Inr. J. Radiation

Oncology

@, Pergamon Pre\\

Rio/.

Phys , Vol.

Ltd.. 1979

5, pp. 905-911

0360.3016/79/0601-0905/$02.0010

Pranked in the U.S.A.

??Technical Innovation

and Note

THE DOSIMETRY

OF “OCo TOTAL

BODY IRRADIATION?

WING-CHEE LAM, Ph.D., BENGT A. LINDSKOUG, Ph.D.,* STANLEY E. ORDER, M.D., DAVID G., GRANT, M.A. The Johns Hopkins Oncology Center, Baltimore,

MD 21205, U.S.A.

The dosimetry characteristics of a single source “Co total body irradiation (TBI) facility have been studied. The dose distribution for AP-PA irradiation in an adult size and an infant size phantom was measured in detail using thermoluminescent dosimeters. A 10% homogeneity relative to the midline dose was found over most of the body. An increase of 10% relative to midline dose was noted in the thoracic region because of lower density lung tissue. A relative decrease of more than 10% was found in the head region because of the reduced beam intensity away from the central axis and reduced scatter volume. Dose in the abdomen was homogeneous to + 5%. Total body irradiation, Phantom, Dose distribution, Dosimetry.

INTRODUCTION Total body irradiation (TBI) is being used in several centers to prepare patients for allogenic bone marrow transplantations for the treatment of leukemia.‘,5.6 A major consideration of these radiation treatments is the degree of dose homogeneity produced in the body. The homogeneity level required to produce consistent clinical results has not been established with any certainty. Experience suggests that a nominal 10% homogeneity is a reasonable goal. However, since the degree of homogeneity in total body irradiation has not been measured in detail the While question remains unanswered. some measurements have been made in separate phantom sections, the critical areas where heterogeneity is most likely to occur, for example, the top part of head, lungs and limbs, have not been measured. The dose homogeneity of these regions could be crucial as leukemia pervades the entire body.. Before guidelines can be established it is important to know the dose homogeneity for conventional patient set-ups where clinical data are being accumulated. TBI. treatments in this institution are conducted in a conventional therapy rooin. A single cobalt source is used and treatments are conducted AP-PA at extended distances. The basic beam dosimetry of these treatments has been determined and

the resultant homogeneity has been measured using adult and child size phantoms complete with extremities. BEAM DOSIMETRY Because the TBI treatment configuration differs radically from conventional radiotherapy, a substantial amount of fundamental beam data had to be measured. Treatments are performed with a @‘Co unit§ with the gantry rotated to project a horizontal beam (Fig. 1). The beam collimator is turned to 45” so that the field diagonal is projected into the horizontal plane. At a source to midline treatment distance of 340 cm, the maximum collimator opening produced a useful treatment length of 2OOcm which is suitable for most adult patients. Patients are treated AP-PA (equal time split) while lying on their sides. This orientation minimizes differences in thickness relative to the beam direction and thereby produces a more homogeneous distribution. The following sections summarize basic beam dosimetry. Beam profile

The ‘j”Co unit we used has a nominal 6,ooO curie cylindrical source, 1.5 cm in diameter and 4.8 cm in length. Relative air exposure rates were measured along the field diagonal at the nominal treatment

TSupport f6r this work was provided by the National Cancer Institute Grants No. CA-15396 and CA-069-73-17. *Present address: Radiofysiska Centrallaboratoriet, Sahlgrenska, Sjukhuset S-41345, GBteborg, Sweden.

DSiemens Gammatron 80s. Reprint requests to: Wing-Chee Lam, Ph.D. Acknowledgements-The authors wish to express gratitude to Jonathan Links and Scott Gruber. 905

their

906

Radiation Oncology 0 Biology 0 Physics

TBI

CLINICAL

June 1979, Volume 5, Number 6

Rclatlve

SET UP

rate

cxpos”rc

%

Stretcher

Cobalt-60 Source,,’

,/

60 .I

40 .-

J 20 ..

k-3.40

meter ~+045

m.

Fig. 1. Clinical set-up for total body irradiation with a single source @‘Co therapy unit. This is a view in the

horrzontal plane. distance (Fig. 2) using an ionization chamber? with build-up cap. The solid line in Fig. 2 represents the measured values relative to the central axis while the dashed curve represents the exposure produced by a hypothetical isotropic radiator located at the same position as the cylindrical source. The reduction in beam intensity at the field edges results #from the angle dependent filtration produced by the distributed source geometry. Other factors contributing to this effect are the changing aspect of the collimator relative to the central axis and also the positionally dependent backscatter produced by the wall. Relative exposure measurements were also made along the central axis and normalized with respect to an ideal inverse square radiator. The results demonstrate that the exposure level does not follow an ideal inverse square law but instead declines initially and then increases as the wall is approached. The initial decline results from collimator effects which tend to make exposure levels measured close to the source artificially high. Relative to the total exposure, the contribution from the collimator diminishes quickly with distance. The excursion above the inverse square law results from wall backscatter. A nominal 5% increase from this effect is noted at the treatment distance for our treatment configuration. The effect of the wall is not large enough to change the general conclusion derived from our other data. Depth dose measurements Percentage depth dose along the central beam axis was measured in a 25 x 25 cm polystyrene phantom at 330cm source-surface distance (SSD) using a Baldwin-Farmer probe and electrometer. The measure-

