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CHAPTER 28
Mammography: Past and Present
Part 1. Patients Often in the Dark about Their Doses
One item has been missing for a long time in the use of medical
x-rays: The ability of a patient to find out (a) whether a
particular facility knows what it is doing, and (b) what dose of
radiation the patient is getting from any particular procedure.
Later (Chapter 42), we will discuss
whose responsibility it is to obtain such information for
patients, for this is not a problem only of the past.
Talking to the general public in Science Digest, two radiation
physicists described the situation rather memorably in 1984.
One of them was Dr. Joel Gray (of the Mayo Clinic), whose admirable
work on reducing radiation dose for scoliosis patients was already
described in Chapter 21. In the
1984 interview, he commented about offices which do not tell
patients the estimated dose of radiation which they will receive
(Gray 1984, p.96):
"My feeling is that if they won't tell you, they don't know, and if
they don't know, they could be among the facilities delivering a
hundred times the necessary dose."
The other physicist commented specifically on mammography. Dr.
Edward Webster, of the Massachusetts General Hospital, said
(Webster 1984, p.96):
"When a woman arrives at a doctor's office for a mammogram, she has
no way of knowing whether she's getting three hundred or three
thousand millirads."
A Lesson from Recent History: The 1960s and 1970s
If we go back to the 1960s and 1970s, the reality of mammographic
doses was startling --- for breast-doses were very high compared
with the current levels. Of course, we have no entry at all for
mammography in our Master Table,
because the table considers only
common procedures of the 1920-1960 period.
We will explore, later in the chapter, what it would have meant for
women if mammography had become common when the doses were so much
higher than today. It did not become common, thanks to some people
with a willingness to say "No!" to various pressures to make it
common. The history of mammography provides women with a reason to
think very seriously about the need for eternal vigilance --- and
for "watchdogging" of some technology buffs who are inclined to
claim that their pet technology is "safe," without any evidence for
such claims.
Part 2. Breast-Doses from Mammography in the 1960s and 1970s
Periodic reports on various aspects of ionizing radiation are
issued by UNSCEAR, which is the United Nations Scientific Committee
on the Effects of Atomic Radiation. The committee consists of
members of the radiation community in interested nations. About
mammography, the UNSCEAR 1977
report stated (p.318, para.70):
"Direct radiography of the female breast, i.e., mammography, is of
particular interest because the technique is also being used in
health investigations. The organization of a number of large
population mass-screening surveys caused concern when high-dose
techniques were in use and when regular re-examinations were
carried out on young women. The justification for such
examinations was questioned because of the increase in
breast-cancer that might be induced by radiation [references
cited]." And:
"In the 1972 report [UNSCEAR 1972] the radiation dose to the breast
per mammography examination was reported to be in the range of
10-35 rad. However, since then considerable progress has occurred
in techniques for reduction of radiation dose ..." And: with "...
the two-film techniques being accepted for screening examinations,
surveys can be undertaken with a breast dose of less than 200-300
mrad ..."
So UNSCEAR 1972 reported 10 to 35 rads as the mammographic dose in
the 1960s. To make the coming comparison, we will say 20 rads. And
UNSCEAR 1977
reported that it was technically possible to
reduce doses to 0.2 rads (200 milli-rads) or less by 1977. That is
a reduction of 100-fold. But as we shall see (Part
5), a large screening project had been undertaken in 1972 with
10-fold higher doses --- about 2 rads per exam.
Part 3. Breast-Doses from Mammography in Current Times
Improvements continue, and it appears that mammography will finally
have sufficient surveillance to insure that low doses become
commonplace. The Food and Drug Administration was asked by
Congress to set comprehensive standards for the nation's 11,000
mammography clinics, and to issue certification when such standards
are met. The FDA requirements, as described in late 1994, include
the following:
- Workers who perform mammography and physicians who interpret
the images must be properly trained and experienced. For
physicians, that includes interpreting at least 40 mammograms per
month. Each clinic must track its accuracy and insure that women
with suspicious results are promptly called for follow-ups.
FDA-trained inspectors will inspect each clinic annually.
