TOPIC 2: Types of Dose

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Let’s explore types of dose

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Obviously, the total dose delivered to a select part of the body, or to the body as a whole, is important.  But the rate at which it is delivered is also important. For example, a whole-body dose equivalent of 50 rem received rather evenly over a 30-year working life sounds rather large, but would be within limits and probably no negative effects would result. However, if the same 50 rem dose is received within a few minutes, it would greatly exceed safe limits.

To begin our discussion on doses, we should learn the different types of dose rates: acute, fractionated and chronic.

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An Acute dose is delivered in a short time period.  Radioactive accidents are generally acute exposures at high doses.  Nagasaki, Hiroshima and Chernobyl are all examples of events with high acute doses.  In all cases, a large amount of energy was deposited into the bodies of those nearby within a short period of time. Fortunately, Most industrial and domestic accidents involving radioactive materials do not reach these dose levels.

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LD50 is the Lethal Dose to Fifty Percent of those that were exposed.

The LD50 for radiation is 350 Rad.  This is the lethal dose to 50% of the irradiated population within 30 days. For example, if a nuclear bomb detonates over a city and a subset population is far enough away to survive the initial blast, but is irradiated with 350 rad, then we would expect half of those people would die within a month or two without medical treatment.

A dose of this magnitude damages bone marrow and reduces the body’s immunity. White blood cells defend us from infection, red blood cells transport oxygen to other cells, and blood platelets are needed to make the blood clot. Furthermore, the cells that line the intestine only live a few days, so injured intestines often bleed internally. Vomiting and diarrhea are common symptoms.  And since the lining of the intestines are damaged the body is unable to absorb the nutrients it needs from food.

Age and health of the exposed population is a factor. A relatively healthy person could withstand a larger dose than a weaker or older individual.

You may have noticed that we used rad instead of rem. Dose equivalent doesn’t mean much at high acute doses. We use absorbed dose instead of dose equivalent because the outcome of concern is short-term death, not long-term risk of cancer.  REM is used to determine long term effects.

The good news is that these doses and effects are far beyond what is possible with the sources you will be dealing with.

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Doses below 100 rad (or if we are using the International Scientific Units …1 Gy) do not cause any recognizable signs of illness. A person exposed to this dose will not feel any effects. If a person feels ill at all, it is coincidental or the result of fear and anxiety. It is, however, possible to detect this excess dose with blood analyses. The number of circulating blood cells may alter temporarily according to the type of cells. The number of white blood cells (leukocytes) may decrease temporarily. The numbers will return to normal in a couple of weeks.

Acute doses of about 200 rad (2 Gy) rarely cause any immediate danger to life. The mildest symptoms of radiation illness may be seen as tiredness, flu-like symptoms, vomiting and lack of appetite for several days or weeks. In more severe cases, patches of hair may fall out. Even with these symptoms, people fully recover with no permanent damage.

If the dose is greater than about 300 rad (remember the LD50 is 350 rad, there will be recognizable signs and symptoms, such as, reddening, hair loss, blistering, ulcers and others.

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After studying atomic bomb survivors, there are three recognized syndromes associated with acute, high doses of whole-body irradiation:

• Cerebrovascular syndrome
• Gastrointestinal syndrome
• Hematopoietic syndrome

Initially, we may have no idea that a person was irradiated, or even if we know, we may not have a good idea of the magnitude of the dose. There are some radiation sickness symptoms that will give clues about the dose. Certain effects are common to all three acute dose categories: nausea and vomiting, malaise and fatigue, increased temperature, blood changes, and others.

Symptoms may occur early after irradiation and last for a limited time

oIndividuals are different so there will be a difference of when the symptoms will begin to show, the severity of the symptoms and the duration

• At doses close to LD50 symptoms are usually anorexia, nausea, vomiting, and fatigue
• At even higher doses symptoms are immediate – diarrhea, fever, or hypertension

Fortunately, any direct health effects, or recognizable signs of illnesses caused by radiation, cannot be detected as long as the dose limits remain below regulatory limits.  Something you will learn soon.

