Renate

Muller-Runkel,

Scatter to the

PhD

#{149} Urmi

Dose from Uninvolved

Thermoluminescent dosimeters (TLDs) were used to measure scatter radiation to the uninvolved breast in 30 patients who received tangential breast or chest-wall radiation with a technique in which the deep edges of the tangential fields were aligned. In most patients, measurements were made during the 1st week of radiation therapy, after port radiographs were obtained to ensure proper field position and accurate alignment of the posterior tangential field edges. Phantom measurements were made simultaneously with TLD measurements to systematically assess the scatter dose as a function of the wedging, number of fields, type of accelerator, beam energy, and bolus used in each treatment. For most patients, the scatter dose to the contralateral breast at a point on the skin 5 cm outside the edge of the medial beam was 8%-13% of the prescribed dose. However, higher doses (up to 36% of the therapeutic dose) were recorded in some patients. Index

terms:

00.33 gy,

Breast

#{149} Breast

cations, effects,

ogy,

00.47

#{149} Phantoms

compli-

#{149} Radiations,

injuri-

of therapeutic

#{149} Radiations,

00.33,

radiolo-

procedures,

complications

neoplastic,

metastases,

therapeutic

#{149}

00.47

ous

neoplasms,

neoplasms,

Interventional

00.129

injurious

00.47

radiol-

effects,

#{149} Radiations,

measure-

ment

Radiology

P. Kalokhe,

1990;

175:873-876

MD

Tangential Breast’

Breast

carcinogenesis is of particular concern in patients breast carcinoma who undergo

ADIATION

with

mastectomy

tion

varies

with

treatment

and

therapy

scopic

conservation surgery followed by radiation therapy. Many radiologists believe that carcinogenesis from exposure to low doses of radiation is minima! (four secondary breast cancers per 1,000 patients who undergo radiation therapy of the breast [1-4]), and latent periods of 10 or more years were reported (1). However, experience has been relatively limited, and few quantitative reports are available (5-7); these involve a vanety of treatment techniques. This study was undertaken to obtam measurements of scatter doses in patients who underwent radiation therapy for carcinoma of the breast. We attempted to identify subgroups of patients in whom scatter radiation to the uninvolved breast might be high and to explore ways to minimize such an unwanted dose. The scatter dose to the contralater-

a! breast

Irradiation

tech-

nique. It has been shown (6,7) that the dose to the medial pant of the contralateral breast can be reduced considerably when the deep edges of the tangential fields are aligned. We conducted measurements with thermoluminescent dosimeters (TLDs) in 30 patients who received tangential breast or chest-wall irmadiation with this technique on either an M-6700 accelerator (Mevatron; Siemens Medical Systems, Iselin, NJ) or a Cl-i800 accelerator (Clinac; Vanian, Palo Alto, Calif).

Among patients 1

From

the

Oncology

Hospital and Aye, Hammond, 1, 1989; vision

Health IN

revision

RSNA,

Centers, 46320.

requested

received

19. Address

Center,

February

reprint 1990

requests

Saint

5454 Received

disease.

treated only,

praclaviculam additional

with with

January

Hohman December 23,

to R.M.R.

1990; February

re-

in this

radiation cGy was

therapy. delivered

fractions with tangential by a boost dose of 1,400 tions, energy, Nine

delivered to the

study,

conservation

with tumor

of 30 patients

A target in 23

ports, followed cGy in seven frac-

electrons bed. had

21

surgery

and two mammary

were

unusually 18-MV

The

treated

either an accelerator. large photons

irradiation

duction breast the The

required field

with

breasts on the

Siddon of this

immadia6-MV

M-6700 acceleraSome patients were treated Cl-1800, in

used (Fig

technique

by

su-

an with

lung

which a metal bolus was appropriate dose buildup described achievement

cGy

Table 1 for the papatients were

excessive

patients

photons from tom or a Cl-1800

of 5,040

tangential fields with a matching

to avoid

tion. Most

madia-

or micro-

dose

opposing treated

field, internal

electrons

with

recurrent

A target

with 14 were

to achieve 1). we

et al (8). technique

used

was

A major is the

me-

of exposure of the contralateral to scatter radiation by alignment

deep edges supraclaviculam

optimize phalic

of the

the M-6700 jaws defined 1800,

accelerator, the the half-field;

a half-field

For

most

were

block

patients,

diogmaphs em field

of the

were position

alignment tangential

lead

wire

posing

1st week

approved

tangential

field

on

ma-

edges.

was visualized port radiograph the

medial

of

port

obtained to ensure propand accurate alignment

posterior

Such emal

On

used.

the

after

to ce-

measurements

during

therapy,

aligned ports. independent on the Cl-

was TLD

performed

radiation

of

tangential beams. field is a half-field

matching with the borders of the tangential

entry

border

tangential

on by

the latplacing

of the

op-

port.

For each dose measurement, three TLD chips (LiF, 3.0 X 3.0 X 0.9 mm) were wrapped in thin plastic foil and placed on the

patient’s

skin

tangential left

in

the

field.