- Only up-to-date mammography machines are allowed. They must be
monitored closely to insure that the images remain clear and that
the radiation dose does not exceed 0.3 rad (300 milli-rads, or 0.3
centi-gray) mean glandular dose per view. The standard exam has 2
views per breast (top to bottom, and side to side). So, on the
average, internal cells of each breast may receive up to 0.6 rad of
absorbed dose per exam --- but actual doses today per exam are
often as low as 0.2 rad for each breast.
Part 4. The Estimated Price in Breast-Cancers
from Full Implementation of Mammography
Although I consider myself an enthusiast for technological progress,
I recognize that the history of applied technology has often been
"act now, think later." Part 5 will describe an
example. So it would be worthwhile to ask what the price in extra
breast-cancer could be from various programs of mammography, if
fully implemented in the United States at a current dose of about
0.2 rads per exam. Mammography, of course, has nothing to do with
prevention of breast-cancer. It has to do with detection of a
problem not prevented.
Because there is no safe dose (risk-free dose) of ionizing
radiation, we can be certain at the outset that mammographic
programs will cause some number of radiation-induced
breast-cancers. The question is: How many?
No one should equate the desire to have a "ballpark" answer, with a
desire to obstruct. A desire not to estimate the consequences
would be peculiar, in our opinion.
So we have prepared an "If-Then" table from which we obtain some
estimates (Table 28-A, on the next
page). Table 28-A provides estimates from annual
mammography at every age-year, starting with
age-30 and ending with age-64. We are well aware that no one today
is proposing annual mammograms starting at age-30. We stress that
Table 28-A is an "If-Then" table. Its purpose is to enable readers
to ask "What if?" about a variety of possible programs with
differing frequencies of mammography for different age-groups.
Later, in Part 8, we will also show how to
estimate an individual's personal risk from mammography today.
Part 5. Why Worry about "Only" 2 Extra Rads to the Breasts per Year?
In 1972, the "Breast Cancer Detection and Demonstration Project"
(BCDDP) was launched in the United States. It included mammography
for the participants. A detailed description of the project was
given by Herbert Seidman and colleagues
(Seidman 1987). We quote
from the "Materials" section of that article (p.266):
"The BCDDP was established in 1972 when the American Cancer Society
funded 12 centers to do annual physical examinations and
mammography on 5,000 women in each center. The program was later
expanded when the NCI [National Cancer Institute] funded additional
centers and the number of women increased to 10,000 per center.
Screening began early in 1973; by 1975 there were 29 centers in 27
locations that had enrolled more than 280,000 women ages 35 to
74." And:
"Almost all of the centers recruited about 10,000 women each over a
two-year period to be screened for breast cancer free of charge at
an initial screening and four subsequent annual screenings. Those
who joined were screened with a combination of medical history,
physical examination, mammography, and thermography."
We doubt that the program could have recruited persons if there
were any suggestion made that breast-cancer could be a result for
some segment of the women enrolled. Seidman and colleagues
reported (at page 266) that about 51 percent of the women who
joined the project completed all five screenings.
The Average Breast-Dose Which Was Received in the BCDDP
In 1976, John C. Bailar III reviewed the dosimetry for the
mammographic studies in the BCDDP. He reported as follows
(Bailar 1976, p.80):
"A very recent survey of the demonstration clinics supported
jointly by the National Cancer Institute and the American Cancer
Society has shown average doses of about 1 to 5 rads with the use
of radiation detectors thought to be significantly more accurate
than those used in the studies cited above. A current average
depth dose to breast tissue of 2 rads per film set seems a
reasonable and conservative estimate after allowance for the use of
xeromammography and other low-dose methods."
Table 28-A
An "If-Then" Table For Mammography.
Assumes all women (age-30 and up) in the United States received 0.2 rad
in Annual Mammographies.
