A versatile electron collimation system to be used with electron cones supplied with Varian's Clinac 18

0

Technical

Innovation

A VERSATILE ELECTRON COLLIMATION SYSTEM TO BE USED WITH ELECTRON CONES SUPPLIED WITH VARIAN’St CLINAC 18 MYRON R. GOEDE, M.S.,S DAVID S. GOODEN, Ph.D.,* ROBERT G. ELLIS, M.D.8 and T. J. BRICKNER, JR., M.D.8 Natalie

Warren

Bryant

Cancer

Center,

Saint Francis

Hospital,

6161 South Yale Avenue,

Tulsa, OK 74136, U.S.A.

A method of fabricating individualized electron shields with openings corresponding to desired field shapes and sizes is discussed. The electron shields are made from an alloy called Lipowitz’ metal. These shields are designed to be mounted on any of six cone sizes provided with the Varian’s Clinac 18 linear accelerator. The effect of these shields on electron dosimetry is shown.

Radiation therapy, Electrons.

INTRODUCTION

sizes with sides of 4, 6, 8, 10, 15 and 25 cm when the face of the cone is positioned 5 cm from the patient’s surface. The shields are mounted on the end of the cone by screwing two or more small bolts into existing holes.

Electron collimation in linear accelerators and betatrons is usually accomplished by providing a set of fixed square field sizes. In order to treat a patient with fields different from those defined by standard cones, lead masks frequently are placed directly on the body surface to reduce dose to uninvolved regions and to define the treated area. While this method is acceptable, it does have drawbacks which may include discomfort to the patient when large pieces of lead must be used. This paper describes a method for fabricating shields which may be attached to the end of the treatment cones for the purpose of defining different field sizes and shapes. These shields are made from a low melting point alloy, Lipowitz’ metal, which may be melted after use and molded into new field configurations. Electron shields discussed in this paper were designed particularly for the Clinac 18, manufactured by Varian Associates. The Clinac 18 is a linear accelerator which is capable of producing electron beams with nominal energy of 6, 9, 12, 15 and 18 MeV. Varian supplies six cone sizes to be used with the electron beams. They produce square field

METHODS

AND MATERIALS

Lipowitz’ metal is composed of 50% bismuth, 27% lead, 13% cadmium and 10% tin. This alloy is marketed under a variety of trade names, including Cerrobend, Ostalloy 1581 and Lomeltoy.tt Cerrobend was used in this study. This material is used commonly in many radiation therapy centers to construct shields for shaped photon ports. To produce electron shields, it is necessary to establish the thickness of alloy which will produce an acceptable attenuation (- 95%). It was decided to determine this thickness for 18 MeV electrons with the assurance that this value also would attenuate the lower energy electrons adequately. The appropriate thickness was determined by “stacking” 2 mm thicknesses to the end of an 8 X 8 cm electron cone. Ionization measurements were made in a Plexiglas phantom with the phantom

TVarian Associates, Radiation Division, Palo Alto, CA, U.S.A. SDepartment of Biomedical Physics, Engineering and Mathematics. SDepartment of Radiation Oncology. BCerro Metal Products, Division of Cerro Corp., Belle-

fonte, Pennsylvania. “Arconium Corporation Co., Providence, Rhode

?tAtomic York. 791

of America, Division of A. J. Oster Island. Development Corporation, Plainview, New

Radiation Oncology 0 Biology 0 Physics

192

surface 5 cm from the face of the cone. A Victoreent 100 R chamber was placed at a depth of 2 cm in the phantom. The readings were normalized to the reading that was obtained with no shielding material in the beam. A thickness of 8 mm was found to reduce the incident dose rate to a 5% level. Plexiglas “forms” were constructed to correspond with each of the six cone sizes. Quarter inch strips of Plexiglas were placed permanently around the periphery of a quarter inch Plexiglas base piece (see Fig. 1). The inside dimensions of the square defined by the form correspond to the outside dimensions of the cone. Strips of l/4 in. x l/4 in. Plexiglas were attached permanently to produce a slot in the shields where the bolts go through to attach to the treatment cone. Three 1116 in. holes were drilled in the base at the corners and at the l/4 in. x l/4 in. strips to allow the shield to be removed more easily after hardening. Once a Plexiglas form has been made, it can be used to produce numerous electron shields. To produce shields, 1/4in. Styrofoam is cut to the desired shape and centered in the Plexiglas form. Two-sided tape may be used to affix the Styrofoam silhouette to the base plate: it also is recommended

July-August 1977, Volume 2, No. 7 and No. 8

that the Styrofoam be weighted down to assure that it does not tend to “float” in the high density alloy. The holes in the base plate are covered with masking tape and melted alloy is poured into the form. After it hardens, the shield is removed by gently tapping a nail punch inserted from the back through the holes in the base plate. The shield then is available for attachment to the electron cone (see Fig. 2). The thickness of the shield is approximately 7.5 mm; this is greater than the 0.25 in. (6.4 mm) edge of Plexiglas because of surface tension effects.

