AAPM T utorial Patient

Dose

Lawrence Patient

dose

gram

assessment different

is the

fect

.

for

Ofthe dose

best and

indicator discusses

PhD

PhD

in mammography especially

and

ment.

Balter,

in Mammography1

N. Rotbenberg,

al reasons,

dose

Stephen

for

is important evaluation and

types

and

of risk

comparison

must

of imaging

ofdosc

that

can

be

of patient

risk.

The

author

measurement

he

to a patient

measured

for

sever-

in a screening techniques

measured,

pro-

and mean

reviews

equip-

glandular

factors

that

af-

of dose.

INTRODUCTION

Patient dose is an important consideration in mammography. Patient dose is measumed for several reasons: (a) to evaluate the risk to the patient, a very important consideration when screening programs are set up in which large numbers of asymptomatic women will be imaged and for which a benefit-risk analysis should be performed; (b) to compare competing imaging techniques, such as screen-film mammography, xeromammography, or new image receptors; (c) to assess the perfonmance of mammographic equipment, both as part of the initial acceptance testing and during periodic quality control testing; (d) to answer questions from the patient and various physicians involved concerning dose; and (e) to comply with various regulations and guidelines related to mammography dose (one from the American College of Radiology [ACR] in its accreditation process, others from vanous states with regulations and guidelines for mammography) This article reviews different dose parameters, factors that affect dose, and measurement of dose. .

U DOSE PARAMETERS Which dose should be measured

and

which

dose

should

be reported?

It may

be

easier to measure certain quantities, yet more appropriate to report a different quantity that can be calculated from those measured quantities. Various dose parametens that might be considered are the in-air exposure at the position of the entrance surface of the breast (X,) the dose to the entrance surface of the breast ,

Abbreviations: = National S’steme Index

From 10021.

to the RSNA.

of Radiology. Cl)RH and Measurements,

= Center br l)evices NEXT Nationwide

and Radiological Evaluation of X-ray

Health. ‘Frends.

NCRI’ SI

Internationale terms:

RadioGraphics I

ACR American College on Radiation Protection

Council

Breast 1990;

the Department From the 1988

radiography.

radiation

dose

l)osimctry

#{149}

Physics

#{149}

l0:’39-6 of Medical Physics. Memorial Sloan-Kettering Cancer (;entcr. I 2’S \ork RSNA scientific assembly. Received and accepted April 10. 1990. Address

Ave. New reprint

York. NY requests

author. 1990

739

(D5) dular

the dose to the midline of the breast (Dmid) and the mean dose to the glantissue of the breast (Dg . av). The in-air surface exposure is easy to measure with an ionization chamber. This measure is useful for quick comparisons of different techniques that employ constant beam quality. However, if the beams are of different quality, surface exposure may not be representative of either the absolute or relative risk. In addition, this measure does not indicate the accumulated dose incurred when multiple ,

,

views

are obtained.

The surface dose is similarly easy to determine from the in-aim surface exposure, and it can also be measured directly on the patient on in phantoms. Measurements of surface dose result in overestimations of the absolute risk and are not repmesentative of relative risk at different beam qualities. Also, if two on more views of the breast are taken, the surface doses for the two views cannot be simply added to get an estimate of the total risk. Midline dose is difficult to measure directly. It is somewhat representative of

the risk

to the glandular

but it represents an underestimation for screen-film mammography. the best indicator of the risk to the patient from a mammographic examination, because it is commonly assumed that the cancem risk is linearly related to the dose and that breast cancers anise in the glandular tissue. Mean glandular dose cannot be measured directly but must be calculated of risk The

with

the

made breast sue,

tissue

for the low-energy mean glandular

results

of simple

in calculations will be firmly not

containing

measurements

to determine compressed glandular

per and lower) surfaces breast tissue composed analyses,

the

tissue,

is made

tissue

and

in that

tabulated

values.

