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“获益”与“风险”,两害相权取其轻

(2021-10-14 08:40:23)
分类: 与大自然通心

(前言:如何平衡“获益”与“风险”?如何做到“两利相权取其轻,两害相权取其重”?这是我们在生活中经常能够遇到的问题。科学家、医学家们怎么解决这些问题呢?下面转发的文章是一个精彩的例子。据称,PET/CT是医学、影像学的一门新技术,下文甚至认为,PET/CT的出现是21世纪一项重要突破”。由于在检查方面,效果显著,已经有一些经济比较富裕的人选择做该检查,以作为癌症以及身体状况的筛查。但是,由于涉及到核辐射问题,不少人对其安全性提出质疑。我从互联网搜索到这篇文章,与大家分享。我感兴趣的角度,还有科学家们解决这些难题的思维方式和推理逻辑,包括“风险-获益”等概念。另外一些超越我们常识的一些研究也很耐人寻味,例如:“有趣的是,航空公司的机组人员在洛杉矶和纽约之间飞行,纽约中央火车站的工作人员每年受到的辐射比核医学技术人员还要多。核医学技术人员的辐射暴露来源于放射性物质的处理以及每天帮助患者上下台做检查时。我自己读后,感觉该文有相当的说服力。但是,一些地方似乎还欠妥。例如,把做pet-ct检查的风险与坐车、开车等的风险作类比是不恰当的。因为:1、前者对于当事人来说是被动的,不可控。后者对于当事人来说是主动的(具有主观能动性),可控的。例如,我们如果坐车,可以选择更安全的车与司机,注意安全带,更加小心,选择出行时间、路线等,可以减少风险概率。2、中枪的原因,前者不清楚,后者相对清楚。例如,后者事故的发生多为酒醉驾驶,疲劳驾驶汽车失修,天气恶劣等。——许金声。)

2019-08-15 06:00 来源: 中华核医学

【分享】核医学马寄晓教授 :PET/CT的安全性

国内对PET/CT等核医学检查的辐射剂量说法不一,有人认为PET/CT 全身扫描是普通 CT 扫描的数倍或十倍,对同道和病人可能形成负担。为了科学地认识PET/CT 对人体的辐射量,我与 Dr. Kai Lee 商讨写下了下文,供参考。 

Dr. Kai Lee 是美国核医学物理学专家,尤其对辐射剂量学有深入研究。他年轻时即与周前教授、刘秀杰教授、屈婉莹教授、潘中允教授、陈盛祖教授、朱承谟教授、林祥通教授、陈可靖教授、陈绍亮教授、常国钧教授等老专家友谊深厚。每次回国都要讲解美国核医学进展。他与 Dr. Yi Li(李沂)等几位在美核医学专家均十分关注国内核医学发展。 

——马寄晓 

作者Kai Lee, PhD ,广州人,七岁时全家带他离开了广州移民美国,他是南加州大学USC放射科核医学的物理学家radiology physicist。他是中山医已故石锐教授的老朋友,石锐教授当年是去USC进修认识他的。八十年代后期起他就常常去中国讲学核医学,我在中山医工作期间得益于他的讲课。今年加州的核医学年会我们十多年没有见面后再次聚会。 

——李沂 

“获益”与“风险”,两害相权取其轻

《PET/CT的安全性》

Kai H. Lee, PhD and Ma Jixiao, MD

PET / CT或任何涉及辐射的医学影像学检查是否安全?这是患者经常会提出的问题。其实他们的担忧是可以理解的,因为公众是由于对原子弹爆炸的恐怖而接触到辐射相关信息的。为了回答有关医疗辐射安全性的问题,我们首先要界定什么是安全的,而人们对安全的理解是不尽相同的。比如,坐过山车安全吗?绝大部分青少年是不会错过这样刺激的机会的,成年人可能不会那么期待,而老年人更有可能拒绝。这些对于坐过山车安全性的观点反应了人们对于风险的承受能力是有差异的。事实上,我们做任何事都会有一定的风险。零风险的活动和事物是不存在的。我们认为某些事物或行为是安全的,即与其相关的风险对我们来说是很低的或者是根本无需担忧的。