Electron

Fig. 2. Electron shield.

::::

RESULTS Characteristics of the electron beams which may be affected by the introduction of Cerrobend shielding include: beam energy (practical range, I&), percent depth doses with possible influence of increased Bremsstrahlung, isodensity curves, surface doses, output factors, field flatness and symmetry and doses to shielded areas. Practical

Fig. 1. Plexiglas form in relation to electron cone. tvictoreen Instrument fiivision, Cleveland, Ohio. SSHM Nuclear Corporation, Sunnyvale, California.

range

measurements

and surface

energies

A comparison of the practical ranges for a standard Varian supplied 10 x 10 cm’ cone was made with a 10x lOcm* shield placed on a 15 x 15 cm’ cone. Measurements were made for nominal electron energies of 6, 9, 12, 15 and 18MeV. A 60x60X 60cm3 water phantom was positioned at the source-to-skin distance (SSD) of 100 cm. The SHM$ dosimetry system was used with the 0.1 cm3 PTWO ionization chambers. Percent depth ionizations were corrected PPTW (P. Pychlau),

Freiburg,

Germany.

A versatile electron collimation system 0 M. R.

for both inverse square and chamber displacement effects. Electron energies at the phantom surface were calculated from the empirical relation of MarCUS.~No significant changes in R, values were noted.

-3cm

GOEDE

et al.

793

I

I

+3cm

Per cent depth dose determination

Central axis depth ionization measurements were made with nominal electron energies 6-18 MeV for various Varian supplied cone sizes and compared with similar sized electron shields mounted to the 15 x 15 cm2 cone. The relative dose values were obtained from the ionization chamber measurements by correcting the data for appropriate C, values. Table 1 compares the depth of 80%, 50% and 20% relative dose in water determined with the Varian supplied 10 x 10 cm2 cone size and a 10 x 10 cm2 Cerrobend plate mounted on a 15 x 15 cm2 cone for each of the five energies. The agreement between these values is representative of those obtained with other Varian supplied cones and electron shields.

-t Fig. 3(a). Isodensity

~3cm

IOcm

curves for 18 MeV electrons 6 x 6 cm’ cone. 1

(

with

+3cm

1. Comparison of depths (cm) of certain per cent depth dose values for 10x lOcm* cone and 10 X 10 cm*

Table

electron Energy (MeV) 80% 50% 20%

6

shield 9

12

15

18

Cone

1.5

2.2

3.3

4.3

5.3

I Shield Cone

1.4 1.8

2.2 2.8

3.4 4.1

4.3 5.2

5.2 6.7

{ Shield Cone

1.8 2.2

2.8 3.4

4.1 4.9

5.3 6.3

6.6 7.8

I Shield

2.2

3.5

4.9

6.3

7.8

7

‘O’

IOcm

t Fig. 3(b). Isodensity curves for 18 MeV electrons 6 x 6 cm2 plate on 15 x 15 cm* cone.

Isodensity

with

curves

Isodensity curves were obtained using the SHM automatic densitometer-plotter and Kodakt RP/V film in a Plexiglas phantom. Comparison of typical curves are shown in Figs. 3(a-f). Note on the 18 MeV electron curves that the 10% curves for the electron shield extend farther than the standard cone produced curves. This may be the result of partial transmission of 18 MeV electrons or Bremsstrahlung produced at the shield edges. This effect disappears for 15 MeV and lower energies and also when smaller amounts of shielding material are needed as shown in Figs. 3(c and d). When the shield is made 1 cm thick, the effect is markedly reduced, indicating the original effect resulted from partial transmission of 18 MeV electrons by 7.5 mm of Cerrobend.

-5cm

I

I

+

5rm

IOcm

t Determination

of Surface

doses

A limited study of surface doses was carried out using a thermoluminescence dosimetry (TLD) method tEastman

Kodak

Co., Rochester,

New York

Fig. 3(c). Isodensity

curves for 18 MeV electrons 10 X 10 cm* cone.

with

Radiation

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Oncology

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Biology 0 Physics

July-August

1977, Volume

2, No. 7 and No. 8

maximum dose and were exposed simultaneously. Results are shown in Table 2. The Cerrobend does

have some effect on increasing surface doses. However, it appears to be only a few per cent at most. Table 2. Surface

T

IOcrn

Fig. 3(d). Isodensity curves for 18 MeV electrons 10 X 10 cm* plate on 15 X 15 cm* cone.