The

assumptions

the mean glandular dose (1 ,2) are that (a) (Fig 1); (b) there is an outer layer of adipose that

of the breast of a uniform

assumption

50% glandular

of the breast,

beams used dose provides

(Fig 2) ; and mix of adipose

that

central

is roughly

there

0.5 (C)

cm

thick

on the

the

tis-

outer

(up-

there is a central portion of and glandular tissue. For most

is a mixture

of 50%

adipose

tissue

and

region.

Exposure measurements are normally made and then converted to dose values by multiplying by the f-factor (the exposure to dose conversion factor) The f-factons for glandular and adipose tissue, which are relatively constant over the range of beam energies used in mammography, are shown in Figure 3 Some calculations assume a single average value of about 0.79 rad/R. The unit used to express exposure in the international system of units (SI) is coulomb per kilogram (C/kg); however, it may be more convenient for our purposes to work in roentgens (R). The dose should be reported in SI units of gray (Gy) on milligray (mGy) and previously has been reported in rad or millirad. .

.

Figure

4 shows

qualities

screen-film dose

due

depth, dular ferent

the exposure

mammography, to the

which

lower-energy

is the midline

and

there beam

dose,

tissue dose for xemomammography. entrance exposures are shown

receptors, layers

how

at xemomammognaphy

and of the

the attenuation

beams

employed

with

and

absorbed

dose

screen-film

is more

rapid

employed.

due

(1 .2 mm

for different

Also,

the

below

is also

absorbed

dose

the average

different

a grid.

of the exposure

For these particular examples, to the different sensitivities of aluminum

beam

without

attenuation

is significantly

depth

vary

mammography

due

at 2 cm

value

of glan-

two difof the image

to the half-value

for xeromammography The mean glandular

0 .3 mm of aluminum for screen-film mammography) for these two exposures is 0.60 mGy (60 mrad) for screen-film 2 1 5 mGy (2 1 5 mrad) for xeromammography.

For

and

and dose

.

mammography

and

.

740

#{149}RadioGrapbics

#{149}Rothenberg

Volume

10

Number

4

x rays

x rays

ICDR

R

1. Drawing demonstrates the effect of firm compression on breast contour. The compressed breast (right) is essentially spread out laterally and made more uniform in thickness, so that x rays traverse less thickness (r) Consequently, a shorter cxposunc time is required, with corresponding reduction in dose. Scattered radiation reaching the image receptor (R) is also reduced, which significantly improves image contrast. The resulting improvement is indispensible in screen-film mammography. (Reprinted, with permission, from reference 3.) Figure

TT7//

T

Uniform

Mix:

[POS:-Gland

Uniform

1’

(i-i)

:LL

Phantom

.

Adipose

Adipose-Gland

Mix

Figure 2. Drawings of the compressed breast and two simple models to estimate dose. Transverse (a) and sagittal (b) diagrams of the firmly compressed breast show how compression makes the outline of the central, gland-bearing portion of the breast more nearly rectangular. (C) Computational model for determining average glandular dose (with a uniform mix of adipose and glandular tissue) r thickness measured in centimeters. (d) Computational model for average whole breast dose. Outer, hatched area in a, b, and C represents skin and outer adipose layer of thickness of 0.5 cm. (Reprinted, with permission, from refer-

Adipose

0

.

.5

‘;.

to

0 0

.0 0 U)

.0

ence

3.)

0

0 U)

Exposure-to-dose conversion factors for monochromatic x-ray beams

0 0.

w

1.(

0

1

2

-

-#{149} water

...