存在于医学影像学检查中的辐射确实有风险。如果使用不当,辐射会对人体造成伤害,并有可能在未来造成问题。根据线性无阈值模型[1],癌症的风险增加与接受的辐射量成正比。一 个人受到的辐射越多,他晚年患癌症的风险就越大。线性无阈值模型的图形表示如图1所示。根据该模型,任何剂量的辐射都有风险。我们必须尽可能的降低辐射暴露。这种保护辐射的方法被称为“辐射安全防护最优化(ALARA)”的原则;ALARA原理有时被称为线性无阈值模型的推论。

“获益”与“风险”,两害相权取其轻

1.预测癌症风险的线性无阈值模型。投影线的斜率是每1 mSv有效剂量增加0.004%的癌症几率。

基于线性无阈值模型,政府和咨询机构为个体允许的辐射暴露设定了标准限值[2,3]。允许的辐射水平称为最大允许剂量(MPD)。在美国,放射科医生和核医学技术人员等职业放射工作人员的MPD每年为50mSv。一般公众的MPD是每年1 mSvmSv是辐射测量的单位。MPD是我们从地球自然接受的本底辐射之上的辐射暴露,不包括医疗照射。MPD并不意味着低于这个水平的人就不会受到辐射的有害影响。它只是意味着根据我们目前的知识,每年暴露于该辐射量的体细胞和基因损伤的风险可以忽略不计。

人们可能会问为什么职业放射工作人员的MPD高于一般公众。如果我们把工作从在办公室做会计换到在实验室处理放射性物质,我们的身体对辐射的耐受就更强吗?答案当然是否定的。辐射暴露限制标准的差异其实和我们日常生活中经常做的风险获益决策是一样的。比如,我们会毫不犹豫地坐上一辆汽车,因为前往目的地的获益远远超过了知道交通事故可造成全世界每年120万人的死亡[4]。由于辐射工作人员从就业中获得的经济利益,他或她则必须承担比 在环境中接受辐射所带来的利益更为间接的公众更大的风险。

基于以上,我们必须从风险-获益的角度来评价PET/CT的辐射安全性。PET/CT的出现是21世纪一项重要突破,它可使医生能够方便地在体内任何地方寻找小至5毫米的肿瘤病灶。如果没有PET/CT,医生将不得不进行探查手术,在体内多个部位采集组织样本,然后在显微镜下进行判断。除了患者必须忍受的疼痛和伤口外,探查手术本身漏诊的概率是很高的,因为外科医生根本无法从患者身体的每个部位采集足够的活检样本。而PET / CT只需注射显像剂,其对肿瘤探测的敏感性和特异性均大于90[5]。敏感性意味着PET/CT在患者中发现癌症的成功率大于90%。特异性意味着,如果患者确实没有癌症,那么诊断患者没有癌症的准确性超过90%。病人在注射过程中只感到轻微的针刺痛。此外,PET/CT不仅避免了许多其他昂贵的检查,还缩 短了对患者进行适当治疗的时间延迟。

如上所述,PET/CT扫描中的x射线和放射性药物确实存在一定风险。辐射风险的一个衡量标准是患癌症的可能性。患癌症的概率被称为辐射暴露的随机效应。前面提到的线性无阈值模型是几个数学模型中的一个,这些数学模型建立是用于预测患癌风险与所接收的辐射量的关系。应用线性无阈值模型和其他模型的困难在于,我们对辐射造成的癌症风险的了解来自辐射事故和原子弹爆炸中个体的全身暴露。由于许多X射线和CT扫描不会暴露患者的整个身体,因此我们必须将部分身体暴露转化为与全身相当的剂量才能使用我们的模型数据。有效剂量的概念即是为了解决这个问题而设计的。它将身体某一部分的剂量转换为整个身体的等效剂量,从而产生相同的患癌几率。有效剂量的目的是利用我们累积的全身暴露数据来评估部分身体暴露的风险。