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  With the Cerebrovascular Syndrome, it is a

• Whole-body exposure of 2,000 rad (gamma) (20 Gy)
• Resulting in unconsciousness within minutes,
• Death within hours
• There are no survivors no matter the remedies applied

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Gastrointestinal Syndrome

Is an effect to the cells that line the small intestine. Nutrients are not absorbed and passed to the blood.

• Damage to these cells occurs with Whole-body exposure of 1,000 rad
• Symptoms include Severe nausea, vomiting and diarrhea very soon after exposure.
• Death in about one or several weeks at the most
• There is no effective medical treatment.

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Hematopoietic Syndrome

• Occurs with Whole-body exposure of 300 – 800 rad (3-8 Gy)
• Sensitive target is bone marrow, which produces blood cells
• Nausea and vomiting appear within several hours of exposure.
• Hair loss is almost always seen 2 to 3 weeks after exposure.
• Symptoms result from a lack of circulating blood cells which is more visible about 3 or more weeks after exposure. This is something we can observe and track with blood samples.
• The first treatment is blood transfusion
• Further treatment could include bone marrow transplant

Complications with bone marrow transplants have led to alternative treatments that stimulate the existing bone marrow to resume production of new blood cells.  Patients receiving radiation therapy for cancer treatments, may experience these types of effects – but only temporarily.

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One last way to get an idea of what radiation injury looks like is to actually see it. The three incidents that follow illustrate the dangers that commercial radiation sources can present if they are not properly safeguarded or used safely.

WARNING: THIS SECTION CONTAINS GRAPHIC IMAGES!

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Radiation Incident 1: The victim unknowingly placed a 164 GBq (4.4 Ci) cesium-137 source in a coat pocket that he later used as a blanket. The picture was taken 4-8 months after the exposure and shows an ulcer in subacute stage and 5 other ulcers that healed but left the skin without pigmentation. This Image courtesy of the World Health Organization.

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Radiation Incident 2: The following injuries occurred after exposure to an iridium-192 source – which is used in industrial radiography.  Contact with the radioactive source was for only a few minutes.  Blistering had occurred within 10 days after the initial contact.  This photo of the left palm was taken on day 20 showing skin reddening (erythema) and painful blistering.

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Radiation Incident 3: The following series of photos show early skin reddening in the front and forward right side of the chest 5 days after exposure to 5 Ci (185 GBq) iridium-192 source.  This is a small source, mounted on a source holder and is about the size of a pencil and used for industrial radiography.  The worker put in in his pocket for about 2 hours. These photos show the progression of symptoms.

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Delayed effects can result from large or small doses that are acute or fractionated. The effects of concern are cancer, genetic, life shortening, and cataracts. There are over 300 different kinds of cancer. Radiation exposure does not induce any unique kind of cancer. Cancers don’t appear immediately. They all have latent periods, or a delayed time until the cancer can be detected. The minimum latent period for leukemia is 2 years and it peaks at about 10 years post-exposure. But solid tumors have much longer latent periods.

Most radiation induced cancers are associated with the blood forming system (bone marrow for instance), thyroid, bone and skin. After x-rays became common place, skin cancer was the first type of malignancy to be observed. At low doses the data for cancer induction is inconclusive, because some data show harm while others indicate a protective effect (radiation hormesis).

Leukemia was observed in physicians who used x-rays to treat a variety of conditions in the 1930s – 1950s. During that period, 14,000 patients were treated with several hundred rem for a spinal condition in Great Britain. There was a small increase in leukemia in this group. Similar outcomes were observed in the atomic bomb survivors.