3

0.E

4

Depth (cm) Schematic shows x-ray exposure and absorbed dose versus depth for screenfilm mammography and xemomammography. The upper set of curves is representative of beams used for xeromammography (half-value layer of I .2 1 mm of aluminum) and the lower set of curves is typical of beams used for screen-film mammography (half-value layer of 0.3 1 mm of aluminum). Solid lines Figure

4.

refer to x-ray exposure; sorbed

dose

the entrance

to glandular

surface,

dashed tissue.

lines to ab-

1990

-

o

-

ar

-

mammary gland

-#{149}--------

V

0 0

,

adipose

U 0

,‘

tissue

-I,

0_i

-

0.4

-

On the left is

with exposure

nepnc-

sentcd by the solid line. The exposure decreases from the surface layer into the mix of adipose-glandular tissue and then into the exit surface layer. The rectangular frame of the graph represents a breast approximately 4 cm thick when compressed. The outer sunface layers (0.5-cm-thick regions on the left and right) do not contain glandular tissue. Computed average glandular doses per view are 0.60 mGy and 2. 1 5 mGy, respectively, for screen-film mammography and xeromammography. (Reprinted, with permission, from reference 3.)

July

o.

10

20 Photon

30

40 Energy

60 (key)

Figure 3. Graph depicts variation tors with photon energy for water, my gland, and adipose tissue.

Rothenberg

of f-facmamma-

U

Ra4ioGrapbics

U

741

FACTORS AFFECTING DOSE

Several following

factors affect discussion,

dose for a properly exposed mammogram. (Throughout a concurrent change in milliampeme seconds may often

the also

be assumed to maintain a properly exposed mammogmam or xeromammogram.) If a longer exposure time is necessary to fulfill increased exposure requirements, an extra increase in the overall exposure may be required because of mcciprocity law failure. In other words, the required film exposure and its accompanying patient dose will not be determined by a single tube current-exposure time

product (mAs); for some greater for a long time-low product. At higher kilovoltages,

screen-film current with

mammography product than

an appropriate

to maintain the same image density, theme penetrating x-ray beam produced but also Three-phase and constant-potential x-ray trating beam than single-phase generators, from a high ripple to a constant-potential

systems, for a short

reduction

the requirement time-high current

in milliampere

is

seconds

will be a reduced dose due to the more some loss of subject contrast. generators will provide a more peneso that as the voltage waveform changes shape, the dose will be somewhat me-

duced. Again, some slight loss in contrast may result from a constant potential waveform, compared with that from more varying waveforms. The effect of changing the x-ray tube target from a tungsten to a molybdenum is that there will be more low-energy trum. In a shift from tungsten to molybdenum, will be improved. There may be a choice offilter type, such target

photons the

in the molybdenum dose will increase,

as molybdenum

and

speccontrast

or aluminum

filters,

with different tube targets. Filters of special metal such as rhodium or of different thicknesses may be used. In general, as the filters are varied to make a beam of higher half-value layer, the dose will be somewhat reduced and so will the contrast. A change of the x-ray tubefocal spot should produce no effect on dose. The compression device, which is used for essentially all mammographic exposures, forms part of the overall filtration of the machine, and its thickness and matenial will affect the half-value layer of the beam. In addition, increased compression on the breast will spread the tissue out further and make the compressed breast less attenuating, a characteristic that results in a reduced dose. Patients with greater breast thickness (of the same composition) will require a higher dose. As thefraction ofadipose tissue to glandular tissue changes toward more adipose tissue as it does in older women, there will be reduced absorption,

which will lead More scattered

to lower

required

dose.

radiation will be removed as the grid ratio goes up; thus, milliampere seconds must be greatly increased to get a properly exposed image. The transmission factor of the grid will be important in determining how much additional exposure is required when a grid is used. In general, compared with nongrid techniques, the doses are raised by a Bucky factor of 2-3 when a typical lowratio grid (5 : 1) is used for mammography. As scattered radiation is reduced by the grid, the dose to the patient must be increased to maintain proper film density. Theme are a variety of screen-film image receptors, including different speed screens, different speed films, single screens, double screens, single-emulsion films, and double-emulsion films. Whatever changes are made to increase the me-

cepton

742

5

Ra4ioGrapbks

speed

#{149} Rothenberg

will

lead

to a reduction

in dose.

Volume

10

Number

4

1.3

i

1 .2 I

Mammography General Diagnostic

-U-

t,

.

0..

