Breast C. Sullivan,

Daniel

MD

#{149} Craig

A. Beam,

PhD

#{149} Suzanne

Measurement ofForce during Mammography’ The discomfort that patients ence during mammography bly

related

to a variety

of which

might

compression sured the

experiis proba-

of factors,

be the amount used.

The

one

of

authors

mea-

amount of force applied to the breasts during mammography and the resulting breast thickness in 560 women and correlated these measurements with the patient’s subjective impression of the examination. The amount of force applied ranged from 49 to 186.2 N (median, 122.5 N). Breast thickness ranged from 10 to 88 mm (median, 46.5 mm). Forty-seven women (8%) rated the examination as painful (ie, mammography was either “very uncomfortable” or “intolerable”). Logistic regression analysis revealed a highly significant relationship between probability of a painful response and ratio of force to thickness (P = .007). Current guidelines

suggest

that maximum

available

force be at least 160 N but not more than 200 N. However, because increasing force is associated with increasing likelthood of pain, technologists should be aware that these recommended maximum limits are

not intended

to be used

I

N two recent studies, women questioned about the degree discomfort experienced during

gists using equipment from different manufacturers. The results of one study suggested that a woman’s prior expectation of discomfort correlated with her actual experience of discomfort during mammography (1). The other study found that mammography performed during the 1st week after the men-

strual

period

higher

was

tionship

(1). Other

breasts.

The

than

was ous sion

00.11 Radiology

1991;

181:355-357

From the Department of Radiology, Duke University Medical Center, Trent Dr, Durham, NC 27710. Received December 18, 1988; revision requested February 10, 1989; final revision received May 23, 1991; accepted May 28. Address reprint requests to D.C.S. RSNA, 1991 See also the editorial in this issue. C

by Eklund

(pp

339-341)

factors,

such

as

applied

amount

to the

of force

applied

not measured in the two previstudies. In one study, “compresof the breast was considered suf-

ficient technology,

a

age, menstrual status, caffeine intake, and history of fibrocystic diagnosis, did not show a strong relationship with discomfort. A patient’s experience of discomfort or pain probably depends on many variables, one of which may be the

women. In this study, high-quality mammograms were obtained in a majority of women with forces less

Breast radiography,

with

of reports of pain or discomfort than mammography performed during other weeks after the menstrual period (2). However, the first study did not find such a rela-

all

Index terms:

associated

percentage

of force

level.

were of

screen-film mammography (1,2). The surveys were performed at several imaging centers, and mammography was performed by different technolo-

amount

available

BSE

Goodman,

#{149} Donald

L. Watt

Applied

for

the maximum

routinely

M.

Imaging

(1), and

when

the

skin

in the other,

became

taut”

“compression

was as much as the consistency of the breast would allow” (2). The amount of compression applied during mammography is controlled manually in some dedicated mammography units, by means of motorized devices in others, and by a combination of manual and motorized means in still others. There is an upper limit for maximum compression that is adjustable either by the technologist or by a service technidan, depending on the unit. In practice, the technologist either applies the maximum force available or stops

when he or she thinks the breast is compressed “adequately.” It is generally agreed that screenfilm mammography requires firm compression of the breast to produce high-quality mammograms, but there are no quantitative guidelines about the optimum amount of force to use. The American Association of Physi-

cists

in Medicine

(AAPM)

recom-

mends a maximum compressor force of 160-250 N (35-55 lb), and the American College of Radiology (ACR) recommends that maximum available compression not exceed 200 N (45 lb) (3,4). However, these recommenda-

lions

refer

to maximum

force

limits.

There is little information available about how much of this force should be applied routinely or what amounts of force might correlate with pain during mammography. Therefore, we performed a study to measure the force used in a sample population and survey the level of patient discomfort.

MATERIALS

AND

METHODS

We used a dedicated mammography unit (Mammomat II; Siemens Medical tems,

Iselin,

force

applied

digital displays and resulting thickness

NJ)

compressed

with

breast.