The

during

For

internal

the

the

(measured the edge

TLD the

treatment

lam field.

contralatemal

in

lateral from

place

including an

on

approximately

plane, 5 cm skin surface)

METHODS

30 patients

underwent

followed by dose of 4,600

Margaret

12; accepted

the

AND

treated

was delivered in 28 fractions. shows the course of treatment tients in our study. Sixteen

breast,

PATIENTS

were

for either

mammary

along the the medial

of

probes

entire

of the two

central

supraclavicu-

patients field,

were treatment,

treated

with

measure-

of suitable

Abbreviations: undergone

TLD

=

SSD

thermoluminescent

source-to-skin

distance,

dosimeter.

873

S DO I00

18 MV

.,

UYERS

WLDP#{128}SH

20

50 40 30 10

18 MV

20

0-1800

6 MV

a

MV

M-6700

4

cm

1.

2.

Figures

skin

1, 2. distance

ments lateral border.

were from

(1) Percentage depth dose (DD) for 18-MV photons with various layers [SSD]). (2) Surface dose for 15 X 15-cm field at an SSD of 100 cm. dmax

made at 5-cm distance the internal mammary

A distance this

contrafield

of 5 cm was chosen

point

usually

falls

because

halfway

between

the patient’s midline and the apex of the contralatemal breast. It is well outside the beam penumbra, and a significant amount of contralatemal breast tissue is present at this distance. The TLD measurements were repeated during the 3rd week of treatment. At this time, an additional TLD was placed in the opposing midaxilla in some patients. The intent was to assess a lower limit of the scatter dose to the opposite breast.

For each measurement, selected with a relative tion

of less

reference equivalent

than

± 2%.

TLD chips were sensitivity variaOn

the

same

day,

TLDs were exposed in a waterplastic phantom to a known

scatter doses at 5 cm contralateral from the medial port were 19.6%, 27.5%, and 36.3%, respectively, of the therapeutic dose to the treated breast. For a tumor dose of 4,600 cGy, 25% amounts to 1,150 cGy to the uninvolved breast. These patients were treated with 18-MV photons; one or more layers of metal bolus were used. As will be shown below, the higher

energy degree

and bolus responsible

scatter dose. protruding

the medial tangential al (6) showed that

creases

read

lam distance

Made

during

Table 2 summarizes the results of our TLD measurements. For most patients the scatter dose to the medial aspect of the contralatenal breast was 8%-13%

of the

therapeutic

dose

de-

livened. The medial gantry angle seems to have had little influence. There was a slight dependence on the size of wedge that was used. Patients treated with 18-MV photons and a metal bolus seemed to receive higher scatter doses than those who were treated with the lower energy and no bolus. Three patients stand out: Their 874

Radiology

#{149}

significant and large

are

the

with

edge. and

field. scatter

increasing from

For a large, shallow

Fraass dose de-

et

perpendicu-

the

geometric

field

protruding

medial

gantry

this distance is shorter shallow, small breast

breast angle,

than for and steep

a medi-

a! gantry angle. The dose to the opposing midaxilla ranged, for all patients, between 1% and 3% of the therapeutic dose. Port

RESULTS Measurements Treatment

More shape

are only to some for such a high

volume of these patients’ breasts, which placed the surface of the contralateral breast relatively close to the edge of

dose.

Reference and patient TLDs were 1 day after exposure on a 4000 TLD system (Harshaw/Filtrol Partnership, Cleveland). The readings from the three chips were averaged.

of metal bolus (10 X 10-cm2 field at 100 cm source-todepth of maximum dose buildup.

nadiographs with a field en for the second exposure the dose to the contralateral

about

3 cGy

Phantom

pen radiograph.

how

the

in a systematic scatter

dose

man-

is influ-

enced by the kinds of accelerator, wedge, beam energy, and bolus that are used, we performed the following

in-phantom

Scatter dose field.-Surface tances

from

measurements: outside open and wedged doses at various disthe geometric field edge

field

were measured parallel-plate

chamber (Markus; PhysikalischTechnische Werkstaetten, Freihurg, Federal Republic of Germany) in a solid ation Wis).

water phantom Measurements, For this purpose,

(RMI 457; RadiMiddleton, the surface of

the chamber was placed at the appropniate distance from the source (SSD), and all readings were normalized to a reading obtained with the chamber at the

Measurements

To investigate ner

opened widincreased breast by

of a i5 X 15-cm with a thin-window

center

the

depth

(1.5

cm

of the

respective

of maximum for

6-MV

x rays

field

dose

buildup

and

3 cm

at for

18-MV x rays). Measurements of the surface doses are shown in Figure 2. The observed difference between machines in doses outside the treated area was due mainly to difference in design: the lower collimators on the M-6700 isocentnic

are

10 cm farther from plane than those on

the the

June

Cl-

1990

Table 2 Measured

Surface

to Contralateral

Dose

at 5 cm Outside

Breast

Field

Wedge

Angle

Dose to Contralateral Breast

(degrees)

(degrees)

at 5 cm*

Medial Field Patient No.

