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June 2010 - December 2013
II - Mammography
16. Radiation primer
Another risk factor of the mammography X-ray screening of asymptomatic women, exposure to ionizing radiation through the screening procedure, is potentially significant and, at the same time, quite obscured and often misrepresented. Let's look at the facts.
There is no doubt that ionizing radiation causes cancer, and that the risk cannot be excluded even at the lowest exposure levels. What is, exactly, the mammography radiation risk? What is, in the first place, the radiation dose delivered by mammography?
Numerical values for the radiation dose published in the variety of official, research and media sources vary from a fraction of 1 to over 5, or so. They come in units of millisieverts, millirems, milligrays, or some others... they come as absorbed, or equivalent, or effective dose... How to make sense of all that?
One step at the time.
Before answering the question of mammography radiation risk, let's take a look at the basic terms and units used for radiation doses. It starts with the absorbed dose, which shows how much of radiation energy - expressed in units of roentgen (R), which is radiation energy that transfers 1 columbus of charge to 1kg of air - has been absorbed by the tissue. The basic unit here is the Gray (Gy), which represents one Joule of energy absorbed per kilogram of tissue (the older unit it had replaced is rad - from roentgen absorbed dose).
But not all radiation energy is created equal. For given absorbed dose, some will inflict more damage, and some less - depending on the rate, or extent of their energy transfer to the tissue. That is expressed by the equivalent dose, which is a product of the absorbed dose of radiation (given in Grays, or rads) and radiation weighting factor WR (formerly called quality factor).
EQUIVALENT DOSE = ABSORBED DOSE x RADIATION WEIGHTING FACTOR
This factor varies with the type of radiation, as mentioned, depending on how much energy (i.e. damage) it actually transfer to biological tissues (specified by the linear energy transfer, or LET). The higher weighting factor, the more extensive damage.
The basic unit for expressing the equivalent dose is Sievert (Sv), or, alternately, rem (from roentgen-equivalent man), the former being internationally recognized unit, and the latter one still commonly used in the U.S.
Official values for the radiation-weighting factor are only estimates. The WR value for alpha particles is set at 20, for neutrons at 5-20, depending on their energy level, while the WR value for X-rays and gamma radiation is 1, regardless of their specific energy level. There is god bit of evidence suggesting that the official estimate in this last assumption may be incorrect,
underestimating the impact of lower-energy X-ray radiation.
An analysis by Brenner (Does Fractionation Decrease the Risk of Breast Cancer Induced by Low-LET Radiation?, 1999) indicates that for a given total radiation exposure breast cancer risk is nearly halved for the fractional, extended in time exposure (such as periodic X-ray mammographic screening), as opposed to receiving the entire dose at once. This is biologically plausible, because it is to expect that the body's repair action is more efficient with a series of relatively small-scale molecular damages widely separated in time, than with a gross one-time damage.
The reason that the damage appears similar with that from fractional exposures is another (likely) incorrect official assumption, which is that both, X-rays and gamma radiation cause similar level of cancer-inducing tissue damage. More likely scenario, proposed by Brenner end others, is that the lower energy X-rays, used for medical treatments and diagnostics, inflict more damage to the tissues due to their higher "stopping power" (i.e. higher energy gamma radiation is more likely to zoom through the tissue without interactions). Brenner concludes that medical X-ray radiation is
nearly twice as damaging as gamma radiation
(which agrees with Gofman's conclusion), hence effectively offsetting damage decrease due to fractionated exposure.
A recent study also points to the evidence that low energy
X-rays (around 30 kVp - peak accelerating voltage - such as
those used for
It is important to keep in mind the likelihood of higher actual risk with lower-energy mammographic X-rays, because it can be significantly higher than what the official view indicates for a single and closely spaced multiple exposures to X-ray radiation.
Internationally, the corresponding units for expressing absorbed and
equivalent dose are Gray and Sievert, respectively. They are
related as shown below:
ABSORBED DOSE UNITS:
The millisievert (mSv) is commonly used to express the equivalent dose. It is also used for expressing the effective biological irradiation dose in the medical diagnostics field.
Since not all body tissues and organs are equally sensitive to radiation, the effect of any given equivalent dose will vary with the part of the body affected by it. For the purpose of making the risk from irradiation of different organs/tissues or their groups comparable, or for calculating the cumulative effective dose for all irradiated organs and/or tissues, the equivalent dose for specific organs/tissue is multiplied by its tissue weighting factor WT, reflecting its estimated relative sensitivity to the cancer-initiating damage from radiation.
