Running head: EXPOSURE IN PET-CT

Exposure in PET-CT

Versus Natural Radiation Exposure

By:

Rodney Barnes

East Tennessee State University

Abstract

The main focus of this paper is addressing the concerns that people express about receiving radiation exposure from a position emission tomography – computed tomography (PET-CT) examination, ignorant to the fact that they are exposed to radiation everyday.  In this paper we will cover what ionizing radiation is, the different types of radiation and the origin of the radiation.  A contrast and comparison will be conducted between exposure received from natural background radiation everyday and the exposure received from PET-CT procedures.  The findings of this research reveal that the majority of the radiation that most people receive on a daily basis comes from exposure to natural background radiation rather than from nuclear imaging procedures which account for only 4% of their radiation exposure.  In addition, the research revealed that patient and technologist education regarding PET-CT procedures needs to be enhanced.

When people hear the words positron emission tomography – computed tomography (PET-CT) they usually ask the question, are you performing CAT scans on animals?  When you tell them that you inject people with radiation most of them “freak out” and say you actually put radiation inside someone’s body, isn’t that dangerous?  What they fail to realize is that people having an exam performed in PET get less ionizing radiation from the isotope when we inject them than they do from just standing around; however, CT does contribute to their dose.

Purpose:

This research was conducted to provide both patients, health care providers, and the general public with the appropriate knowledge they need to develop an adequate opinion of PET-CT.

Methods:

            The three sources of information in addition to work related experiences provided more

than adequate information for this research paper.  I researched various articles and publications

written by different physicians who have compared the human exposure to radiation from both

medical exposures and natural exposures. I have included charts detailing the breakdown of

radiation exposure according to their sources.  In addition, I researched books and conducted two

surveys.  The first survey requested information from radiologic technologists.  The first

question was “Do you know what a PET-CT scan is?”  Second question was “What have you

heard about the dose of radiation to patients from a PET-CT whole body procedure?”  The third

question was “Which procedure do you feel has the higher dose to patients: a PET–CT whole

body exam or a nuclear medicine bone scan?”

The other survey requested information from patients who come into have a PET-CT

procedure.  The questions asked of the patients include “You have come in for a PET-CT

procedure, what have you heard from your physician about from this procedure?”  Another

question asked “What have you heard about the dose of radiation you will receive from this

procedure?”  The third question asked was “Do you know what nuclear medicine is?  Have you

had a bone scan?”  The fourth question asked was “Which procedure do you feel has the higher

dose to patients: a PET–CT exam that you are having now or a nuclear medicine bone scan?”

Results:

First let’s find out what ionizing radiation is, where it comes and what it does.  Ionizing radiation is radiation that has sufficient energy to remove electrons from atoms.  From this point on, we will just say radiation.  One source of radiation is the nuclei of unstable atoms.  For these radioactive atoms (also referred to as radionuclides or radioisotopes) to become more stable, the nuclei eject or emit subatomic particles and high-energy photons (gamma rays).  This process is called radioactive decay.  Others are continually being made naturally or by human activities such as the splitting of atoms in a nuclear reactor.  Either way, they release ionizing radiation.  The major types of radiation emitted as a result of spontaneous decay are alpha and beta particles, and gamma rays.  X rays, another major type of radiation, arise from processes outside of the nucleus.    

Alpha Radiation.

Alpha particles are energetic, positively charged particles (helium nuclei) that rapidly lose energy when passing through matter. They are commonly emitted in the radioactive decay of the heaviest radioactive elements such as uranium and radium as well as by some manmade elements.  Alpha particles lose energy rapidly in matter and do not penetrate very far; however, they can cause damage over their short path through tissue.  These particles are usually completely absorbed by the outer dead layer of the human skin and, so, alpha emitting radioisotopes are not a hazard outside the body.  However, they can be very harmful if they are ingested or inhaled.  Alpha particles can be stopped completely by a sheet of paper.

Beta Radiation.

 Beta particles are fast moving, positively or negatively charged electrons emitted from the nucleus during radioactive decay.  Humans are exposed to beta particles from manmade and natural sources such as tritium, carbon-14, and strontium-90.  Beta particles are more penetrating than alpha particles, but are less damaging over equally traveled distances.  Some beta particles are capable of penetrating the skin and causing radiation damage; however, as with alpha emitters, beta emitters are generally more hazardous when they are inhaled or ingested.  Beta particles travel appreciable distances in air, but can be reduced or stopped by a layer of clothing or by a few millimeters of a substance such as aluminum and is known to be able to be completely stopped by a book.