诚然,有效剂量的概念是令人困惑和有争议的。因为有效剂量不是物理上可测量的量,但它使用相同的辐射单位毫西弗(mSv)来表示剂量。有效剂量是用来比较辐射随机效应大小的,并不代表均匀分配给全身的实际剂量。举例来说,一名核医学技术人员每年接受的辐射量约为2mSv;这种全身剂量是由技术人员佩戴的剂量计测量和记录的。而头部CT扫描的患者通常也 接受2mSv的有效剂量。2mSv的有效剂量是由计算机模拟得到的全身剂量。因此,比较接受头 部CT扫描的患者的有效剂量和技术人员所接受的全身剂量的结果是,两者患癌症的风险是相同的。有效剂量虽然在使用上存在混乱和争议,但它是比较涉及辐射的不同医学检查的随机效应风险的一个有用的参考量。

1列出了各种核医学检查和不同职业的一些典型全身剂量,以及患者可能通过不同的医 学影像学检查所受到的有效剂量[6,7]。请记住,有效剂量是一种假想的全身剂量,用于比较辐射致癌风险。

“获益”与“风险”,两害相权取其轻

“获益”与“风险”,两害相权取其轻

有趣的是,航空公司的机组人员在洛杉矶和纽约之间飞行,纽约中央火车站的工作人员每年受到的辐射比核医学技术人员还要多。核医学技术人员的辐射暴露来源于放射性物质的处理以及每天帮助患者上下台做检查时。航空机组人员则在高空飞行时受到宇宙辐射。纽约中央火车站是由巨大的花岗岩建成的,花岗岩中天然存在的放射性物质使那里的员工每年都受到辐射。PET/CT检查,接受低剂量CT扫描进行解剖定位的患者,其有效剂量为11 mSv(7 + 4),而诊断性CT则为19 mSv(7 + 12)。根据线性无阈值模型,每1mSv有效剂量获得癌症的几率为0.004%[8,9,10], 使用这个癌症风险概率,一次PET/CT扫描病人将会增加0.044%0.076%的患癌风险。美国的统计数据显示,一个人一生中患癌症的几率为38.4%[11],且暴露于辐射后,癌症发展的潜伏 期约为20-30年。因此,一个人在PET/CT扫描后罹患癌症的额外风险是微不足道的。PET/CTCT扫描对于治疗生病或受伤患者的益处远远超过了远期癌症的额外风险。事实上,由于害怕 辐射而拒绝接受PET/CT扫描,让他们的疾病或癌症在没有得到诊断或治疗的情况下继续发展,其风险远远大于多年后患癌的风险。尽管如此,对病人的辐射也不容忽视。PET/CT的从业者应 认识到病人的风险,并尽量减少可能的辐射[12]。这一考虑对辐射更敏感的儿童和妇女以及未来几十年寿命较长的年轻患者尤其重要。

必须强调的是,表1中的有效剂量是15年前根据20年或更早前发表的数据公布的。虽然没有已发表数据,但过去10年的技术进步已经大大减少了CTPET扫描对患者的辐射。制造商开发了更有效的辐射探测器和更抗噪声的图像重建软件,以减少PET/CT扫描所需的辐射量。对于那些需要多次检查的患者,医生应注意限制PET/CTCT扫描的次数。如允许,医生应选择核磁共振或超声,而不是具有辐射的检查。我们还必须认识到,某些影像学检查比其他检查更适合用于评估某些疾病。那些因为害怕辐射而坚持用核磁共振或超声而不是PET/CT的患者,其结果往往弊大于利。

总之,生活中的风险是无处不在的。虽然PET/CT扫描具有一定的辐射,但PET/CT显像在肿瘤诊断、确认手术的必要性、监测化疗疗效等方面的获益远远超过了数十年后可能存在的任何风险。

参考文献

1. Weber, W and Zanzonico, P, “The Controversial Linear No-Threshold Model,” The Journal of Nuclear Medicine. Vol 59, No. 1, January 2017.

2.U.S. Nuclear Regulatory Commission:10 CFR 20.Standards for protection against radiation. Washington DC, 1996.

