Total Effective Dose Equivalent Question

Asked on YouTube:

"The question still remains: how does the test account for the concentration of energy, i.e., the difference of being zapped by x amount of energy (expressed in gamma) while flying in an airplane vs letting the same amount of energy (x) accumulate in your thyroid? Radiation therapy is based on this - the energy is concentrated and directed to a specific spot (cancer); in case of Fukushima, one absorbs radiation internally, which accumulates in a certain organ and kills it"


First, the radiation levels associated with Fukushima are not high enough to kill any organs.  The biological effect of concern in both cases (airplane or Fukushima) is cancer.

Second, dose is absorbed energy, units are Gray (1 Gy = 1 Joule/kg).

Let's take the two examples:  Case 1 = whole body gamma radiation,   Case 2 = alpha emitter in lungs


Step 1 - Determine Dose

Case 1:  The "kg" is the weight of the whole body.  To determine the Joules, one has to estimate the energy the body is exposed to and then calculate how much is absorbed in the body versus how much passes through the body (this is estimated based on the energy of the gamma radiation and the known density of the body).  After this step we have a dose rate of Gy/unit time.  So we have to multiply by how long one is in the radiation field to get Gy.  Let's just assume in this case we have 1 Gy.

Case 2:  The "kg" is the weight of the lungs.  To determine the Joules, one has to know the energy of the alphas being emitted, and calculate how much is absorbed in the lungs (which will be virtually all of it).  After this step we have an estimate of the number of Gy/unit time. We have to look at the radioactive half-life and biological clearance half-life to determine how long the material resides in the lungs and multiply by this time frame (actually it's an integration).  After determining this, we have Gy. Let's just assume in this case we have 1 Gy.

Step 2- Determine Equivalent Dose

Case 1: Gamma radiation is taken as the baseline.  It has a radiation weighting factor of 1 by definition.  The Sievert is the unit of equivalent dose.  1 Gy x 1 = 1 Sv.

Case 2:  Alpha radiation deposits its energy in a more concentrated fashion.  It has a radiation weighting factor of 20.  1 Gy x 20 = 20 Sv.


***For high doses, like above we would stop there and not add the two doses.  We would be concerned about acute effects most importantly.  But once the person is given treatment for the acute effects, we next look at cancer risk. Since very, very few people ever receive such high doses, we normally continue to next step, because cancer is the only risk of concern.***


Step 3 - Determine Total Effective Dose Equivalent (also has units of Sv)

Case 1:  The whole body is taken as the baseline.  It has a tissue weighting factor of 1.  1 Sv x 1 = 1 Sv.  This is called the effective dose equivalent.

Case 2:  A little more complicated.  We perform a 50-year integration (we look at radioactive half-life and biological half-life of the isotope and integrate the dose received over an expected 50 years into the future).  Even though the dose is ACTUALLY delivered over 50 years, we ASSUME it is all delivered in the year the intake took place.  This is called the committed equivalent dose.  But the whole body isn't irradiated over the 50 years, only the lungs are.  So we compare the cancer sensitivity of the lungs to the whole body.  This has been found to be 0.12.  20 Sv x .12 = 2.4 Sv.  This is called the committed effective dose equivalent.

If one person had both of these Cases happen to him/her, the TEDE would be (1Sv + 2.4 Sv) = 3.4 Sv.

Hopefully you get it!!