Radiation Physics

The Influence of Bone on 45 MV Photon Dose Distributions 1 John R. McLaren, M.D., Arthur B. Kirchner, M.D., and Patton H. McGinley, Ph.D. In an attempt to assess the perturbation of 45 MV depth dose curves by bone we have studied the dose distribution in a water phantom equipped with various thicknesses of bone-equivalent plastic. An increased dose was observed immediately behind the bone material and a reduced dose was found for points located greater than 5 cm behind the bone-equivalent plastic. The most significant change of the depth dose curve was produced by bone located in the dose build-up region. INDEX TERMS:

Bones, photon absorption • Dosimetry • Treatment planning

Radiology 125:817-820, December 1977

of depth dose distributions produced by bone for orthovoltage and megavoltage radiation beams has been investigated (1-4). Most studies in the high energy region have been conducted with photon beams generated at an accelerator potential of 22.5 MV or less. Haas and co-workers (2) demonstrated that bone located near the surface will alter the dose build-up region for 22.5 MV photons. It was also found that within 2 em of boneequivalent material, the dose level was approximately 30/0 greater than the dose value observed without the bone material. At points more distant than 2 em from a 1-cm thick bone slab, a shadowing effect or dose reduction of the order of 2 % was experienced. Meredith (3) has made theoretical calculations of the enhanced dose of high energy photons to soft tissue located in or near bone. Based on this analysis, an increased dose of approximately 26 % should be experienced by soft tissue near or in bone when 45 MV photons are employed. This increased dose was attributed to the higher degree of pair production interaction in bone as compared to soft tissue. Kitabatake at al. (4) have criticized the use of high energy photons for radiation therapy due to this effect. Meredith (3) also predicted that bone will produce a shielding effect which should result in the per cent depth dose being reduced by a factor of 1.9 % per em of bone for points located behind bone. The purpose of this study was to demonstrate the influence of bone on the dose distributions resulting from 45 MV photon beams. Information of this type is needed for treatment planning if dose perturbations of the order of those derived by Meredith occur in the presence of bone irradiated by high energy photons.

T

HE DISTORTION

PHOTON IC BEAM

o

BO N E - E QUI VA LEN T

~

IONIZATION

CHAMBER

~

CONTAINER

OF

Fig. 1.

MAT E R I A L

WATER

Simplified diagram of phantom.

bone) and the composition of bone plastic in t~ms of per cent by weight (7). The mean atomic number (Z) for Type B-100 plastic and ICRU bone shown in TABLE I was calculated by the authors using the technique suggested by Jones (8). As can be seen from the values in TABLE I, Type B-100 plastic is well suited as a bone phantom material for photon dosimetry. In order to simulate several thicknesses of bone, cylinders of bone plastic of diameter 4.95 em and lengths of 0.5, 1, 2 and 4 em were constructed. A phantom was fabricated of 0.635-cm thick sheets of Lucite in the form of a cubical container (30 X 30 X 30 em) and water was used to fill the phantom when depth dose measurements were carried out. Figure 1 shows the general experimental arrangement of the phantom, photon beam, and dosimeter. A Lucite tube with 5-cm internal diameter and 0.635-cm wall thickness passing through the center of the cube allowed the introduction of cylinders of bone plastic, containers of water, and dosimeters into the phantom. Small thicknesses (~3 em) of water were simulated by the use

MATERIALS AND METHODS

Shonka at al. (5) have devised a plastic (Type B-100) which has a photon absorption coefficient similar to bone for photons in the energy range 0.01 to 20 MeV. TABLE I shows the composition suggested by the International Commission on Radiological Protection (ICRU) for the skeletal region (this includes marrow as well as mineral

~om the Division of Radiation Therapy, Emory University Clinic, Atlanta, Ga. 30322. Accepted for publication in July 1977.

817

shan

818

R.

JOHN

MCLAREN AND OTHERS

110

TABLE

100

III

10

I:

ELEMENTAL COMPOSITION (PER CENT BY WEIGHT) AND DENSITY OF BONE EaUIVALENT PLASTIC AND REFERENCE MAN SKELETON

ELEMENT

0

December 1977

REFERENCE MAN SKELETON (7)

BONE EaUIVALENT PLASTIC TYPE B-1 00 (6)

CI

% ~

~-

60

- - W I T H O U T lONE - - - W I T H lONE

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7.2 25.0 3.0 47.0 0.3 0.1 7.0 0.2 10.0 0.1 0.2

H C N

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40

20 lONE

0 0

1

10

12

14

16

18

20

DEPTH(em)

Fig. 2. Depth-dose distribution with and without 2-cm bone cylinder, 10 X 10-cm field size, and 110-cm target-to-skin distance. The bone cylinder is located at the surface of the phantom.

Na Mg P S Ca CI

K

E.

17.69

16.77

Z Photoelectric lCompton Z Pair production Density (g/ cm'') 8

110

b

100

6.39

54.41 2.67 3.06

9.7 8

4.208 5.888 1.5

10.3 8 4.208 5.968 1.46 b

Based on calculations by authors. Density obtained by direct measurement of samples used in this study.