The

displayed

Sys-

of of metric

values of kilograms force were converted to newtons by using a conversion factor of 1 kp = 9.8 N (5). The maximum force that could be applied with the motor was arbitrarily set at 118 N (12 kp). The technologist could add up to 186.2 N (19 kp) of force accuracy displayed

manually and

purchased,

by turning a knob. The reliability of the force value was measured with a newly commercially available bath-

room scale with a digital display (Sears Roebuck, Chicago). This scale, in turn, was calibrated in the range of 10-45 lb with a

Abbreviations:

AAPM

= American

Associa-

hon of Physicists in Medicine, ACR = American College of Radiology, LMP = last menstrual penod. 355

help us serve our patien better, we would like your opinion about how comfortable or uncomfortable your mammogram procedure was. Please mark the answer that describes your experience today. To

z

02

U)

HI)

14

mammogram

___ 1. 2. 3. 4. 5. 6.

-

Figure

D.

scale (model

8138; Toledo),

which

the scale (Fig 1). Each measurement was repeated three times. The scale readings were converted from pounds to newtons by using a conversion factor of 1 lb = 4.448 N (5). Accuracy and reliability of the thickness measurements were checked with a standard centimeter ruler. Patients were randomly assigned to the mammography unit used in this study by

a receptionist study grams gist,

was

who was unaware being

were

produce

for her

by a single ability

high-quality

to the

consecutive

mammograms

mammography

patients

mography the study.

while

(Fig

question

unit,

undergoing

those used was asked

before

about

response

all

mam-

her experience categories

Patient

she dressed

and drop

the

fled by type of hormone) was obtained part of our normal procedure at patient

#{149} Radiology

10-

At

0 . 05

85

125

165

FORCE

questionnaire.

Patient

Figure

3.

breasts women.

during

Distribution

of force

mammography

205

(N)

applied

to

in 560

0.20

2U)

accrual

swered.

Those

continued

15 patients

from this analysis.

0

for

women

ranged

(median,

0

excluded 560 28 to 83 years

.

15

0.10 .

05

were

0

The remaining in age

from

THICKNESS

Figure

53 years).

The probability of a “painful” response (ie, rating the procedure as very uncomfortable or intolerable) as a function of compression force, breast thickness, and patient’s age, menstrual status, and previous hormone use was modeled with logistic regression with use of the CATMOD

4.

obtained

(mm)

Distribution of breast thicknesses with compression in 560 women.

procedure (SAS, Cary, NC). The linearity in logits required for this analysis was supported by examination of the data. A “best” model was sought that included only significant factors (P < .05) and that fit the data “well” (likelihood ratio x2 with P > .05).

RESULTS The

distribution

shown

of forces

in Figure

used

3. The range

N (11-41.8

lb); the

is shown

in Figure

thicknesses

ranged 1 standard

(mean

mm

mean

46.5

±

as

4. The

is

was 49± 1 stan-

12.8). The median mm, and the mode

The

distribution

shown similar

in Table to those

formation of women

compressed

small, we sis by dose. Preliminary

from 10 to 88 mm deviation = 46.3

±

demonstrated significant

thickness was was 45 mm.

of responses 1. Our reported

al (2). For purposes

duplicated

questionnaire in a box provided. The patients were told their responses would be anonymous and confidential. The questionnaires had sequential numbers that permitted later matching with the mammographic data. Information about age, menstrual status, and hormone use (classi-

356

.

45

deviation was 127.4 N ± 2.95 (28.6 lb ± 0.66). The median force was 122.5 N, and the mode was 107.8 N. The distribution of breast thicknesses obtained

by Jackson et al (2). The patient to mark her answer privately

or after

0

was:

3 months. Data were collected from 575 women, but 15 returned questionnaires unan-

186.2

with this unit were included in The technologist was told to

2). The

0

dard

perform mammography in her usual way, explaining the necessity for compression to the patient and applying vigorous compression. The same technologist recorded the compression and breast thickness data and gave the patient a questionnaire with

a single

2.

technolo-

to consistently

maintaining excellent patient rapport. This technologist knew she was participating in a study assessing patient discomfort. Whenever this technologist was assigned

15-

All mammo-

performed.

obtained

chosen

that a

.

