness)来描述这种关系:两种不同治疗束产生同样的生物效应时两者的吸收剂量比。所以最终的RBE值跟剂量、生物效应的评价点、还有LET有关。前面我们看到质子的LET值是随着速度的降低不断增加的7,但是实际的质子治疗我们假设它是常数。例如质子的RBE(相对于高能光子)为1.1,那么意味着要达到相同的生物反应,质子计划的处方是光子计划的90%(光子计划剂量50Gy,那质子计划剂量45Gy即可)。这些RBE值都是在体内和体外实验的细胞存活曲线中得到的平均值。

RBE值的不确定性使得一些TPS在优化时避免重要组织附近有高RBE值,下面一张图显示了即使是同样的剂量分布,从LET层面看来是多么不同。

PT

联合治疗

放射治疗不是采取的唯一手段,它往往和其他治疗手段(化疗、靶向治疗、免疫治疗等)同时实施,一些靶向药和免疫治疗的方法可以造成一定的放射损伤和细胞修复机制的影响,从而影响到放射治疗疗效。不同治疗模式的搭配(如给多少药量?什么时间给?),加上个体的差异,使得治疗效果的评估非常复杂。

临床挑战

一种新的化疗药上市前,药厂要花费巨大精力和重金去做临床试验,周期长,最后得到的结果也未必正面。放射治疗则完全不同,做临床试验的职责在临床科室,往往是设备已经在临床安装完毕,就可以直接使用,原因是因为我们仅仅使用“剂量”这个概念,我们假设只要剂量学上有优势,那么疗效应该会更好。但是基于上述的挑战可以看出,质子设备相对光子设备有很多的不同,质子设备也未必适用于所有类型的肿瘤(主要适用于儿童肿瘤、或者靶区体积较大无法给足剂量的病例),选择是否治疗前也要算一笔经济账,费用应不仅包括放疗整个疗程,而且还得包括副反应引起的再次入院检查和治疗的费用,因正常组织受量高引起的生活质量降低的成本,这是未来质子设备是否能广泛使用的一个重要因素。临床疗效则应靠标准周密的临床试验得到,标准化数据存储和分析方法可以为有效分析治疗疗效打下基础,也有助于基于知识经验的自动计划系统的应用。

PT

展望

现在,1%的放射治疗采用质子加速器,随着质子中心的增加,这一比例会快速提高,与此同时,人员培训也成为需要关注的问题。

除了质子外,重离子也受到人们的关注(主要是碳离子),重离子较质子的优势在于旁向散射少(半影较窄),对于乏氧细胞(对方射线不敏感)更为有效,劣势在于它的布拉格峰有一个“尾巴”,RBE值不确定性更高,更为重要的是,重离子的成本比质子加速器成本高出很多。

  1. 虽然多个治疗室只配置一台质子加速器,然而对于工作流程并不会带来多大限制,因为Beam-on 时间相对病人治疗摆位往往只占一小部分。 

  2. 对大多数治疗而言,不需要使用360度旋转的机架,可以利用六维床与病人体位的变化增加治疗的灵活性。 

  3. 例如,如果覆盖远端靶区需要10cm的水等效深度,那么加外扩后实际要在10.45cm左右,正常组织有近5mm进入到靶区中。 

  4. 可直接和间接地产生伽玛射线。直接:原子核退激以顺发伽玛辐射(Prompt Gammas)的方式释放能量;间接:原子核反应产生一些碳、氧、氮同位素,他们发出的正电子泯灭释放伽玛射线(类似于PET)。 

  5. 如可以探测高能伽玛射线探测板的研制;探测得到的信号有一定的不确定性,一是因为物理截面积计算是不精确,二是信号不是一对一的关系(剂量基于电磁反应,探测利用的是核反应)。所以不能直接对比原计划的剂量分布,需要使用蒙卡算法模拟出类似于PET一样的剂量分布,再和探测得到的分布比较。 

  6. 例如肝脏的一部分承受很高的剂量,其余部分仍然可以正常的工作。靶区中的不均匀性目前看来不仅没有坏处,而且对于放射不敏感的区域(如靶区中心)更有剂量学优势。 

  7. LET的变化导致了RBE的变化,估计在布拉格峰附近,RBE值的变化从1.0~1.5,所以RBE是一个非常复杂的概念,射线能量、剂量、剂量、细胞类型(生物效应)、氧合状态、放射敏感性等因素都会影响最终的结果,这些因素目前都不能定量地考虑进去,其中一些因素跟个体有关,必须借助血液或者基因标记才能确认。