7 Fig. 3(e). Isodensity

with

7cm

curves for 12 MeV electrons 6 x 6 cm2 cone.

with

Energy (MeV)

6

12

18

6x6cone 15 X 15 cone 6 x 6electron shield on 15 X 15 cone

77 75 83

88 87 88

95 94 98

Output factors Output factors are defined as the ratio of the dose rate at II,,,,, for any field size to the dose rate at ID,,, for a reference field size. At this facility, the reference field size is that one produced by the Varian supplied 15 x 15 cm2 cone. When electron cones are used with the Clinac 18, the manufacturer recommends that the collimator setting be 5 cm larger than the cone size. For instance, one would use a 20 x 20 cm2 collimator setting for a 15 x 15 cm* cone. This variation in collimator setting for different cone sizes seems to cause a rather pronounced effect on output factors, particularly for the lower energies. When using our electron shields the collimator setting is that suggested for the particular cone regardless of the shield size. This decreases the marked variation in output factors (see Fig. 4). I I 09

-

06

-

I 03

-

I 00

-

097

c

094

-

091

-

088

-

0.85

-

082

-

079

-

076

-

073

-

II

11

1

’

11

-

0.67

t-

064

I,

I 4

I 6

!

1 8

II

11 IO Port

similar to that outlined by Rao et al.’ Lithium fluoride extruded ribbons (0.030 x l/8 x l/8 in.) were employed in a lucite phantom. Eight chips were placed on the surface and eight chips were placed at the depth of

”

”

”

IA) (8)

25

with

1

”

’ ”

1”

Comparison Of scatter COrreCtlO” f.x.fOr5 for 9 MeV electrons

070

0.61

Fig. 3(f). Isodensity curves for 12 MeV electrons 6 X 6 cm* plate on 15 x 15 cm* cone.

doses (% of maximum dose)

11

‘1 15

”

‘ones Cerrobend Plates 15 x 15 cones

1’

”

0”

’ “1

size. cm

Fig. 4. Output factors for 9MeV electrons.

Field flatness and symmetry Field flatness and symmetry were compared for a given Varian supplied cone and corresponding electron shield. This was done with the SHM dosimetry

A versatile electron collimation system 0 M. R.

I

I

GOEDE

et al.

795

L

100% -

(B)

(A)

Fig. 5. Field flatness, 12 MeV electrons. (a) 10 x 10 cone; (b) 10 x 10 plate on 15 x 15 cone. system by driving an ion chamber perpendicular to the beam at D,,,. A typical comparison of these scans is shown in Fig. 5. The beam with the electron shield is somewhat less flat.

indicates that there is some transmission of 18 MeV electrons through 7.5 mm of Cerrobend and negligible transmission of the lower energies. DISCUSSION

Doses to shielded areas

Dose to shielded areas was measured with a Victoreen 100 R chamber placed at D,, in a Plexiglas phantom. A 10 x 10 cm* plate size was mounted on a 15 x 15 cm* cone. Percentage of the dose in the shielded area to the dose in the unshielded area was determined. The results are presented in Table 3 for each energy. 18 MeV electrons produce the greatest dose to shielded areas. As mentioned previously, when the shield thickness was increased, the dose was reduced markedly. The reduction for the lower energies was of a consistant and lesser degree. This

Table 3. Transmission of electrons through 7.5 mm thickness of cerrobend Energy (MeV)

Transmission (%)

6

1.2 2.3 2.6 3.7 6.4

9 12 15 18

The method described for fabrication of electron shields can be accomplished easily in most radiation therapy centers. There appear to be no pronounced adverse dosimetry effects when using these shields. Isodensity curves, depth dose values and field flatness are little affected by use of these devices. It is recommended, however, that output factors be determined for each individual shield size used in clinical conditions. This may be accomplished through thermoluminescence dosimetry which generally is available in most centers. It also is recommended that the shields be made to mount on one of the cone sizes, rather than varying cone sizes every time a new shield is made. A 15 x 15 cm* cone generally is large enough to accommodate most shields. Occasionally, the 25 x 25 cm* cone must be used for chest wall ports. Similar methods may be developed independently for use with other machines. A method has been described by Bjarngard et al.’ for use with another linear accelerator, the Mevatron XII.? These systems conceivably may be adapted to a variety of manufacturers’ treatment devices.

REFERENCES 1. Bjarngard, B.E., Piontek, R.W., Svensson, G.K.: Elec-

tron scattering and collimation system for a 12 MeV linear accelerator. Med. Phys. 3(3): 153-158, 1976. 2. Marcus, B.: Energie Bestimmung Schneller Elektronen Aus Tiefendosiskurven. Strahlen Therapie 116: 280-292, l%l. ISiemens

Corp., Walnut Creek, California.

3. Rao, P.S., Pullai, K., Gregg, E.C.: Effect of shadow trays on surface dose buildup for megavoltage radiation. Am. J. Roentgenol. 117(l): 168-174, 1973.