Col. A Col. B Col. C Col. D Col. E Col. F
Breast-
Dose Cancers Annual
Age Number of Medical Person- per 10,000 Breast-
Year Women* Rads Rads Person-Rads** Cancers
30 1,670,816 0.2 334,163 44.95 1,502
31 1,669,424 0.2 333,885 44.95 1,501
32 1,667,946 0.2 333,589 44.95 1,499
33 1,666,364 0.2 333,273 44.95 1,498
34 1,664,697 0.2 332,939 44.95 1,497
35 1,662,910 0.2 332,582 26.38 877
36 1,660,984 0.2 332,197 26.38 876
37 1,658,904 0.2 331,781 26.38 875
38 1,656,652 0.2 331,330 26.38 874
39 1,654,176 0.2 330,835 26.38 873
Annual Mammogram-Induced Breast Cancers (30-39 year-olds). Sum = 11,873
40 1,651,443 0.2 330,289 26.38 871
41 1,648,418 0.2 329,684 26.38 870
42 1,645,083 0.2 329,017 26.38 868
43 1,641,422 0.2 328,284 26.38 866
44 1,637,399 0.2 327,480 26.38 864
45 1,632,982 0.2 326,596 26.38 862
46 1,628,151 0.2 325,630 26.38 859
47 1,622,856 0.2 324,571 26.38 856
48 1,617,098 0.2 323,420 26.38 853
49 1,610,858 0.2 322,172 26.38 850
Annual Mammogram-Inducod Breast Cancers (40-49 year-olds). Sum = 8,619
50 1,604,120 0.2 320,824 24.56 788
51 1,596,848 0.2 319,370 24.56 784
52 1,588,992 0.2 317,798 24.56 781
53 1,580,517 0.2 316,103 24.56 776
54 1,571,372 0.2 314,274 24.56 772
55 1,561,522 0.2 312,304 24.56 767
56 1,550,950 0.2 310,190 24.56 762
57 1,539,588 0.2 307,918 24.56 756
58 1,527,332 0.2 305,466 24.56 750
59 1,514,026 0.2 302,805 24.56 744
60 1,501,100 0.2 300,220 24.56 737
61 1,483,790 0.2 296,758 24.56 729
62 1,466,720 0.2 293,344 24.56 720
63 1,448,518 0.2 289,704 24.56 712
64 1,429,452 0.2 285,890 24.56 702
Annual Mammogram-Induced Breast Cancers (50-64 year-olds). Sum = 11,280
Notes: Col.D = Col.B x Col.C
Col.F = (Col.D / 10,000) x Col.E
* Column B entries are for the number of women present at each
age for an equilibrium population of 250,000,000 total persons. Taken
from Table 24 of Gofman 1981.
** Conversion factors taken from Chapter
39, Master Table, Column V.
The Status of Knowledge at the Time of the BCDDP Initiation
When the Project was initiated in 1972, was there any reason to
worry that two extra rads of breast-irradiation every year might
cause some breast-cancers?
Seven years earlier, in 1965, MacKenzie had published his findings
on radiation-induced breast-cancer (discussed in
Chapter 1). In
1968, Wanebo had published
his findings concerning breast-cancer in
A-bomb survivors. In 1970,
Tamplin and Gofman had published
estimates, based upon MacKenzie and Wanebo and a relative risk
analysis, of a doubling dose for breast-cancer of about 20 to 50
rads. In 1972, the Committee on the Biological Effects of Ionizing
Radiations (the BEIR Committee of the National Academy of Sciences,
USA) had estimated a doubling dose for radiation-induced
breast-cancer of 28 to 120 rads
(BEIR 1972, p.141).
Despite all this, a project was launched to give women an extra 10
rads to their breasts (2 extra rads per year for 5 years). It
would seem that someone must have had misgivings about initiation
of this project, in light of the information on radiation-induced
breast-cancer already published and available to the ACS and NCI.
The Breast Cancer Detection and Demonstration Project (BCDDP) was
an illustration of the "technological imperative" at work, coupled
with all the political propaganda in those years about the "war on
cancer."
Mammogram-Induced Breast-Cancers: An "If-Then" Table Based on BCDDP
We will set up another "If-Then Table" to explore the consequences
if this program had "carried the day" and had expanded
indefinitely, with all U.S. women enrolled at age-35 to receive an
annual mammogram thereafter at 2 breast-rads per exam. This is
Table 28-B. Since it uses the same
population-size as Table 28-A,
with a dose ten-fold higher, it shows estimates which are ten-fold
higher for the mammogram-induced breast-cancers.
