What is Radiation?

To understand the effects of radiation in general, one must first understand radiation itself.  Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials. Light, radio, and microwaves are types of radiation that are called non-ionizing. The kind of radiation discussed here is called ionizing radiation because it can produce charged particles (ions) in matter.  There are many types of ionizing radiation; alpha, beta, gamma, neutron and X-rays are examples.  Most of these types of radiations are emitted from unstable atoms such as may be found in nature (uranium, thorium, radium, etc.), or those made by man (plutonium, americium, curium, etc.).  Some very high energy radiations bombard earth from space, and are known as cosmic radiation.  Other types of radiation are generated by machines such as X-Rays.  X-Rays are produced by bombarding a metal “target” (“anode” in Figure 1[1]) with electrons.

How is Radiation Measured?

Radiation is measured in units of energy absorbed by the media it impinges upon[2].  Units of measure include the Roentgen (R), Roentgen Adsorbed Dose (RAD) and Roentgen Equivalent Man (rem). For all practical purposes gamma and X-Ray radiations 1 R = 1 rem.  “Scalars” are often used to conveniently describe the amount of radiation.  The most common scalars are “milli” (1/1000, or 0.001), “micro” (1/1000000, or 0.000001), and kilo (1000).  Therefore, 1 milli Roentgen (abbreviated mR) = 0.001 Roentgen, 1 micro Roentgen (abbreviated µR) = 0.000001 Roentgen.

Radiation is measured by instruments that convert the effects of radiation impinging on a “detector” (typically ions or ion pairs) to electronic signals.  Radiation detectors measure either an exposure- or dose rate (e.g., milli-roentgens per hour, or mR/hr), or total exposure or dose (e.g., mR).  Dose rates are typically used to calculate total dose.  If as an example, a dose rate of 5 mR/hr is measured, exposure to this dose rate for 2 hours would result in a total dose of 10 mR (5 mR/hr X 2 hr = 10 mR).  Instruments that measure total dose perform this calculation for the user for as long as the detector is exposed to the radiation field.

Radiation Effects

The subject of radiation effects has filled volumes.  Consequently, the following discussion has been greatly generalized and simplified.

Radiation effects can be generally classified as one of two different manifestations; somatic effects, and genetic effects.  Somatic effects are those appearing in the person exposed; whereas genetic effects are those appearing in the offspring of the person exposed.  Somatic effects can be further [sub-] classified into two categories; early and late effects. Early effects in humans include nausea, hair loss, fatigue and changes in blood chemistry (notably lowered white blood cell and lymphocyte counts).  The most common late effects are various types of cancer (most commonly leukemia, with lesser instances of stomach, breast and lung cancer).

Genetic effects in humans generally manifest themselves as cancer in the offspring.  Radiation causes a phenomenon known as "genomic instability", a tendency for genetic material in the cells to recombine to cause unpredictable mutations that in turn could cause cancers.  Other genetic anomalies observed on offspring of exposed human parents include reduced brain size and reduced mental acuity.

How Much Radiation is Safe?

The onset and severity of radiation effects vary substantially with several factors, including magnitude of the exposure, how fast the dose was received, the part of the body exposed to the radiation, and to some degree, age.  While there are several theories concerning the effects of exposures to low levels of ionizing radiation (linear-no-threshold {any amount of radiation exposure is harmful}, hormesis {a little radiation is good for you}, and threshold {negligible effects below a given radiation dose level or threshold}); international and US regulators and standards bodies recommend the linear-no-threshold theory.  Because of these factors, radiation effects as a function of dose cannot be precisely predicted.  As a consequence effects are generally stated in a statistical fashion.  As an example, 100-200 rem received over a very short period of time will result in serious radiation sickness effects and hemorrhage; untreated this exposure is a lethal dose to 10-35% of the population after 30 days (LD 10-35/30).

Because of these levels of uncertainty, radiation exposure limits are set at low levels (0.1 rem per year for members of the public, as compared to 5 rem for radiation workers[3]).  Special limits are placed on the embryo/fetus because rapidly dividing cells in the embryo/fetus are much more sensitive to radiation than cells in an adult.  Pregnant radiation workers are limited to a total of 0.5 rem through the entire 9 months of the pregnancy.  The risks at this or higher levels are still controversial.  For this reason, a philosophy of limiting radiation exposures to levels that are As Low As Reasonably Achievable (AKA, ALARA) is imposed by US regulators.  As implied in the foregoing, radiation exposures are considered to be cumulative in so far as radiation effects are concerned.

Are Dogs Different?

Little data are available which describe the effects of low levels of radiation on puppies in utero.  In humans, the following risks have been published by Duke University:

·         “Fetal Dose Less Than 1 rem -- There is no evidence supporting the increased incidence of any deleterious developmental effects on the fetus at diagnostic doses within this range.

·         Fetal Dose between 1 rem and 10 rem -- The additional risk of gross congenital malformations, mental retardation, intrauterine growth retardation and childhood cancer is believed to be low compared to the baseline risk. However, the lower limits (in terms of statistical confidence intervals around the mean) for threshold doses for some studies, especially those related to cancer induction, fall within this range.

·         Fetal Dose Exceeding Than 10 rem -- The lower limits (in terms of statistical confidence intervals) for threshold doses for effects such as mental retardation and diminished IQ and school performance fall within this range. Overall, exposure at levels exceeding 10 rem could be expected to result in a dose-related increased risk for deleterious effects. For example, the lower limit (95% confidence interval) for the threshold for mental retardation is about 15 rem, which an expectation value of about 30 rem.”

Generally, most human X-Ray film procedures for the abdomen result in radiation exposures of less than 0.5 rem.  Fluoroscopy procedures result in higher exposures in the 0.5 to 2 rem range.  Veterinary administered X-Rays may well fall within the same ranges.  However, vets typically like to take multiple exposures, which obviously increase the dose to fetuses.  It is possible that multiple X-Rays of a pregnant bitch could well result in a dose to the puppies of in excess of 1 rem, which may be harmful.

Recommendations

If your vet believes that there is a health concern for which an X-Ray would prove to be beneficial, by all means have the procedure performed. You should, however, ask the vet to minimize the number of X-Rays.  However performing X-Rays to determine the number of puppies present in utero, or any other trivial reason, should be avoided.

E-Mail Nuke.Advisor@GMail.com for additional information.

[1] http://www.physics.isu.edu/radinf/xray.htm

[2] “Ergs per gram” was originally used for describing the R, RAD and rem; where an erg is defined as the amount of energy required to move 1 gram a distance of 1 centimeter.  There are approximately 454 grams in a pound, and 2.54 centimeters in an inch.  Currently the units have been changed to joules per kilogram.