A particle related to beta decay is the position, otherwise referred to as β⁺.  In beta decay, a neutron located within a radioactive nucleus breaks down into a protron and a beta particle (referred to as a positron or electron).  A positron or β⁺ is a positively charged electron, and an electron or β^- is a negatively charged electron.  The beta particle (β⁺ or β^-) is emitted from the nucleus of the radioactive material.  A positron is the anti particle or counterpart of an electron.  The positron has an electric charge of +1 and a spin of 1/2.  It has the same mass as an electron.  Positron emitters include ¹¹C, ¹³N, ¹⁵O, ⁶⁸Ga, ⁸²Rb, and ¹⁸F.  ¹⁸F is the work horse of PET-CT.   The positron can be generated by positron emission radioactive decay.  During PET imaging, a positron meets with an electron and their mass is converted into kinetic energy.  The kinetic energy is referred to as gamma radiation. 

Gamma and X Radiation.

 Like visible light and X rays, gamma rays are weightless packets of energy called photons. Gamma rays often accompany the emission of alpha or beta particles from a nucleus. They have neither a charge nor a mass and are very penetrating.  One source of gamma rays in the environment is naturally occurring potassium-40.  Manmade sources include plutonium-239 and cesium-137.  Gamma rays can easily pass completely through the human body or be absorbed by tissue, thus constituting a radiation hazard for the entire body.  Several feet of concrete or a few inches of lead may be required to stop the more energetic gamma rays.  X rays are high-energy photons produced by the interaction of charged particles with matter.  X rays and gamma rays have essentially the same properties, but differ in origin; for example, x rays are emitted from processes outside the nucleus, while gamma rays originate inside the nucleus.  They are generally lower in energy and therefore less penetrating than gamma rays.  Literally thousands of x-ray machines are used daily in medicine and industry for examinations, inspections, and process controls.  X rays are also used for cancer therapy to destroy malignant cells.  Because of their many uses, x rays are the single largest source of manmade radiation exposure.  A few millimeters of lead can stop medical x rays or gamma rays.

Then there’s background radiation also referred to as natural radiation which humans are primarily exposed to from the sun, cosmic rays, and naturally occurring radioactive elements found in the earth's crust. Radon, which emanates from the ground, is another important source of natural radiation. Cosmic rays from space include energetic protons, electrons, gamma rays, and X rays. The primary radioactive elements found in the earth's crust are uranium, thorium, and potassium, and their radioactive derivatives. These elements emit alpha and beta particles, or gamma rays.

Most of the population’s exposure comes from naturally occurring background radiation, 55% percent of our exposure to natural sources of radiation usually comes from radon (Levchuck, 2000).  Radon is a colorless, tasteless, and odorless gas that comes from the decay of uranium found in nearly all soils.  Levels of radon vary throughout the country.  Radon usually moves from the ground up and migrates into homes and other buildings through cracks and other holes in their foundations.  The buildings trap radon inside, where it accumulates and may become a health hazard if the building is not properly ventilated. When you breathe air containing a large amount of radon, the radiation can damage your lungs and eventually cause lung cancer. Scientists believe that radon is the second leading cause of lung cancer in the United States. It is estimated that 7,000 to 30,000 Americans die each year from radon-induced lung cancer. Only smoking causes more lung cancer deaths and smokers exposed to radon are at higher risk than nonsmokers.

We receive about eight percent of our exposure to radiation from other radioactive elements in the earth's crust, such as thorium and potassium. Radiation levels from these sources vary in different areas of the country. This is called terrestrial radiation. Given that, this radiation is in the soil and also in our vegetation.

Another eight percent of our radiation exposure comes from outer space. This cosmic radiation originates in our galaxy, other galaxies, and our own sun. Our exposure to cosmic radiation depends in part on the elevation where we live. For example, people who live in Denver, Colorado, which is more than 5,000 feet above sea level, are exposed to more cosmic radiation than people living in Chicago, Illinois. Because Chicago is only approximately 1,000 feet above sea level, it has a thicker atmosphere, which can filter out more cosmic radiation than Denver's thinner atmosphere.

We get 11% of radiation exposure from inside our own body. All people also have radioactive potassium-40, carbon-14, lead-210, and other isotopes inside their bodies from birth. The variation in dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources.