Because of the variable latency period (Chapter
2), the mammogram-induced breast-cancers would not have shown
up all at once. Table 28-B estimates the
number which would have been put "on the shelf" each year, for
delivery later. After a few decades of this program, at two rads
per examination, the annual production of mammogram-induced
breast-cancers and the annual "delivery" (incidence) of
mammogram-induced breast-cancers would have become about equal in
number (Chapter 4). And what would the
equilibrium number have been?
Ages at Mammogram-Induced Breast-Cancers:
Production Annual "Delivery" at Equilibrium
(Chapter 4)
35-39 43,757
40-49 86,187
50-64 112,803
Total 35-64 242,747
Table 28-B
An "If-Then" Table For Mammography.
Assumes all U.S. women (age-35 and up) received 2 rads per year in a
universal, continuous "Breast Cancer Detection and Demonstration Project."
Col. A Col. B Col. C Col. D Col. E Col. F
Breast-
Dose Cancers Annual
Age Number of Medical Person- per 10,000 Breast -
Year Women* Rads Rads Person-Rads** Cancers
35 1,662,910 2 3,325,820 26.38 8,774
36 1,660,984 2 3,321,968 26.38 8,763
37 1,658,904 2 3,317,808 26.38 8,752
38 1,656,652 2 3,313,304 26.38 8,740
39 1,654,176 2 3,308,352 26.38 8,727
Annual Mammogram Induced Breast Cancers (35-39 year-olds). Sum = 43,757
40 1,651,443 2 3,302,886 26.38 8,713
41 1,648,418 2 3,296,836 26.38 8,697
42 1,645,083 2 3,290,166 26.38 8,679
43 1,641,422 2 3,282,844 26.38 8,660
44 1,637,399 2 3,274,798 26.38 8,639
45 1,632,982 2 3,265,964 26.38 8,616
46 1,628,151 2 3,256,302 26.38 8,590
47 1,622,856 2 3,245,712 26.38 8,562
48 1,617,098 2 3,234,196 26.38 8,532
49 1,610,858 2 3,221,716 26.38 8,499
Annual Mammogram Induced Breast Cancers (40-49 year-olds). Sum = 86,187
50 1,604,120 2 3,208,240 24.56 7,879
51 1,596,848 2 3,193,696 24.56 7,844
52 1,588,992 2 3,177,984 24.56 7,805
53 1,580,517 2 3,161,034 24.56 7,763
54 1,571,372 2 3,142,744 24.56 7,719
55 1,561,522 2 3,123,044 24.56 7,670
56 1,550,950 2 3,101,900 24.56 7,618
57 1,539,588 2 3,079,176 24.56 7,562
58 1,527,332 2 3,054,664 24.56 7,502
59 1,514,026 2 3,028,052 24.56 7,437
60 1,501,100 2 3,002,200 24.56 7,373
61 1,483,790 2 2,967,580 24.56 7,288
62 1,466,720 2 2,933,440 24.56 7,205
63 1,448,518 2 2,897,036 24.56 7,115
64 1,429,452 2 2,858,904 24.56 7,021
Annual Mammogram-Induced Breast Cancers (50-64 year-olds). Sum = 112,803
Notes: Col.D = Col.B x Col.C
Col.F = (Col.D / 10,000) x Col.E
* Column B entries are for the number of women present at each
age for an equilibrium population of 250,000,000 total
persons. Taken from Table 24 of Gofman 1981.
** Conversion factors taken from from Chapter
39, Master Table, Column V.
The BCDDP program was intended to help women. But if it had been
expanded and continued, it had the potential to have produced
mammogram-induced breast-cancer at the rate of about 243,000 cases
every year --- a number to be compared with the 182,000
breast-cancers in 1994 from all causes combined.
Of course, there will be readers who believe our estimate above
should not be 243,000 cases per year --- but "only" 50,000 or
100,000 or 150,000 or 200,000. This is not the chapter where we
show the basis of the "conversion-factors" in the table's Column E
(see Chapter 40). But
those who believe that the potential number
of mammogram-induced breast-cancers would have been "only" 50,000
cases every year, can surely join in some gratitude to the people
who prevented the BCDDP project from expanding and continuing at
"only" 2 rads per mammographic exam.
