Friday, January 27, 2012

The Intermediate Physics for Medicine and Biology Tourist

A map of the path of an Intermediate Physcis for Medicine and Biology tourist.
Over the Christmas break I was browsing through the Guidebook for the Scientific Traveler: Visiting Physics and Chemistry Sites Across America, and it got me to wondering what sites a reader of the 4th edition of Intermediate Physics for Medicine and Biology might want to visit. Apparently having too much time on my hands, I devised a trip through the United States for our readers. (Perhaps I’ll prepare an international edition later.) The trip starts and ends in Rochester, Michigan, where I work, but the path consists of a large circle and you can begin anywhere. I have not visited all these places, but I know enough about them to suspect you would enjoy them all. Tell me if I have forgotten any important sites. Happy travels!

Friday, January 20, 2012

Radiation Risks from Medical Imaging Procedures

On December 13, 2011 the American Association of Physicists in Medicine issued a position statement (PP 25-A) about radiation risks from medical imaging procedures. It is brief, and I will quote it in its entirety:
The American Association of Physicists in Medicine (AAPM) acknowledges that medical imaging procedures should be appropriate and conducted at the lowest radiation dose consistent with acquisition of the desired information. Discussion of risks related to radiation dose from medical imaging procedures should be accompanied by acknowledgement of the benefits of the procedures. Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent. Predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged. These predictions are harmful because they lead to sensationalistic articles in the public media that cause some patients and parents to refuse medical imaging procedures, placing them at substantial risk by not receiving the clinical benefits of the prescribed procedures.

AAPM members continually strive to improve medical imaging by lowering radiation levels and maximizing benefits of imaging procedures involving ionizing radiation.
News articles discussing this position statement appeared on the Inside Science and Physics Central websites.

The 4th edition of Intermediate Physics for Medicine and Biology discusses the risk of radiation in Section 16.13. Dose is the energy deposited by radiation in tissue per unit mass, and its unit of a gray is equal to one joule per kilogram. A sievert is also a J/kg, but it differs from a gray in that it includes a weighting factor that measures the relative biological effectiveness of the radiation, and is used to measure the equivalent dose (although often, including in the remainder of this blog entry, people get a little sloppy and just say “dose” when they really mean “equivalent dose”). A sievert is a rather large dose of radiation, and when discussing medical imaging or background radiation exposure, scientists often use the millisievert (mSv).

Table 16.7 of Intermediate Physics for Medicine and Biology lists typical radiation doses for many medical imaging procedures. For example, a simple chest X ray has a dose of about 0.06 mSv, and a CT scan is 1–10 mSv. The average radiation dose from all natural (background) sources is given in Table 16.6 as 3 mSv per year (primarily from exposure to radon gas). A pilot logging 1000 hours in the air per year receives on the order of 7 mSv annually.

Perhaps the most interesting sentence in the AAPM position statement is “Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent.” To me, the phrase “may be nonexistent” seems to cast doubt on the linear nonthreshold model often used when discussing the risk of low-dose radiation. Russ Hobbie and I discuss this model in Intermediate Physics for Medicine and Biology.
In dealing with radiation to the population at large, or to populations of radiation workers, the policy of the various regulatory agencies has been to adopt the linear-nonthreshold (LNT) model to extrapolate from what is known about the excess risk of cancer at moderately high doses and high dose rates, to low doses, including those below natural background.
We also consider other ideas, such as a threshold model for radiation effects and even hormesis, the idea that very low doses of radiation may be beneficial. The controversy over the biological effects of low-dose radiation is fascinating, but as best I can tell the validity of each of these models remains uncertain; getting accurate data when measuring tiny effects is difficult. I assume this is what motivates the word “may” in the phrase “may be nonexistent” from the position statement (although, I hasten to add, I have no inside information about the intent of the authors of the position statement—I’m just guessing). In our book, Russ and I come to a conclusion that is fairly consistent with the AAPM position statement.
Some investigators feel that there is evidence for a threshold dose, and that the LNT model overestimates the risk [Kathren (1996); Kondo (1993); Cohen (2002)]. Mossman (2001) argues against hormesis but agrees that the LNT model has led to ‘enormous problems in radiation protection practice’ and unwarranted fears about radiation.
Although I find the AAPM position statement to have a slightly condescending tone, I applaud it primarily as an antidote for those “unwarranted fears about radiation.” My impression is that many in the general public have a fear of the word radiation that borders on the irrational, stemming from a lack of knowledge about the basic physics governing how radiation interacts with tissue, and a poor understanding of risk analysis. I hope the AAPM position statement (and, immodestly, our textbook) helps change those concerns from irrational fears to reasoned and fact-based assessment. I would not discourage analysis of public safety, but I definitely encourage an intelligent and scientific analysis.

Friday, January 13, 2012

Open Access

The journal Medical Physics is one of the leading publications in the field of physics applied to medicine. Recently, many articles in Medical Physics have become free to everyone (open access) (see the editorial here). This is great news to those readers of the 4th edition of Intermediate Physics for Medicine and Biology who do not have a personal or institutional subscription to Medical Physics. Some of the articles that can now be downloaded for free are the ever-popular point/counterpoint debates, review papers, award papers, and something called the “editor’s picks.” Also available free are the special 50th anniversary articles published as part of the celebration of half a century of contributions by the American Association of Physicists in Medicine in 2008. Several of these were cited by Russ Hobbie and me in our American Journal of PhysicsResource Letter MP-2: Medical Physics” (Volume 77, Pages 967–978, 2009). To access this wealth of free material, just go to the home page of the Medical Physics website and click on the Open Access Tab.