0

Very Comfortable Comfortable Mildly Uncomfortable Uncomfortable, but Tolerable Very Uncomfortable Intolerable

registration.

has an accuracy of ±0.01 oz in the 0-75-lb range. Accuracy of the bathroom scale was determined to be ± I lb (because the digital display gives weight to the nearest pound). The bathroom scale was placed on the cassette holder of the mammography unit so that the compression paddle was centered on the weighing surface of

0 14

HI)

measurement.

Toledo

today

20-

you for your cooperation.

Thank

1. Maximum force of compression device measured with a bathroom scale. Readings are reproducible if force is applied to center of scale and if alignment of scale, machine, and paddle is consistent at each

Figure

procedure

.

H

04

A

The

0

14

is

a “pain-

ful response” was defined as response numbers 5 (very uncomfortable) or 6 (intolerable). Forty-seven women (8%) gave a painful response. Ninety women had not undergone

menopause.

The number

of days

“best”

since

were

hormone

classified preparation.

according Because

to type dose

since

of the

data

but statistically between force

logistic

regression

model

in-

LMP

“best”

was not a significant factor it had to be included in the model to ensure an adequate fit

to the

data.

(P = .4337),

of in-

examination

a small correlation

cluded force and weeks since LMP (Table 2). Force was found to be highly significant (P = .0084). Although weeks

their last menstrual periods (LMPs) were grouped into weeks since LMP. One hundred forty-eight women gave information about hormone use, and

they

unreliable and numbers in each group would be did not undertake any analy-

applied and breast thickness (Spearman rank correlation = .182; P = .0001). Because these independent variables were correlated, we first analyzed the data excluding breast thickness as a possible factor. We then analyzed the data with thickness included and force excluded. After excluding breast thickness, the

results are very by Jackson et

of analysis,

was

Exclusion

of this

factor

November

gave

1991

required explain

in the painful

the natural

model to satisfactorily response. This factor,

logarithm

force to thickness, tion of force and

breast

thickness

reached

of the ratio

suggests thickness.

(ie, when

a minimum

increasing

force

ity of a painful

a given

thickness

for a given

increases

has

patient),

the probabil-

response.

level

of

a joint acAt a given

Conversely,

of force,

women

at

with

thin-

ner breasts at compression likely to have a painful

are more response. of 122.5 N (12.5

The median force 27.5 lb) used in this population

kp;

suggests

that the lower end of the AAPM recommendation is reasonable. That is, a maximum compressor force of at least 160 N

(16.3 kp; 36 lb) should

be available.

However, since increased ated with pain, the upper AAPM recommendation

kp; 56 lb) is probably a model based on force not fit the data well. Analysis of all factors

a “best” breast

model thickness

that

alone, but

force

included

(Table

together

led

with

well

part,

solution by of the

demonstrated

techniques

were

used

and/or in both

amounts

since

she knew

patient

should

graphic

units

If there

discomfort

because

ratings. significant

(a) the quality

We doubt degree

of mammograms

A suggested

used

a force

compression tients. R

the probability of a painful (P = .0069), and inclusion

alone very

yields a model that well (P = .9999).

response

of this factor fits the data

DISCUSSION Although it is generally agreed vigorous compression is essential

tam high-quality mammograms, knowledge the actual amount used

routinely

for

modern

that to ob-

to our of force screen-film

mammography We measured

has not been reported. the force applied to the

breasts

consecutive

of 560

women

one skilled technologist. The fact that our distribution

by

of re-

181

#{149} Number

2

That

is, she

tried

to apply

the pawould

of 118 N. Then,

the we

in the ma-

is no means

for adding

manual

compression, the motorized limit should be set close to 160 N (36 lb), but technologists should understand that they would apply less than maximum

in the majority

of pa-

allow. Tautness of the skin was a central factor in this decision. The amount of force used was found

References 1.