Part 6. The Gradual Development of Mammography: 1913-1964
In a very interesting summary article published in 1964, entitled
"Mammography of Cancer," Dr. J. Gershon-Cohen and his colleague,
Myron Forman, described the early origins of the mammography
technology. Dr. Gershon-Cohen is one of the real pioneers of
clinical mammography. We quote from it briefly
(1964, p.674):
"Mammography is not new. Fifty years ago, when the science of
radiology was still in its 'teens, Salomon in Germany produced
credible mammograms of excised breasts (1913). His observations of
some 3,000 mastectomy specimens correlated the roentgen findings
with the gross and microscopic anatomy of tumors. His descriptions
and roentgenograms contain a wealth of interesting information.
The differences in the roentgen appearances of the most common
forms of mammary cancer were recognized with amazing accuracy and
he could differentiate scirrhous and infiltrating types of
malignancy from the circumscribed or the nodular forms. He even
described the punctate calcifications seen in many cancers,
especially of the duct variety. Finally Salomon is to be credited
with the first roentgenographic detection of an occult cancer found
in a breast which was removed because of the presence of a large
cyst."
Nonetheless, Dr. Gershon-Cohen related that it was only in the
later 1920s and early 1930s that references began to appear
concerning mammography. Despite excellent work being reported from
several clinics in the United States and abroad, he stated that
interest in the subject was simply not sustained.
In 1938,
Gershon-Cohen and Strickler published a paper on the
roentgen examination of the normal breast which emphasized that
knowledge of the normal breast at all ages and stages of activity
was a prerequisite for recognizing pathologic conditions.
"The Rising Sweep of Interest" in the 1950s and 1960s
Raul Leborgne's work in 1951 was cited by Gershon-Cohen
(1964) for
bringing to the attention of roentgenologists the importance of
punctate calcifications as a pathognomonic [uniquely predictive]
sign of malignant processes. Then Gershon-Cohen named a host of
workers who stood out prominently with "the rising sweep of
interest in this subject during the 1950s" --- all of whom shared
in developing techniques and diagnostic criteria.
Speaking of his own institution, Gershon-Cohen cited the
collaboration of the pathologist, Dr. Helen Ingleby, as the key
event which led to a sustained interest in the subject of
mammography. About Dr. Ingleby, Gershon-Cohen stated: "She made
notable progress in bringing order out of the chaotic pathologic
nomenclature so that the roentgenologist could reconcile his
findings with pathologic conceptions."
Dr. Robert L. Egan was cited by Gershon-Cohen
(1964, p.679) as
another important pioneer in this field:
"More recently, Egan's efforts [1962,
1963] to improve technology
and his report of excellent results generated intense interest in
the subject. The impact of his work seems to have centered on his
preoccupation with technique. It goes without saying that without
optimum technique, no substantial reliability can be placed on any
roentgenographic procedure. Good contrast and the best possible
detail are essential if maximum information is to be obtained from
the x-ray film ..."
Efforts to Prove the Value of Mammography
The work described above led to many investigations and
consultations between the surgeons and the roentgenologists, to
ascertain if mammography could improve the outlook in the
breast-cancer problem. At a time when little progress was apparent
in therapy, the hope was widespread that perhaps earlier diagnosis
of breast-cancer might make treatments more successful.
Radiologists such as Dr. Robert Egan realized that, in order for
mammography to be accepted as a valuable technique, capable of
providing information over and above techniques already being
applied, the radiologists had to prove several things, among which
were:
(a) Ability to provide their diagnosis without any knowledge of the
clinical findings. They were very wise in setting up this hurdle
for themselves, for otherwise it could be stated that they made
their analysis of the mammogram after knowing what the clinical
picture was.
(b) Ability to diagnose cases by mammogram which could not be
diagnosed by the clinicians either by history or physical
examination or both. This was a major key, for the clinicians were
always very worried about their own "false negatives" --- failures
to recognize a case as being a cancer when it truly was a cancer.
(c) A low rate of "false positives" from mammography. The more
often a mammogram caused a biopsy to be taken, with the finding of
a negative result with respect to malignancy, the less useful would
the mammogram be.