... III

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10

12

14

16

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DE PT H (em)

Fig. 3. Depth-dose distribution with and without 2-cm bone cylinder, 10 X 10-cm field size, and 11O-cm target-to-skin distance. The bone cylinder is located in depth intervals of 5 to 7 cm.

110, 100

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--WITHOUT ---WITH

40

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Fig. 4. Depth-dose distribution with and without 2-cm bone cylinder, 10 X 10-cm field size, and 110 cm target-to-skin distance. The bone cylinder is located in depth intervals of 10 to 12 cm.

of Shonka muscle-equivalent plastic (Type A-150). Thicknesses of water greater than 3 em were produced by using water-filled Lucite containers with 1mm wall thickness. When muscle-equivalent plastic was employed in the phantom a simple density correction was used to adjust the depth to that equivalent to unit density material. An E G & G extrapolation chamber was utilized in conjunction with a Keithley 610 C electrometer to obtain depth dose data near the bone-equivalent material with a pre-

cision of ±0.5 0/0. The ionization chamber was operated with a fixed plate separation of 2.28 mm and a 180 volt battery was used to establish saturation voltage. The t-ern diameter collecting electrode was constructed of muscle-equivalent plastic and the front plate of the ionization chamber was fabricated from conducting polyethylene 2.9 mg/cm 2 thick. The phantom and dosimeter were exposed to beams of 45 MV photons generated by a Brown Boveri 45 MeV Betatron. A target-to-phantom distance of 110 cm and beam sizes of 6 X 6, 10 X 10, and 20 X 20 cm were employed for this work. Depth dose measurements were made with and without the bone cylinders located at the following depths: surface, near the maximum dose build-up point (5 cm depth), and at 10 em depth. Thermoluminescent dosimeters (TLD-700, Harshaw Chemical Co.) were used to assess the dose received by a small mass (3 X 3 X 0.9 mm) of soft tissue embedded in bone. Type TLD-700 was selected for these measurements due to the low response of this dosimeter to neutrons produced by the interaction of high energy photons in the phantom and accelerator structure. An Eberline model TRL-5 TLD reader was employed to obtain the TLD response produced by an irradiation with a standard deviation of ±3 0/0. The TLD dosimeters were exposed in the 10 X 10 cm photon beam at a depth of 12 cm in order to approximate the dose received by a small mass of soft tissue located in bone. Irradiations were conducted with 2 cm of bone plastic in front and behind the'TLD dosimeter, and with muscle-equivalent plastic surrounding the TLD dosimeter. RESUL1S AND DISCUSSION

Figures 2, 3, and 4 show results of depth dose measurements made with and without the 2-cm thickness of bone plastic placed at various depths in the phantom. Each of the curves is normalized to 100 % based on the maximum dose obtained without bone material in the phantom.

INFLUENCE OF BONE ON 45 MV PHOTON DOSE DISTRIBUTIONS

Vol. 1-25

As can be seen from Figure 2 bone located near the surface gives a faster build-up of dose with depth and an increase in dose above the maximum value observed without bone. The enhanced dose levels found for points located behind different thicknesses of bone-equivalent material varied from 105 % to about 100 % of the maximum dose measured without bone for the 4 cm and 0.5 cm thick bone cylinders, respectively. At points more distant than 5 cm from the bone/water interface, a slight shadowing effect was seen due to the increased absorption of radiation by bone. As would be expected, the maximum dose reduction was found for the thicker bone cylinder (4 ern) and was of the order of 4 % . Figures 3 and 4 show the dose perturbation produced by the 2-cm long bone cylinder located in the depth regions 5 to 7 cm and 10 to 12 cm for the 10 X 10-cm photon beam. Once again a higher dose was observed for points immediately behind the bone material as compared to the dose at the same location without bone. Values of 40/0 and 2 % dose increase were obtained for the 2-cm long bone cylinder with the proximal surface located at 5 and 10 cm depth. The dose increase was of the order of 3 % for both positions of the 4-cm long bone cylinder. TABLES II and III summarize the magnitude of bone correction factors (CF) which are defined as the ratio of the dose with bone present to the dose observed at the same depth without bone. Each table is based on measurements made in a 10 X 10-cm photon beam with and without the 2-cm thick bone cylinder present. As can be seen from TABLE II, corrections in the build-up region as large as 1.16 are required to account for the increased dose near bone and dose reductions of approximately 0.97 are needed at points far from the bone/water interface. When the bone cylinder is positioned in the depth interval 10 to 12-cm smaller correction factors varying from 0.984 to 1.02 were observed. It should be pointed out that the maximum value of the correction factor depends on the thickness and location of the bone slab and occurs at the distal bone/water interface. TABLE IV indicates the maximum value of the correction factor for a number of configurations. The field size dependance of the bone correction factors was evaluated by conducting depth dose measurements for field sizes of 6 X 6 cm and 20 X 20 cm in addition to the 10 X 10-cm field. Based on this series of observations it was concluded that the field size has less than 1 % effect on the bone correction factor. It was found that the correction factor for bone located at depths greater than or equal to the depth of maximum dose build-up could be expressed as a simple exponential function as shown below: OF