3. International Commission on Radiological Protection; 1990 Recommendations of the ICRP. Publication No. 60. New York: Pergamon Press, 1991

4. WHO, ed. (2018) Global Status Report on Road Safety 2018.Geneva: World Health Organization (WHO).

5. Freudenberg, LS, et. al, “FDG-PET/CT in re-staging of patients with lymphoma,” Euro. J. Nucl. Med. And Molec Imag, March 2004, Vol 31, No. 3, pp:325-329.

6. Bailey, S., “Air Crew radiation exposure – An Overview,” Nuclear News, January 2000, Last http://www2.ans.org/pubs/magazines/nn/docs/2000-1-3.pdf, last assessed July, 2019.

7. Mettler, FA, Huda, W, Yoshizumi, TT, Mahesh,M, “Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog, “ Radiology, July 2008.

8. Brenner, CD, Elliston, E, Hall, EJ, Berdon, WE, “Estimated risks of radiation-induced fatal cancer from pediatric CT, “ Am. J. Roentgenol. 176, 289-296 (2001)

9. Brenner, DJ, Elliston, CD, “Estimates of the cancer risks from pediatric CT radiation are not merely theoretical,”Med. Phys. 28 (11), November 2001.

10. Smith-Bindman, R, et al, “Radiation Dose Associated with common computed tomography examination and the associated lifetime attributable risk of cancer,” Arch Intern Med. 2009; 169 (22):2078-2086.

11. National Cancer Institute, NIH, “Cancer Statistics,” https://www.cancer.gov/aboutcancer/understanding/statistics;accessed July 13, 2019.

12. Limacher, MC, et al, “ACC expert consensus document.Radiation safety in the practice of cardiology,”J. Am. Coll. Cardiol. 1998; 31:892-913

Radiation Risk from PET/CT

Kai H. Lee, PhD and Ma Jixiao, MD

Is PET/CT or any medical imaging that involves radiation safe?This is a question Frequently asked by patients.Their concern is understandable considering the public was introduced to radiation by the horror of atomic bomb blasts.In order to answer the question of medical radiation safety, we must first define what is safe.People have different notions of what safe means.Is it safe to ride on a roller coaster?Most teenagers would not pass up a chance for the thrill; adults may not be so eager; and senior citizens would more likely say no.The opinions on safety of roller coaster illustrate the point that people have different levels of risk tolerance.The truth is, everything we do carries risk.Activities or things with zero risk do not exist.We label things or actions that we deemed safe for the reason that the risks associated with their use are of little or no concern to us.

The radiation in medical imaging does carry risks.If used improperly, radiation can induce bodily harms, and has potential to cause problems in the future.According to the linear no threshold model [1], the risk of getting cancer increases in direct proportion with the amount of radiation received.The more radiation a person is exposed to, the greater the risk of developing cancer later in life.Graphical representation of the linear no threshold model is shown in Figure 1.According to the model, no amount of radiation is without risks.

We must strive toward minimizing our exposure to radiation.This approach to radiation protection is known as the “As Low As Reasonably Achievable (ALARA)” principle; the ALARA principle is sometimes called the corollary of the linear no threshold model.

“获益”与“风险”,两害相权取其轻

Figure 1. Linear no-threshold model for projection of cancer risk.Slope of the projection line is 0.004% increase in chance for cancer per 1 mSv effective dose.

Based on the linear no threshold model, governmental and advisory agencies set standard limits for allowable radiation exposure to individuals [2,3].The allowable levels of radiation are known as the Maximum Permissible Doses (MPD).In the United States the MPD for occupational radiation workers such as radiologists and nuclear medicine technologists is 50 milliSiverts (50 mSv) a year to the whole body.MPD for the general public is 1 mSv per year.A milliSivert (mSv) is a unit of radiation measurement.The MPD is exposure above background radiation we receive naturally from the earth, and excludes medical exposures.The MPD does not mean the level below which a person is safe from the deleterious effects of radiation.It simply means that according to our current knowledge the risks for somatic and genetic injuries from exposure to that amount of radiation annually are negligibly small.