Gershon-Cohen appreciated the need to have true screening tests of
mammography, where no clinical symptoms or signs caused the women
to come in for mammography. That is not easy to accomplish. But
he and his colleagues tried, in January 1956, to set up such a
program in the Radiology Department of Albert Einstein Medical
Center. All x-ray examinations in the survey were provided free to
the volunteers --- with a single condition: Every candidate
wishing to participate in the program must be free of breast
symptoms. However, one can be at the mercy of the candidates, with
respect to their truthfulness about absence of symptoms.
With All This Activity: Why No Entry in Our Master Table?
With only minor exceptions, we found that the studies done before
1960 were not suitable for inclusion in our Master Table. Most of
the work involved women who were really under suspicion for
possible breast-cancer and were having a mammogram as part of their
work-up. As we have said before, we want not to count breast-doses
administered to women who were already at the stage of clinical
breast-cancer. We do not want to consider them as candidates for
radiation-induced breast-cancer. We are trying to evaluate what
breast-dose of radiation was being accumulated in the population
not under study for possible breast-cancer. This, in essence, is
why several major studies in the late 1950s were not appropriate
for our purposes.
Part 7. Early 1960s: "No Concern about Excessive Radiation Is Entertained"
In 1961, Gershon-Cohen and colleagues made a report on their
screening program, mentioned in Part 6. It
was entitled "Detection of Breast Cancers by Periodic X-Ray
Examinations: A Five-Year Survey." In it, they reported:
"At present 1,055 women still remain in the program [out of 1,312
enlisted in the program]; 257 dropped out after one or more
examinations. Each volunteer is examined every six months." What
about the dose to the breasts in these repeated mammographic
examinations? Gershon-Cohen and associates did indeed consider the
safety of their program. They stated:
"Since we find nonscreen films satisfactory for our purposes and
since each exposure entails less than 1.5 rad to the breast alone,
no concern about excessive radiation is entertained."
It was not even "entertained" in 1961.
By contrast, we have illustrated in Table 28-B
that people had better be very concerned
about doses of 1.5 rads to the breasts
twice a year, in any program which enlists large numbers of women.
In Table 28-B, the dismal entries in Column F would be 50 % higher
from an annual dose of three rads rather than two.
Annual Breast-Doses Well above 10 Rads: Called "Safe"
In 1964,
Dr. Robert Egan --- whose work we also admire --- was
apparently making the same mistaken assumption about safety as Dr.
Gershon-Cohen. Dr. Egan described his evaluation of 1,217
consecutive mammograms on patients who came to biopsy. He used
three views, rather than two. And he assumed the total dose was
"safe." He stated (p.125):
"Mammography is a highly accurate, reproducible, safe, simple, and
non-traumatic roentgenographic technique that provides a new
objective approach to the diagnosis of breast diseases. It is the
only means by which cancer of the intact breast can be demonstrated
before signs and symptoms are present. In a series of 1,217
consecutive mammograms on patients who came to biopsy, 85
clinically unsuspected carcinomas were detected. The mammograms
were interpreted without knowledge of clinical findings." And:
" ... From all three views, the central point of the average breast
receives a total of 3.3 rads. Radiation to the eyes and gonads is
negligible. Since only a small volume of tissue, which can be
considered an appendage, is in a low-energy x-ray beam, this dose
is quite acceptable for use in frequent re-examinations." How
frequent? In a 1963 paper,
Dr. Egan indicated that this dose would
safely allow repeating the examination several times during each
quarter year. So he was talking about annual breast-doses well
above 10 rads per woman.
Such advice from a pioneer expert in mammography is just the
opposite of what might have set the entire mammography effort on a
good track, in our opinion. Three rads, in frequent
re-examinations, widely practiced, would not be acceptable at all
to that small-volume "appendage," the breasts. Such a regime, if
applied to all women in the United States, would have breast-cancer
consequences more than five times the magnitude estimated in
Table 28-B.
The Only Target of Our Criticism
We most certainly do not fault Dr. Gershon-Cohen and Dr. Egan for
not knowing in 1961 and
1964 what no one
knew. Dr. MacKenzie's ground-breaking paper (see
Chapter 1) did not even appear until
1965. And we certainly recognize an individual woman's right to
accept voluntary medical risks of unknown sizes for herself.