Stomper PC, Kopans DB, Sadowsky NL, et a). Is mammography painful? a multicenter patient survey. Arch Intern Med 1988; 148:521-524.

to be significantly associated with the subjective experience of discomfort or pain. The more force used to produce

2.

Jackson

the desired compression, the more likely the patient was to report discomfort or pain. Age, hormone replacement therapy, and menstrual status did not show association with the level of discomfort in this group of women. Breast thickness was another factor influencing painful response. When force and breast thickness were combined, we obtained a highly significant

factor.

Volume

(1,2).

the maximum compression that tient and her breast consistency

neces-

for the motor-

jority of women, the technologist would apply the maximum motorized force and add a small amount of manual force to tailor the compression to the patient.

pression

high and (b) the (122.5 N) and meobtained (46.5

value

ized limit would be slightly less than median force (122.5 N); in this study,

model to

motormotor160 the

sary.

ratio of force to breast Table 2 gives results

with

for both

nologist consciously or unconsciously used less force during this study to im-

If there

associated

a compression

is a provision

mm) are consistent with those found routinely in our practice. The technologist in this study used criteria similar to those previously described to judge the adequacy of com-

significantly

rec-

mammo-

ized and manual application, the ized limit could be set to less than N, with the additional force up to maximum added manually when

days of the

be highly

to deliver

these

their

force of at least 160 N (36 Ib) but not more than 200 N (45 lb). The maximum force available in the machine used in this study was 186.2 N.

response and age, hormone use, since LMP, and natural logarithm

analysis. The combination breast thickness is shown

with

by setting

remained consistently median force used dian breast thickness

in this latter of force and

comply

omrnendations

the previous analyses, we used the ratio of force to breast thickness. We analyzed the relationship between painful

thickness. for a best

and

was being monitored; or may support our impression that the technologist was highly skilled in putting patients at ease. Another possibility is that the tech-

prove the discomfort this occurred to any in

unnecessary

excessive. The ACR recommends that maximum available compression not exceed 200 N (20.4 kp; 45 Ib). Thus, radi-

ologists

studies.

intolerable category in our study. This may be due to random difference; may reflect extra effort on the technologist’s

of independent

thickness

similar

There was a slightly higher percentage of responses in the very comfortable category and a lower percentage in the

two variables (6). Given the positive relationship with force and negative relationship

(Table 1) was similar to those by Jackson et a! (2) suggests

of force

2), which was (P = .0488). How-

We followed a standard situation of multicollinearity a suitable combination

sponses obtained

that

to

of the preceding two analyses that both force and breast were associated with painful However, because they were they could not be included

in the group

factors. for this seeking

did

only

marginally significant ever, the model fit the data (P = .9140). Results suggested thickness response. correlated,

which

force is associend of the of 250 N (25.5

None

of the other

factors

3.

VP,

during

diology

1988;

Yaffe,

M.

New

AM,

Smith

screen-film

DJ. Patient dismammography. Ra-

168:421-423.

Equipment

ity control ciation of

requirements

and

qual-

for mammography: American AssoPhysicists in Medicine Report no. 29.

York:

in Medicine, 4.

Lex

comfort

Committee

American

Association

47. on Quality

of Physicists

1990;

Assurance

in Mammog-

raphy. Mammography quality control for radiologic technologists. Reston, Va: American College of Radiology, 1990; 32. 5.

Lide

DR. ed. CRC handbook 71st ed. Boca Raton,

of chemistry

#{182}‘ ics. 6.

Draper ysis.

NR, Smith 3rd

ed.

New

Fla:

H.

Applied

York:

Wiley,

and

CRC Press, regression

anal-

1959.

were

Radiology

#{149} 357