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To put things in perspective, the Japanese atomic bomb survivors have been intensely monitored for decades. First, let’s look at somatic effects (effects to the tissues). Of the 100,000 survivors who received the very highest doses, a cancer increase of 6% was observed. Most of these cancers were confined to specific types: leukemia, cancers of the thyroid gland, and breast cancer. No excess cancers were found among the survivors who received a dose of 30 rem or less (which we will soon learn is 6 times our annual occupational dose limit). The Health Physics Society – the society of radiation protection specialists – states that below 10 rem there is absolutely no adverse health effect.

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Let’s look at genetic effects. Hiroshima and Nagasaki survivors exposed to high radiation doses have had more than 80,000 children and grandchildren during the 70+ years since the bombings. No hereditary effects above normal probabilities, including mutations or genetic deformities, have been found. Radiation does not produce new and unique genetic mutations. So even with close scrutiny of this large population of people exposed to large radiation doses, no information has been gained to help establish a numerical risk factor for genetic damage.

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The unborn may suffer in-utero effects. Meaning the unborn was conceived and the mother was pregnant at the time of exposure.  Much of our knowledge of in-utero effects comes from the pregnancies at the time of the Hiroshima and Nagasaki nuclear bombings. Obviously, these were very high, acute doses. In-utero effects are not statistically evident above the background level at the 5 Rem annual dose limit for a radiation worker (you’ll learn more about that later). However, regulators are extremely cautious and limit the dose for the duration of the pregnancy for a worker to 500 mrem, which is 10% of the annual radiation worker dose.

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And finally, let’s look at the delayed effects. These include somatic and genetic effects that do not occur soon after irradiation, and can also include effects that occur much later in life due to in-utero exposure. These delayed effects are the source of much scientific uncertainty that complicates our ability to determine the risk of mortality from doses of radiation.

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Let’s talk a little more about delayed effects of radiation.  A famous study was conducted on hundreds of women who painted radium on luminous watch dials up to 1925. They would use their mouths and tongues to shape their radium-laden brushes. All isotopes of radium are radioactive. These women received from 100 to 600 mrem per week via ingested radium. Radium is chemically similar to calcium and will be used by the body in bones. As these women aged, there was a higher incidence of bone cancer.  Many of these women experienced necrosis (deterioration) of the bone at a faster rate than the development of cancer.  They were losing their teeth, As the tooth was pulled, the injured area would not heal, or the jaw bones would break apart.  Many of these women died of bone cancers before they were 30 years old.  The study of these women and their sacrifices was the beginning of establishing federal standards for using specific toxic and radioactive materials in the workplace.

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The Radium Girls book written by Kate Moore describes the Dark Story of America’s “Shining” women.

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Fractionated doses are aa series of discrete, acute doses.  Radiation treatment for cancer is an example of fractionated doses. In other words, instead of one massive dose, the dose is broken up into a series of smaller doses given in time intervals – usually over several weeks. A single high dose would kill the cancer cells, but would also harm healthy cells.  Fractionated doses target the tumor at different angles for different treatments.  The tumor receives a dose with every treatment but the healthy tissue only when it is in the path of the beam. Both normal cells and cancer cells can repair and repopulate, but they do so on different timelines that can be exploited with the fractionation strategy. Cancer cells are killed and healthy cells are spared to a greater extent. It isn’t all that simple and other factors are involved, but that’s information for another course. Remember, the oncologist is in a race to kill the cancer before the cancer kills the patient.  Many times the patient will have radiation sickness symptoms. But these are temporarily.

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A chronic dose is a constant dose that is present over a long period of time.  All humans are constantly being bombarded by radioactive materials from the sky (cosmic), from building materials, from the air we breathe and the food we eat.  Radon is a source of radiation that is breathed in and out constantly and would be considered chronic dose.  Radiation workers that receive minute amounts of radiation from their workplace over a long period of time are considered to receive chronic doses.  While there are no known effects from chronic doses, if there is any effect at all, we may see it in many years in the form of cataracts, cancer, or some other life shortening effect.

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Breathing radon is considered a CHRONIC dose of natural radiation because it is all around us and we get a dose from it over a long period of time.