Medical x rays also account for 11% of our radiation exposure. Nuclear medicine procedures, however, only account for 4%, which is just 1% more than consumer products (Sources of ionizing radiation, 1990).  PET-CT is a sister modality of nuclear medicine.  As will be explained later in this report, PET-CT does have a higher dose than nuclear medicine.

PET-CT is a diagnostic procedure that incorporates the use of nuclear medicine and computed tomography technology.  Before the procedure begins, a radioactive substance is produced in a cyclotron and placed in a syringe shielded with tungsten or lead.  The most commonly used radiopharmaceutical for clinical PET imaging is ¹⁸F labeled fluorine ions to form a sugar analog (glucose).  Once this substance is administered to the patient, the radioactivity localizes in areas of high metabolism and is detected by the PET scanner.  This will provide information about chemical activity of normal and abnormal tissue.  This procedure involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons.

During PET imaging, a positron meets with an electron and their mass is converted into kinetic energy.  The ¹⁸F, which is the work horse of PET, emits a positron from the injected radioisotope.  The positron meets with an electron within the patient and their mass is converted into two 511 keV gamma photons.  The photons strike the PET detector at 180 degrees from each other.  The detectors are constructed in a ring around the patient so that many slices of data that can be obtained at one time. 

In addition, the procedure involves the use of computed tomography (CT) since it provides detailed, cross-sectional anatomical views of the body.  It utilizes a computerized tomography to obtain image data from various angles around the patient and then uses computer processing of the information to reveal a cross-sectional image of body tissues and organs.  The computer software in PET-CT fuses both images together, which provides images of both function and anatomy.  It enables physicians to evaluate a variety of diseases. 

Metabolic or biological activity of disease can be demonstrated on PET-CT prior to it being detected on a diagnostic x-ray, CT, or MRI procedure.  Unfortunately, the cost of a PET-CT scan is extremely high in comparison to other imaging modalities.  The PET-CT can cost approximately $5,000 in comparison to a CT procedure that would cost roughly $800 - $1000. 

In PET-CT, the technologist primarily works with gamma and x-ray photons which range in energy from 100 to ___ keV.  This is ionizing radiation to which the technologist is exposed. These photons can cause damage to cellular DNA.  At low doses, which are comparable to natural background radiation levels, the cell is fully capable of repairing any damage it may have sustained.  At higher doses of approximately 100 rem, the cells will either die or be permanently altered.  Exposure to ionizing radiation may produce biological effects such as cataracts, growth impairment, erythema, genetic effects, and epilation. 

The most commonly talked about health hazard of low-level exposure for a technologist is the potential for cancer.  The risk of cancer is increased as exposure to radiation is increased. There are, however, no forms of cancer that are unique to exposure to ionizing radiation.

The majority of the exposure a technologist receives actually comes from the patient after injection rather than from the radiopharmaceutical.  The most common ways of receiving this exposure are injecting without using a syringe shield and not maximizing your distance from your patient.  Also, a lead or tungsten shield should be used for all radiopharmaceutical injections.  The PET-CT technologist receives an average of 300 mrem occupational dose (Thompson, 2001).

Once the link between cancer induction and high doses of ionizing radiation became evident, there was a shift in radiation protection philosophy (Ron, 2003).  This relationship between occupational dose and risk led to the implementation of the ALARA concept by stressing the importance of time, distance, and shielding.  This principle enables the technologist to keep his/her exposure at the lowest level possible.  Because of current regulatory safeguards, it is very rare for a radiation worker to exceed the annual occupational dose of 5 rem or 5000 mrem.

In PET-CT, the patient receives an average dose of 18 mCi from the PET.  The minimum

dose received is 10 mCi, and the maximum received is 20 mCi.  In addition, the patient receives 800 to 1000 mrem from the CT procedure (Radiology Rounds).  According to the Journal of Nuclear Medicine, the combined exposure is 2.5 rem.   On the other hand, the average dose from a nuclear medicine procedure is 15 mCi.  Therefore, the dose level from a PET-CT procedure is higher than that of nuclear medicine procedures.  Nevertheless, the dose is still lower than the background radiation received by individuals. 

One of the basic principles of radiation protection in all imaging modalities is the benefit v.s. risk relationship.  This principle states that the benefit of having a procedure must outweigh the risk of the dose received during the exam.  Fortunately for both the physician and the patient, the PET-CT procedure provides immediate results to the physician in the form of a diagnosis. 