The resulting value is called effective dose for that particular organ or tissue. Simply put,
it relates the risk of causing cancer in a specific organ or tissue due to the radiation dose absorbed to it, to the whole-body dose that would have comparable risk of inducing cancer anywhere in the body.
That gives to the organs and tissues with different radiation sensitivities a common denominator, enabling the comparison of respective risks.
If, for instance, breasts had absorbed 10mGy of X-ray radiation, hence the absorbed dose is 10mGy, the equivalent dose is obtained by multiplying it with the X-ray radiation weighting factor WR=1, as 10mSv, and the effective dose is obtained by multiplying the equivalent dose with the breast tissue weighting factor WT=0.12, giving 1.2mSv. The latter means that the risk of inducing breast cancer by exposing them to 10mSv equivalent radiation dose is estimated to be comparable to the risk of inducing cancer anywhere in the body by exposing the whole body to an effective dose of 1.2 mSv.
Likewise, if for instance, in addition to the breasts, brain also absorbed 10mGy, i.e. 10mSv as the equivalent dose, its effective dose, after multiplying with 0.01 tissue weighting factor for the brain, is only 0.1mSv. Now we can add the two together, which results in 1.3 mSv combined effective dose. This means that the risk from 10 mGy absorbed radiation by each, breasts and brain, poses as much of a risk to inflict cancer to either of the two, as it does the whole body exposure to 1.3 mGy absorbed (i.e. 1.3 mSv equivalent or effective) radiation.
This way, the risk from irradiating different tissues/organs, or body portions, can be expressed as a total risk, in terms of the risk associated with whole-body exposure as the common denominator. At least to the extent allowed by the (very) approximate nature of tissue weighting factors.
The latest WT
values set by ICRP (International Commission on Radiological
Protection) in 2007 are as follows:
1Remainder tissues: Adrenals, extrathoracic (ET) region, gall bladder, heart, kidneys, lymphatic nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen, thymus, uterus/cervix
Officially, the radiation weighting factor WR for both X-rays and gamma rays is 1, but it amounts to a gross approximation. The actual energy delivered to human tissues
tends to be significantly higher for X-rays which,
due to their generally lower energy level (i.e. higher stopping power), are more likely to interact with body molecules, instead of just zooming through. In his book, Gofman - who had both, medical degree and Ph.D. in nuclear chemistry - considers medical X-rays about twice as harmful as gamma radiation from atomic bomb.
Studies specifically designed to find what the facts are on this subject support his view. For instance, Brenner in his 1999 study finds that fluoroscopy X-rays are 1.6 to 1.9 times more damaging than the atomic bomb gamma radiation, with the apparent similarity of the degree of damage coming due to the typical X-ray exposure having about as much lowered harmful effect due to its fractionation (i.e. due to the dose in comparison - the cumulative dose - being delivered not at once, but through multiple exposures separated in time).
A series of studies in the 1960s and 1970s indicated that the lower energy X-rays are significantly more efficient in killing cells than higher energy gamma rays. A 1975 study found that 50kVp X-rays were nearly 1.5 times more efficient in killing cells, and nearly twice more efficient in causing double-strand DNA breaks than 660keV gamma radiation (Bonura et al.).
With mammography X-rays, at about 25kVp, having nearly three times lower energy than fluoroscopy X-rays (60-80 kVp), and about half the energy of 50kVp X-rays (used in Bonura et al.), their potential to cause biological damage, again, appears to be
nearly double that of gamma rays.
It suggests that the (unknown) accurate radiation weighting factor WR value for the mammography X-rays is also nearly twice higher than the current estimate, i.e. closer to 2.
The arbitrary nature of the current values is well illustrated by the magnitude of the last ICRP revision (2007), reducing, for instance, WR value for protons to less than a half the previous value, from 5 to 2.
Similarly, in the latest 2007 revision, the ICRP changed tissue weighting factor WT for the gonads from 0.20 to 0.08 and for breast from 0.05 to 0.12. It clearly indicates that WT values assigned to specific tissues and organs are an educated guess at best.
the actual risk also greatly varies individually;
genetic vulnerability, DNA repair efficiency, hormonal activity and level of oxidative protection are some of the major factors determining what the actual degree of damage will be.
After this long interlude, which should help understand the radiation-related terms, we can look at the mammography radiation exposure.