Figure

a,

:

,

5.

variation

lI

_n%D

1 .0

..

measurements

#{149}#{149}a

.

I

-

0.1

10.0

(mm Al)

There are various development tunes of the development chemicals

chemicals, forfllm

(solid

times of development, processing. As these

are

and temperaadjusted to

to have

mately

and

source-image

distance.

techniques,

the overall

As the

changed. An increase in the magnification tient, since the breast will be relatively However, in magnification radiography,

tened

radiation

is already

In that

geometry.

case,

reduced theme

due

may

choose

used in the and filters

an ionization

is moved

factor

and

up

the

source-

approxi-

down

for the image

will

to the aim gap built

be only

a small

increase,

into

designed

be

any magnification

no change,

compared with with a grid.

that

on even

resulting

measurement of dose include the ionization chamber, for the half-value layer measurements. It is desirable

chamber

for

will lead to increased dose to the pacloser to the target for the same technique. the grid may not be needed because scat-

duction in dose for magnification radiography conventional screen-film radiography performed

Instruments electrometer,

breast

magnification

and

(dotted ranges. layer.

increase the speed of development, dose will be reduced. Adjustments of distance can be made on many of the machines, image distance varies from one unit to another. Newer units tend a 65-cm

mam-

line)

general diagnostic line) x-ray energy HVL halfvalue

I

#{149}

1.0 HVL

magnification

factor

in the

mography 0.9

shows

with beam quality for ionization chambers designed for

Ia.,. (

Graph in correction

specifically

for the low-energy

a re-

from

to

MEASUREMENT OF DOSE

beams

used for mammography, particularly screen-film mammography. The ionization chamber should have an appropriate collection volume to give a reasonable size signal for the exposure being measured, and it must have a well-known, preferably almost constant, energy response in the mammography energy mange. This may me-

quime that the materials of the chamber walls be different from those used in chambers employed for routine diagnostic x-ray measurements in the 60- 1 50 kVp range. Figure 5 shows correction factors for a chamber designed specifically for mammography measurements

July

1990

dosimetry versus those at higher half-value

for a chamber layers.

designed

for

general

diagnostic

Rothenberg

#{149}RadioGrapbics

#{149} 743

TLD

Relative sensitivity

1.0

-

C C 1.01 c’J

0.9

_f

0.9c

Figure 6. Graph shows variation in sensitivity of a thermoluminescent dosimetcr (in this case, lithium fluoride

TLD-

1 00

shaw/Filtrol,

with

0.8

.

0.8C C,)

ci

[Har-

0.7

7

Clevelandi)

beam

half-value

quality.

HVL

=

0.25

0.50

Table 1 Average

.00

1.25

FirstHVL

Glandular

Breast

Dose

(D5N)

Molybdenum

Thickness (cm)

per

Unit

Target

and Beam (mmAl)ofo.3P

Exposure

Tungsten

in Air

Target

and

1.50

1.75

2.00

225

ImmAl)

(rad/R) Beam

HVLs

(mm

Al)

of

HVL 0.30

0.80

1.00

1.20

1.40

1.60 0.710 0.665 0.630 0.595 0.565

3.0 3.5 4.0 4.5 5.0

0.220 0.195 0.175 0.155 0.140

0.220 0.200 0.185 0.170 0.150

0.470 0.430 0.396 0.365 0.335

0.535 0.490

0.595 0.550

0.455

0.515

0.425 0.395

0.480 0.450

0.645 0.605 0.570 0.540 0.510

5.5

0.125

0.140

0.315

0.375

0.425

0.485

0.540

6.0 6.5

0.115 0.105 0.095

0.125 0.110 0.100

0.295 0.275 0.260

0.350 0.330 0.310 0.290 0.275

0.400 0.380 0.360 0.340 0.325

0.460 0.435 0.415 0.395 0.375

0.515 0.490 0.470 0.445 0.425

7.0 7.5

0.245

8.0

0.230

Note.-HVL - Only one

beam

half-value quality

lybdenum

spectra

does

layer. Reprinted, with permission, is given, since the half-value layer not

Thermoluminescent in the

change

dosimeters

measurements served

0.75

layer.