= ae- b x + 0.97

where CF is the bone correction factor (dose with bone present divided by the dose without bone), x the distance in cm from the distal bone water interface to the point in question, and the a and b terms are fitting constants. For the 2 cm and 4 cm thick bone cylinder, the following values

819

Radiation Physics

TABLE II: BONE CORRECTION FACTORS (CF) FOR 2-CM THICKNESS OF BONE PLASTIC POSITIONED AT SURFACE OF PHANTOM (45 MV PHOTON BEAMAND 10 X 10 CMFIELD SIZE) DEPTH (CM)

CF

o 0.5

1 1.5 1.16 1.12 1.11 1.08 1.06 1.04 1.02 1.01 1.00 0.992 0.984 0.983 0.978 0.972 0.972 0.972

2 2.5 3 3.5

4 4.5 5 6

7 8 10 12 14 16 18 20

TABLE III: BONE CORRECTION FACTORS (CF) FOR 2-CM THICKNESS OF BONE PLASTIC POSITIONED AT 10-CM DEPTH (45 MV PHOTON BEAM AND 10 X 10 CM FIELD SIZE)

TABLE IV:

DEPTH (CM)

CF

12 12.5 13 13.5 14 14.5 15 15.5 16 18 20

1.02 1.02 1.01 1.01 1.01 1.00 1.00 0.933 0.990 0.985 0.984

MAXIMUM VALUE OF BONE CORRECTION FACTOR (CF) FOR 45 MV PHOTONS

BONE THICKNESS (CM)

DEPTH OFDISTAL ENDOF BONE CYLINDER (CM)

MAXIMUM CF

0.5 1.0 2.0 7.0 12.0 4.0 9.0 14.0

1.32 1.27 1.16 1.04 1.02 1.07 1.03 1.03

0.5 1

2 2 2 4 4 4

for a and b will yield OF values accurate to within

±1%:

a

= 0.06

b = 0.35 cm- 1 As a result of TLD measurements it was found that wlthln the experimental uncertainty of ±3 % the dose to soft tissue located adjacent to a 2-cm thickness of bone was unaltered when the soft tissue was completely surrounded by a 2-cm thickness of bone.

JOHN R. McLAREN AND OTHERS

820

SUMMARY Large increases of per cent depth dose occur when bone-equivalent material is located in the dose build-up region of 45 MV photon beams. In general, this distortion of the depth-dose distribution was observed for points within 5 cm of the bone-water interface. On the other hand, a slight shadowing effect of less than 4 % dose reduction was found for points located more than 5 cm from the bone-equivalent material. When bone is located at depths greater than the maximum dose build-up point then dose changes of the order of a few per cent were observed for points behind the bone-equivalent material. The results of this work are similar to those reported by Lewis et ale (2) for 22.5 MV photons. However, the theoretical values of Meredith (3) seem to overestimate both the increase of dose to soft tissue near bone and the shadowing effect produced by bone absorption.

fluence of bone on dose distributions in radiotherapy. Radiology 92: 1-10, Jan 1969 2. Haas Ll, Sandberg GH: Modification of the depth dose curves of various radiations by interposed bone. Br J Radiol 30: 19-26, Jan

1957 3. Meredith WJ: Some aspects of supervoltage radiation therapy. Am J Roentgenol 79:57-63, Jan 1958 4. Kitabatake T, Hatj,ori H, Okumura Y: Optimum energy in supervoltage x-ray therapy. Strahlentherapie 137: 158-161, Feb 1969 5. Shonka FR, Rose JE, Failla G: Conducting plastic equivalent to tissue, air and polystyrene. Second International Conference on Peaceful Uses of Atomic Energy. [In] Progress in Nuclear Energy Series 12: 160-166, 1958 6. Wingate Cl, Gross W, Failla G: Experimental determination of absorbed dose from x-rays near the interface of soft tissue and other material. Radiology 79:984-1000, Dec 1962 7. International Commission of Radiological Protection Report No. 23. Report of the Task Group on Reference Man. Oxford, Pergamon Press, 1975, p 273 8. Jones TD: Distributions for the design of dosimetric experiments in a tissue equivalent medium. Health Phys 27:87-96, Jul 1974

ACKNOWLEDGEMENT: We thank Miss Ann Jinks and other members of the Atlanta West Hospital, Department of Radiation Therapy for their aid and the use of the Brown Boveri 45 MeV Betatron.

REFERENCES 1.

Debois JM, De Roo M: Experimental demonstration of the in-

December 1977

Division of Radiation Therapy Emory University Clinic 1365 Clifton Road, NE Atlanta, Georgia 30322