Accel-

Energy

emator

(MV)

T T

M-6700

6 6

30 30 30

60 39 53

8.3 6.8 10.9

6

30

56

13.4

6 6

45

52

8.7

6 18

45 30 15

57 46 56

9.8 8.3 19.6

M-6700

6

30

M-6700 M-6700 M-6700

6 6 6

30 Large 30 30

56 52

10.1 14.1

60 49

10.6 6.5

6

15

59

12.4

30

56

27.5

15 15

50 50

8.3 11.8

45 45

53 58

12.5 36.3

45 Large 60 Large 45 Large 30

55 59 56 58

10.0 12.3 11.6 10.9

11 12

T + S/Cl T

M-6700 M-6700

6 6

14

T

M-6700

6

15

T

M-6700

16

T

M-6700

17 18 20

T T T

M-6700 M-6700 Cl-1800

21

T

22 25

T + S/Cl T + S/Cl T

26

T

M-6700

27

T + S/Cl

Cl-1800

28 29

T T

1 5

30 45

6 45 Large 6 30 18 + 6 + 16 30

18 + 6

6 6

M-6700 M-6700

T + S/Cl T + S/Cl T + S/Cl T + S/Cl

18 + 6 18 + 6

Cl-1800

10

T + S/Cl

13

T+S/Cl+IM

Cl-1800 M-6700 M-6700 M-6700 Cl-1800

19

T+S/Cl

M-6700

6

30

53

12.2

24

T + S/Cl

M-6700

6

45

Cl-1800

6

45

52 67

12.1 7.5

6 7

30

Table

12.9 10.6 13.3

T 1 + S/Cl + IM

patients

=

49 57 45

T + S/Cl

8 9

Mastectomy

Percentage

10.0 10.0

M-6700 M-6700 M-6700 Cl-1800

23

*

48 52

2 3 4

Lumpectomy patients

Note.-T

Gantry

Arrangement

tangential, of dose

T S/Cl

supraclavicular,

prescribed

6 6 6 6+6+12

IM

for treated

internal

breast.

mammary.

wedge

material.

Dose

at 5 cm Outside

Field

for Open

and Wedged

15

Field

X 15-cm

at 100

cm SSD 300

Accelerator (Energy) M-6700

(6 MV) Cl-1800

(6 MV) Cl-1800

(18MV) Note-Values

Open

Small

Open Field

300 Small

with

with

Depth

Large

Small

Large

Tray

Tray

SurfaceI cm Surface1 cm Surface1cm

5.1 2.8 8.8 5.5 11.6 11.0

5.3 3.4 9.4

6.5 6.8 10.0

6.6 5.6 11.0

9.1 11.9 11.8

7.4 3.3 10.7

7.4 3.6 12.8

7.7

8.9

10.8

11.6

5.8

8.2

11.9 11.8

12.0 12.0

15.5 16.6

16.0 16.3

12.8 11.2

13.0 12.3

are

percentages

of dose

30#{176}

60#{176}

60#{176}

at dmax.

slope

were

obtained

has “small-field” up to 20 cm wide Volume

175

(9).

Number

#{149}

The

M-6700

wedges for fields and “lange-field” 3

wedges for fields the wedge slope. tients, the small

whenever As can scatter

gle and

up to 30 cm across For treatment of pawedges are used

possible.

be seen from Table 3, the dose increases with wedge an-

is higher

for the M-6700 ence between

for the accelerator. smalland

Cl-1800

than

The differlarge-field

wedges is less pronounced at the sunface than at a depth of 1 cm, indicating that the scatter component from large-field wedges is more energetic than that from small-field wedges. A block tray increases the scatter dose at the patient’s skin but has little effect on the scatter dose at depth.

of 1 cm,

a

will

receive

a considerably

higher dose to the contralateral breast than a patient treated with a steep medial gantry angle and no wedge. This effect is enhanced for a large, protruding breast. Influence offield separation.-The medial gantry angle is influenced to some extent by the separation be-

tween points fields. only angle,

the

medial

and

lateral

entry

of the opposing tangential A large sepanation may not imply a shallow medial gantry it also means a reduced dis-

tance

1800. For the same energy, scatter doses outside the treated area were generally twice as high on the Cli800 as on the M-6700, and even higher when 18-MV photons were used. Table 3 lists scatter doses at 5 cm outside the geometric field edge for open and wedged fields with and without a block tray. The standard wedges on the Cl1800 are designed for fields up to 15 cm wide; custom-made wedges for fields up to 30 cm across the wedge

At a depth

dependence on gantry angle and wedges was seen. In addition, the effect of beam hardening in the large-field wedges could be observed. These measurements demonstrate that a patient treated with a shallow medial gantry angle and a thick similar

wedge

3

Scatter

No significant increase (