Open Access publishing is becoming more common, and has been championed by many leading scientists, such as former NIH director and Nobel laureate Harold Varmus (listen to Varmus talk about open access here). Nevertheless, the topic is hotly debated. For instance, see the point/counterpoint discussion in the November 2005 issue of Medical Physics, titled “Results of Publicly Funded Scientific Research Should Be Immediately Available Without Cost to the Public.” Additional debate can be found in the journal Nature and at

 Harold Varmus discussing open access publishing.

Open Access to journal articles should benefit readers of Intermediate Physics for Medicine and Biology, because it will allow those readers immediate access to cutting-edge papers that otherwise would require a journal subscription. Another source of open access papers is BioMed Central:
BioMed Central is an independent publishing house committed to providing immediate open access to peer-reviewed biomedical research. All original research articles published by BioMed Central are made freely and permanently accessible online immediately upon publication. BioMed Central views open access to research as essential in order to ensure the rapid and efficient communication of research findings.
BioMed Central journals that will be of interest to readers of Intermediate Physics for Medicine and Biology are BMC Medical Physics, Biomedical Engineering Online, and Radiation Oncology.

A third source of papers is the Public Library of Science. Specific journals are PLoS One (the flagship journal, covering all areas of science), PLoS Medicine, PLoS Biology, and especially PLoS Computational Biology. Also of interest is PLoS Blogs.

The Open Access movement continues, slowly but steadily, to remake scientific publication. There are now hundreds of Open Access journals. Even some of the most prestigious leading publishers are getting into the act: the American Physical Society recently initiated the open access, all on-line journal Physical Review X to go along with its other Physical Review journals.

In the spirit of Open Access, I’m pleased to announce that the 4th edition of Intermediate Physics for Medicine and Biology will now be given away, free of cha... just kidding. Maybe someday the Open Access movement will reach to textbooks, but not yet. At least this blog is free. ;)

Friday, January 6, 2012

Destiny of the Republic

Destiny of the Republic: A Tale of Madness, Medicine and the Murder of a President, by Candice Millard, superimposed on Intermediate Physics for Medicine and Biology.
Destiny of the Republic:
A Tale of Madness, Medicine
and the Murder of a President,
by Candice Millard.

Regular readers of this blog know that I am in the habit of listening to audio books while I take my dog Suki on her daily walks. My tastes lean toward science, history, and biography, and I always keep a watch out for biological or medical physics in these books. Over the Christmas break, I listened to Destiny of the Republic: A Tale of Madness, Medicine and the Murder of a President, by Candice Millard, about the assassination of President James Garfield in 1881, shot by madman Charles Guiteau.

The book tells the fascinating story of Garfield’s nomination at the Republican National Convention in 1880, back in a time when conventions were less choreographed and predictable than they are today. Garfield nominated his fellow Ohioan John Sherman (General William Tecumseh Sherman’s brother), who was running against Senator James Blaine and former president Grant. After many ballots in which no nominee obtained a majority, the delegates turned to Garfield as their compromise choice. After being chosen the Republican nominee, he defeated Democrat and former Civil War general Winfield Scott Hancock in the general election.

A few months after being sworn in, Garfield was shot by Guiteau, who had applied for a job in the new administration but had been turned down. The bullet did not kill Garfield immediately, and he lingered on for weeks. At this point, medical physics enters the story through one of the book’s subplots about the career of Alexander Graham Bell, inventor of the telephone. Millard tells the tale of how Bell set up one of his early telephones for demonstration at the 1876 Centennial Exposition, but was ignored until a chance meeting with his acquaintance, Emperor Pedro II of Brazil, who drew attention to Bell’s display. Upon hearing that the President had been shot, Bell quickly invented a metal detector with the goal of locating the bullet still lodged in Garfield’s abdomen. The detector is based on the principle of electromagnetic induction, discussed in Section 8.6 of the 4th edition of Intermediate Physics for Medicine and Biology. A changing magnetic field induces eddy currents in a nearby conductor. These eddy currents produce their own magnetic field, which is then detected. Essentially, the device monitored changes in the inductance of the metal detector caused by the bullet. Such metal detectors are now common, particularly for nonmedical uses such as searching for metal objects buried shallowly in the ground. At the time, the device was rather novel. Michael Faraday (and, independently, Joseph Henry) had discovered electromagnetic induction in 1831, and Maxwell’s equations summarizing electromagnetic theory were formulated by James Maxwell in 1861, only twenty years before Garfield’s assassination. Being a champion of medical and biological physics, I wish I could say that Bell’s invention saved the president’s life, or at least had a positive effect during his treatment. Unfortunately, it did not, in part because of interference from metal springs in the mattress Garfield laid on, but mainly because the primary physician caring for Garfield, Dr. Willard Bliss, insisted that Bell only search the right side of the body where he believed the bullet was located, when in fact it was on the unexplored left side.

Another issue discussed in the book is the development of antiseptic methods in medicine, pioneered by Joseph Lister in the 1860s. Apparently the direct damage caused by the bullet was not life-threatening, and Millard suggests that if Garfield had received no treatment whatsoever for his wounds, he would have likely survived. Unfortunately, the doctors of that era, being skeptical or hostile to Lister’s new ideas, probed Garfield’s wound with various non-sterile instruments, including their fingers. Garfield died of an infection, possibly caused by these actions.

I enjoyed Millard’s book, and came away with a greater respect for President Garfield. Bell’s metal detector was used to locate bullets in injured soldiers throughout the rest of the 19th century, until X rays became the dominant method for finding foreign objects. It is an early example of the application of electricity and magnetism to medicine.

Listen to Candice Millard speak about her book.

 Candice Millard speaking about Destiny of the Republic.