on the actual use

the dosimeter

measurements,

made, which

particularly are used

much

may

also

patient

of thermoluminescent

chips

raphy

very

and

their

reference

3.

molybdenum-mo-

kilovoltage.

be used

for

Several

mammographic

precautions

exposure

have

to be ob-

materials. In particular, the selection of and annealing are important. In mammogenergy correction, shown in Figure 6, must be

handling

a significant

for beams for screen-film

surfaces.

with

from of the

with half-value mammography.

layers of 0.3-0.4 mm of aluminum, Fading corrections are also me-

quimed if the dosimeters are exposed and not read out for a significant length of time. Aluminum filters used in mammography measurements should be thin, about 0. 1 mm. The aluminum filters that may come in a standard quality assurance pack for routine diagnostic measurements may be as thick as 0.5 on 1 .0 mm, which is inappropriate for measuring half-value layers in mammography, particularly for the beams for screen-film imaging. The filters should have uniform thickness and also be of high purity: 99% purity (type 1 100 aluminum) on better. Impurities of

high-atomic-number ment

744

#{149}RadioGrapbics

of beam

#{149}Rothenberg

materials quality

for x-ray

can significantly beams

used

change

the results

of the measure-

in mammography.

Volume

10

Number

4

Table 2 Typical Values

of Mean

Glandular

Dose

Mammographic Technique Screen-film’ All examinations Examinations

Mean

Average Glandular

0.93 with

Examinations

grids

without

1.28

0.55

grids

Xenomammographyt All examinations Positive Negative

3.94

mode mode

examinations examinations

4.08 3.40

Note-Adapted from reference 8. #{149} Doses for a 4.7-cm-thick compressed tissue. t

Doses

Value of Dose (mGy)

for a 5.0-cm-thick

compressed

breast

of 50% adipose

tissue

and

50% glandular

breast

of 50% adipose

tissue

and

50% glandular

tissue.

Although cases,

the average

it is convenient

glandular

dose

to tabulate which is the

glandular dose DgN, face of the breast. Measurements exposure

the

in air.

tables

to the

to arrive

Tables

(4)

.

are then

measured

made

values

at reasonably

to be reported

in most

are

sun-

machine

of

at the mammography

multiplied

accurate

average entrance

by the

estimates

values

of DgN from

of the average

glandular

dose

patients.

sources,

ments

These

is the quantity

on to obtain tables of the normalized dose pen unit exposure in air at the

of values

of

including

the

(NCRP) With these

breast,

which The types

posune

match

it with

the

target

and

provided

,

half-value

the measured in-air used either to generate

of testing,

such

control,

can

as the

by a number

Protection

and

be made

used

of the

of a variety

used,

and

obtain

If only

of acrylic and into the beam.

related

should be of materials whose composition with a range of thicknesses from about

a factor

dose. cx-

reproducibility

look

for

is be-

a consistent

to the adipose-glandular

mimics 2 to 8 cm.

with

glandular

on for other of the automatic

reproducibility

of materials.

block is put

in measurements

layer

of

Measure-

Health (CDRH) of the compressed

exposure to obtain average new tables of DgN values

evaluation

one could use a simple each time the block

Phantoms

1 have been on Radiation

as Table Council

(3) and the Center for Devices and Radiological tables, one can locate the appropriate thickness

to multiply phantoms

ing tested, sure reading

such National

DgN,

expo-

tissue

the actual Among the

mix

tissue composition, phantom materials

commercially available for this purpose are BR- 1 2 (5) which closely simulates a 50% adipose tissue-50% glandular tissue mix, and RF-1 and RM-1 (6), which simulate fat and muscle, respectively, such that an appropriate combination of the two can simulate different compositions of the breast. The acrylic phantom used ,

as part

thick

of the

ACR

compressed

In summary,

accreditation

process

(7)

represents

an approximately

4.5-cm-

breast.