The procedure accurately pinpoints the precise location of the cancer, immediately provides location of metastases, and provides standard uptake values.  Standard uptake values indicate the activity level of the cancer cells.  Another benefit of PET-CT procedures is that radiation therapy planning can take place immediately following completion of the PET-CT procedure.  No other imaging modality can accomplish all of the above items.  Therefore, the benefit of the procedure does outweigh the risk of the dose.

In terms of patient and technologist knowledge of PET-CT procedures, the cognitive level is low for both groups.  It is unfortunate that an imaging modality that has been around for more than thirty years has resulted in very little information to patients nor health care providers.   A total of one hundred patients and fifty technologists were surveyed as to their knowledge of PET-CT as well as the dose received by patients during the procedure.

Out of one hundred surveys of patients, it was determined that no patients knew about the dose being received during the PET-CT procedure.  Even though twenty percent of the patients had nuclear medicine bone scan procedures, one patient thought he was receiving less dose from the PET-CT procedure than from former nuclear medicine procedures.  This information was inaccurate and the dose was explained to the patient.  The remaining 99 patients had no idea of radiation dose from either procedure. 

In addition, only thirty six percent of the patients were well informed about the PET-CT procedure.  Of the thirty six patients, thirty gained information as a direct result of having prior PET-CT procedures.  The other six patients conducted their own research on the world wide web about the procedure.  The remaining fifty four percent of the patients surveyed were obviously uninformed and knew absolutely nothing about their procedure.  Forty eight percent of the patients were given incorrect information from their physician’s office about the procedure.  Not surprisingly, the remaining six percent of the patients showed up for the procedure but had no clue as to what exam they were going to have completed. 

            A separate survey was conducted on radiologic technologists who are employed at area hospitals in northeast Tennessee.  Survey results reveal that twenty percent of the technologists had heard of PET-CT but had no idea of what it entailed.  The remaining eighty percent of the technologists indicated that they knew absolutely nothing about PET-CT.

            Survey results also indicated that ninety percent of the technologists surveyed were not aware of the dose received by patients during the procedure.  The remaining ten percent of the radiologic technologists assumed that the dose was higher in PET-CT than in nuclear medicine.  These technologists made the assumption due to the combination of the PET and CT.

            Therefore, thirty six percent of the patients were aware of the PET-CT procedure; however, thirty percent of the patients were returning for the same exam.  On the other hand, none of the technologists had a clear understanding of the PET-CT procedure.  Only twenty percent had even heard of PET-CT.  Ten percent of the technologists assumed that the dose was higher in PET-CT than in nuclear medicine.  On the other hand, ninety percent of the technologists and zero percent of the patients were aware of the dose compared to nuclear medicine.

Discussion:

 Studies show that approximately 20% of the adult population will die of cancer resulting from causes other than occupational exposure. These causes include smoking, alcohol, drugs, pollution, natural background radiation, and genetics. Although genetic effects are often linked to radiation exposure, there is no direct evidence of radiation-induced genetic effects in humans at high doses.

As a result of surveys conducted, it has been determined that patients, health care providers, and the general public needs an education of the positron emission tomography – computed tomography procedure as well as the dose received by the patient.  Patient education should come initially from physician’s offices in the form of brochures and information from the physician and his office staff.  Radiologic technologists need information in the form of conference sessions and education during radiography programs. 

Conclusion:

Medical radiation exposure accounts for a very small portion of our total radiation exposure. Therefore, having a PET-CT scan is similar to using your microwave and watching television, which is something people do everyday.  Every individual is exposed to a certain level of radiation on a daily basis.  As long as ALARA principles are in place to minimize radiation exposure to both patients and technologists, we should not be alarmed by the use of radiation for PET-CT procedures.  PET-CT procedures are not well known in the healthcare community nor among the general public.  Therefore, information must be disseminated to increase the awareness of this imaging modality.

References

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Levchuck, C.M. (2000).  Environmental Health. Healthy Living.

Ron, E. (2003).  Cancer risks from medical radiation. Health Physics, 85, 47-59. Retrieved

October 4, 2004, from PubMed database.

Saha, Gopal B.  (2005).  Basic of PET Imaging.  Cleveland:  Springer.

Task-specific monitoring of nuclear medicine technologists’ radiation exposure. (2004). National

Library of Medicine. Retrieved October 4, 2004, from Medline database.

Thompson, M. A. (2001). Maintaining a proper perspective of risk associated with radiation

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