One may ask why the MPD is higher for people occupationally exposed to radiation than for the general public.Will our body become more resistant to radiation if we change our job from doing accounting in the office to working with radioactive materials in a laboratory?The answer is no, of course.The rationale for difference in the exposure limits goes back to the risk versus benefit decisions that we make in our daily lives.We do not hesitate to get on a car because the benefits from going to places far outweighs knowing traffic accidents in the world caused 1.2 million deaths each year [4].Because of economic benefits a radiation worker receives from the employment, he or she has to take a greater risk than members of the public whose benefits from the use of radiation in the environment are more indirect.

Given the above preamble, safety of radiation in PET/CT has to be evaluated from the risk versus benefit perspective.The advent of PET/CT at the turn of the 21th century was a breakthrough that enabled physicians to conveniently look for cancerous tissues as small as 5 mm anywhere in the body.Without PET/CT, physicians would have to perform exploratory surgery to take tissue samples in multiple parts of the body to study under the microscope.In addition to the pain and wounds the patient had to endure, chances for missing cancerous tissues were high in the brutal exploratory surgeries; the surgeon simply could not possibly take enough biopsy samples from every imaginable part of the patient’s body.On the other hand, with a simple injection, a PET/CT scans has sensitivity and specificity of detecting cancer greater than 90% [5].Sensitivity means PET/CT has a success rate of greater than 90% to find cancer in the patient.Specificity means it is better than 90% accurate to state a patient has no cancer if the patient indeed does not have cancer. The patient suffers no other pain than a tinge of needle pain during injection.In addition, PET/CT not only obviates many other costly tests, it cuts short the time delay in giving proper treatments to the patient.

As mentioned, patients do incur risks from the x-rays and radiopharmaceutical in PET/CT scans.One metrics of risk from exposure to radiation is the probability of getting cancer.The probability of getting cancer is known as the stochastic risk of radiation exposure.The aforementioned linear no threshold model is one of several mathematical models devised to project the probability of getting cancer as a function of the amount of radiation received.A difficulty in applying the linear no threshold model and other models is that our knowledge of cancer risk from radiation came from total body exposure to individuals in radiation accidents and atomic bomb blasts.Because many x-ray and CT procedures do not expose the entire body of the patient, we have to translate a partial body exposure to an equivalent dose to the whole body in order to use our database.The effective dose concept was devised to address this issue.It translates the dose to a part of the body to an equivalent dose to the whole body that would produce the same probability of getting cancer. Purpose of the effective dose is to permit utilization of our accumulated whole-body exposure data for evaluation of risks from partial body exposures.

Admittedly, the effective dose concept is confusing and controversial.It is confusing and controversial because the effective dose is not a physically measurable quantity, and yet it uses the same radiation unit mSv for measurable dose.The effective dose is a scale for comparison of stochastic risks from radiation, not an actual dose imparted uniformly to the whole body.As an illustration, a nuclear medicine technologist receives about 2 mSv a year to the total body; that whole-body dose was measured and recorded by a dosimeter worn by the technologist.A patient who had a CT scan of the head typically received 2 mSv effective dose.That 2 mSv effective dose is an imaginary whole-body dose derived from computer simulation.The way to compare the effective dose to the head CT patient and the whole body dose received by the technologist is that CT of the head gives a patient the same risk of getting cancer as a nuclear medicine technologist working for one year.In spite of the confusion and controversies with its use, the effective dose is a useful reference scale for comparison of stochastic risks of different medical examinations involving radiation.

Table 1 lists some typical whole-body doses to workers in various occupations, and the effective doses a patient may receive from different medical imaging procedures [6,7].Keep in mind that the effective dose is an imaginary whole-body dose for comparison of cancer risk from radiation.