What we fault is something quite different. We fault the policy of
making sweeping assertions about safety --- instead of warning
everyone about the actual state of ignorance at any given time.
Part 8. How to Estimate a Personal Radiation Risk from Mammography
Women who want a "ballpark" estimate of their personal radiation
risk, per mammographic exam, can easily obtain it by borrowing from
Table 28-A, Column E. By answering some
questions, we will illustrate how to do it.
- Question One. I am age 33. What would be my risk from having
a baseline exam at this age?
The lifetime rate at age 33 is 44.95 radiation-induced
breast-cancers per 10,000 women who each receive 1 rad
(Table 28-A,
Column E). But the dose per exam is 0.2 rads, not a whole rad, so
the rate will be lower. It will be (44.95 cases x 0.2) per 10,000
women = 8.99 radiation-induced cases / 10,000 women.
To make this fraction easier to grasp, just divide both the top and
the bottom of the fraction by 8.99. The result is: 1
radiation-induced breast-cancer per 1,112 women --- or a lifetime
individual risk of 1 chance in 1,112. Your chance of not
developing a radiation-induced breast-cancer from the one exam is
1,111 chances out of 1,112.
- Question Two. How does my radiation risk accumulate from a
series of several mammographic exams?
If you have 15 exams, you add up the risks from each one. Suppose
you have 15 mammograms beginning at age 50. The lifetime rate is
24.56 radiation-induced breast-cancers per 10,000 women who each
receive 1 rad (Table 28-A, Column E). But
the dose per exam is
only 0.2 rad, not a whole rad, so the rate will be lower. On the
other hand, we need to consider 15 exams. Multiplying by 15 is
equivalent to adding the rate 15 times, of course. So the rate
would be:
(24.56 radiation-induced cases x 0.2 x 15) per 10,000 women = 73.68
radiation-induced cases / 10,000 women. Then divide both the top
and the bottom of the fraction by 73.68. The result is 1
radiation-induced breast-cancer per 136 women, or a lifetime
individual risk of 1 chance in 136. Your chance of not developing
a radiation-induced breast-cancer from the series of 15 exams is
135 chances out of 136.
- Question Three. How do I combine my risk from the baseline
exam with the risk from the later exams?
Using the illustrations above, we just do the addition while both
fractions still have a denominator of 10,000 women. So: (8.99
cases / 10,000 women) + (73.68 cases / 10,000 women) = 82.67 cases
/ 10,000 women. Then we can divide both the top and the bottom of
the fraction by 82.67. The result is 1 radiation-induced
breast-cancer per 121 women, or a lifetime individual risk of 1
chance in 121. Your chance of not developing a radiation-induced
breast-cancer from all 16 exams combined is 120 chances out of
121.
Variable Doses, and Variable Risks
The illustrations above use the approximation that "one size fits
all." Throughout this book, we are dealing with average doses and
average risks. In reality, there is variation around the average
values. Neither the dose nor the risk is uniform. For example,
dose has to vary with breast-thickness. And even at exactly the
same dose, two women of the same age do not necessarily have the
same risk of radiation-induced breast-cancer. One reason: Each
woman has a unique genetic heritage.
There is debate over whether women born with one faulty copy of the
gene for ataxia telangiectasia (the AT gene) have a truly elevated
risk of breast-cancer and whether they are especially vulnerable to
radiation-induced breast-cancer
(Swift 1991;
Boice 1992;
Easton 1994). Resolution
will require additional evidence. And the AT gene is just
one of many genes which --- if faulty at conception
--- might predispose their carriers to breast-cancer. For example,
let us consider people who inherit genes which mean an elevated
rate of non-repair or misrepair of new genetic injuries, including
new injuries induced by ionizing radiation. It is probable (but
not proven) that such people have a higher rate of
radiation-induced cancer, per rad of radiation exposure, than
people who inherit a better repair-system.
# # # # #
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"From error to error, one discovers the entire truth."
- Sigmund Freud, 1856-1939,
founder of psychoanalysis.
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