to calculate

mean

glandular

dose,

one

should

(a)

measure

the

expo-

sure in air at the breast entrance surface, (b) measure the half-value layer of the beam, (c) determine the average thickness of the compressed breast for the range of patients being examined, and (d) estimate the composition of the breast tissue (the fraction of adipose and glandular tissue) With these values determined, the

SUMMARY AND RECOM-

MENDATIONS

.

mean glandular Typical values

dose can be calculated and of dose have been reported

reported. from a survey

the Nationwide

Evaluation

(NEXT)

CDRH

July

1990

and

the

Food

and

of X-ray Drug

Trends

Administration.

These

program values

performed

(8) are

as part

carried given

out in Table

Rothenberg

of

by the 2.

#{149} RadioGrapbics

#{149} 745

Some changes will be seen in these values in the future: We may expect the mean glandular dose for screen-film mammography to decrease somewhat because faster screen-film combinations have been introduced. Of the single-screen, single-emulsion film combinations, several have been introduced in recent years that are on the order of 25%-50% faster. Double-screen, double-emulsion systems now in use reduce the exposure by a factor of two or more, compared with some of the older systems used when the NEXT data in Table 2 were calculated. Table 2 shows that xemomammogmaphy requires significantly higher mean glandular dose than that needed for screen-film mammography.

The

NCRP

has recommended

that

for an examination

of a 4.5-cm-thick

com-

pressed breast, the average glandular dose should be less than 1 mGy (1 00 mnad) for one view for screen-film mammography without a grid, less than 4 mGy (400 mrad) for screen-film mammography with a grid, and less than 4 mGy (400 mnad) for xemomammognaphy (3) New York State has decided in its regulations that the .

average than

glandular 3 mGy

(300

dose mrad)

Acknowledgments: Sloan-Kettering eral helpful

REFERENCES

1

.

2.

3.

.

rather

mammography

than

the

4 mGy

The author

Cancer discussions

Center on this

is grateful to Mary and Leonard Stanton, topic and for providing

Hammerstein GR, Miller DW, Masterson ME, Woodard HQ, JS. Absorbed radiation dose mography. Radiology 1979;

White DR, Laughlin in mam130:485-

be less (9).

MS, of Memorial

1977;

Hermann Lietz

50:814-82

K-P, Geworski

R, Harden

D.

1.

L, Hatzky

Muscle-

and

T, fat-

Stanton L, Villafana T, DayJL, Lightfoot DA. Dosage evaluation in mammogra-

tube voltages below 1 00 kV. Biol 1986; 31:1041-1046.

phy. Radiology

1984;

Mammography:

a user’s

no.

85.

Bethesda,

7.

150:577-584.

guide. Md:

NCRP National

Council on Radiation Protection and Measurements, 1986. Rosenstein M, Andersen LW, Warner GG. Handbook of glandular tissue in mammography.

DHHS

8.

publica-

FDA 85-8239. Rockville, Md: U.S. Department of Health and Human 5cr-

vices, 1985.

#{149} RadioGrapbics

Masterson,

should NCRP

MS, of Hahnemann University for sevmost of the original drawings.

Radiol

6.

White DR, Martin RJ, Darlison R. oxy resin-based tissue substitutes.

#{149} Rothenberg

EpBrJ

Phys

Med

Mammography accreditation program. Reston, Va: American College of Radiology, 1987. Conway BJ, ed. Nationwide evaluation of x-ray trends (NEXT) , tabulation, and graphical summary of surveys, 1984 through 1987. CRCPD publication 893 Frankfort, Ky: Conference of Radiation Control Program Directors, 1989. Quality assurance programs for providens of mammography services. Publicaiion no. PH-7. Albany, NY: New York State Department of Health, 1987. .

iion

746

Ellen

a grid by the

equivalent polyethylene-based phantom materials for x-ray dosimetry at

doses

5.

with

suggested

491.

report

4

for screen-film

9.

Volume

10

Number

4