“获益”与“风险”,两害相权取其轻

“获益”与“风险”,两害相权取其轻

It is interesting to note that airline crews fly between Los Angeles and New York, and workers in the New York Grand Central train station receive more radiation annually than nuclear medicine technologists who handle radioactive materials and help patients get on and off the table each day for imaging.Airline crews receive radiation in high altitude flying from the cosmic radiation.The Grand Central train station in New York is constructed of massive granite.The naturally occurring radioactive materials in granite gave workers there the annual radiation.Patients who received a PET/CT scan with a low dose CT for anatomical localization exposed to 11 mSv effective dose (7 + 4).Patients who took a diagnostic CT in conjunction with PET received 19 mSv (7 + 12).According to the linear no threshold model, chances for acquiring cancer is 0.004% per mSv effective dose [8,9,10].Using this cancer risk probability, a PET/CT patient would incur an additional 0.044% to 0.076% risk for a PET/CT scan.Statistics in the U.S. shows the chance for a person getting cancer is 38.4% [11].The latent period for cancer to develop after exposure to radiation is about 20-30 years.Thus, the additional risk for a person to develop cancer after a PET/CT scan is insignificant.The benefits of PET/CT or CT to help treatment of patients who are sick or injured far outweigh the additional risk for cancer in the distant future.In fact, the risks from refusing a PET/CT scan for fear of radiation and let their disease or cancer advance not diagnosed or untreated are much greater than the small chances a new cancer is to develop many years later.Notwithstanding, radiation to the patient is not to be ignored.Practitioners of PET/CT are cognizant of risks to the patient and strive toward using as little radiation as possible [12].This consideration is especially important for children and females who are more sensitive to radiation and young patients who have many decades of life ahead.

It must be emphasized that the effective doses in the Table 1 were published 15 years ago based on data published 20 or more years prior.Though no data have been published, technological advances in the past 10 years have substantially reduced the radiation to patients from CT and PET scans.Manufacturers developed more efficient radiation detectors and more noise tolerant image reconstruction software to reduce the amount of radiation needed for PET/CT studies.Physicians are made aware to limit the number of PET/CT or CT studies for patients who have medical problems that may require multiple imaging.Whenever possible, physicians would select MRI or ultrasound instead of procedures involving radiation.We must also recognize that some imaging tests are better suited than others for evaluation of certain diseases.A patient Insists on getting MRI or ultrasound instead of PET/CT for fear of radiation actually gets more harms than benefits.

In conclusion, everything we encounter in life carries risks.Although the radiation received from PET/CT studies is not trivial, the benefits of PET/CT for cancer diagnosis, confirming the need for surgery, and monitoring the progress of chemotherapy far outweighs any potential risks in the distant future.

References

1. Weber, W and Zanzonico, P, “The Controversial Linear No-Threshold Model,” The Journal of Nuclear Medicine. Vol 59, No. 1, January 2017.

2.U.S. Nuclear Regulatory Commission:10 CFR 20.Standards for protection against radiation. Washington DC, 1996.

3. International Commission on Radiological Protection; 1990 Recommendations of the ICRP. Publication No. 60. New York: Pergamon Press, 1991

4. WHO, ed. (2018) Global Status Report on Road Safety 2018.Geneva: World Health Organization (WHO).

5. Freudenberg, LS, et. al, “FDG-PET/CT in re-staging of patients with lymphoma,” Euro. J. Nucl. Med. And Molec Imag, March 2004, Vol 31, No. 3, pp:325-329.

6. Bailey, S., “Air Crew radiation exposure – An Overview,” Nuclear News, January 2000, Last http://www2.ans.org/pubs/magazines/nn/docs/2000-1-3.pdf, last assessed July, 2019.

7. Mettler, FA, Huda, W, Yoshizumi, TT, Mahesh,M, “Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog, “ Radiology, July 2008.

8. Brenner, CD, Elliston, E, Hall, EJ, Berdon, WE, “Estimated risks of radiation-induced fatal cancer from pediatric CT, “ Am. J. Roentgenol. 176, 289-296 (2001)

9. Brenner, DJ, Elliston, CD, “Estimates of the cancer risks from pediatric CT radiation are not merely theoretical,”Med. Phys. 28 (11), November 2001.

10. Smith-Bindman, R, et al, “Radiation Dose Associated with common computed tomography examination and the associated lifetime attributable risk of cancer,” Arch Intern Med. 2009; 169 (22):2078-2086.

11. National Cancer Institute, NIH, “Cancer Statistics,” https://www.cancer.gov/aboutcancer/understanding/statistics;accessed July 13, 2019.

12. Limacher, MC, et al, “ACC expert consensus document.Radiation safety in the practice of cardiology,”J. Am. Coll. Cardiol. 1998; 31:892-913返回搜狐,查看更多

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