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Fig. 2. The diamond figure on the right is the field shape in a vertical plane at 3.4Om from the source. The beam exposure profile scanned along the diagonal of the field is indicated by the solid line. The dash curve is the expected beam variation from a point source. The solid dots are the average center line dose of a 25 cm cubic phantom, normalized to 100 per cent at the central ray.

ments were converted to equivalent water measurements and demonstrated that the depth dose fall-off from 3 to 20cm depth is nearly linear. Hence, a combined parallel opposed irradiation produces a dose uniformity across a homogeneous transverse section of better than 5%. A comparison of this depth dose data was made with the depth dose of a 25x 25 cm field measured at 100 cm SSD and transformed via Mayneord formula4 to the TBI distance. The comparison shows that the transformed data is 26% lower than the measured data. The midline absorbed dose measured as a function of phantom thickness at a probe distance of 34Ocm is presented in Fig. 3. The percentage depth dose was also measured at various locations along the field diagonal but displaced from the central ray. These measurements were made using the diode system-? and a 25 x 25 x 25 cm3 polystyrene phantom. Simultaneous measurements were taken on the entrance surface to establish the entrance dose, and at 5, 10, 15 and 20 cm depths. The surface diode was subsequently placed on the exit side of the phantom and re-irradiated to establish the exit dose. A tabularized summary of the readings and their averages over lateral position are presented in Table 1. The per cent depth dose appears nearly constant irrespective of lateral position implying beam energy is essentially uniform across the

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The dosimetry of MCo total body irradiation 0 W. C. LAM et al.

MIDLINE

ABSORBED

DOSE

05-I 20

10

Phantom

Thickness

40

30

(cm-water)

Fig. 3. The relative absorbed dose in the midpoint of a 25 x 25 cm2 phantom as a function of phantom thicknesses, at 34Ocm STD. It is normalized to unity at midline for the phantom thickness of 10 cm. The error bar represents twice the probable error of the data.

field. It is noted also that the integrated data over 6 depths in the phantom, when normalized to the central ray, corresponds quite closely to the relative exposure curve indicating that relative exposure can be used to normalize TBI dose calculations.

tribution are very important in TBI. The effect of the concrete wall appears in our measurements and must be accounted for when comparing results with other institutions. Measurements of exit dose characteristics were made by placing a diode on the exit side of a 12 cm thick polystyrene phantom. Variations in depth, measured from the exit side, were effected by placing additional slabs of phantom material on top of the detector. The diode itself had an equivalent backscatter thickness of 3 mm. The exit dose at a phantom exit depth of 3 mm is 93% of the percentage depth dose of that measured at the same point in a semi-infinite phantom. The diminution of dose compared to a semi-infinite phantom as one approached the exit plane results, of course, from the missing backscatter volume. It is noted that the presence of the wall somewhat compensates for this effect. This backscatter is not expected to vary substantially with phantom thickness. The skin dose for combined parallel-opposed irradiation equals the sum of entrance and exit dose. For cobalt irradiation this is about 90% of the midplane dose. Skin dose may be smaller with high energy accelerators. Efect

Surface dose and exit dose

The entrance dose build-up region was measured at the treatment distance using a parallel plate thin wall chamber? and e1ectrometer.S The ion chamber assembly was imbedded in a 25 x 25 x 25 cm3 polystyrene phantom such that the thin chamber window was flush with the phantom surface. Variable thicknesses of lucite sheet were placed over the chamber and measurements were taken and corrected for the slight SSD variation. Measurements were reproducible to 0.5%. The results show a broad maximum centered at 4 mm and a relative surface dose of 87%. The characteristics of the exit beam dose dis-

0

of patient length

Absorbed dose at a point depends upon the scatter volume surrounding the point. Quantification of this effect in TBI treatments cannot be handled by a field size parameter but rather is determined by the physical size of the patient. Accordingly, measurements were made to determine the effect of patient length on absorbed dose. The central axis of a polystyrene phantom was monitored at several depths while the longitudinal extent of the phantom was changed. Results presented in Fig. 4 show the effect upon dose measured at the surface and at 5, 10, 15 and 25 cm as a result of increased scattering volume added in the patient length direction. Initial measurements started

Table 1. Depth dose variation

Distance from central ray(cm)

across radiation field

Diode readings normalized to average of 6 positions in phantom Entrance 5 10 15 20 Exit (25 cm)

32 60 77 90

1.42 1.43 1.41 1.43 1.44

1.34 1.29 1.31 1.31 1.31

1.08 1.07 1.10 1.13 1.11

0.92 0.92 0.93 0.92 0.92

0.70 0.75 0.73 0.70 0.71

0.53 0.54 0.51 0.51 0.52

Average

1.43

1.31

1.10

0.92

0.72

0.52

tThe average of the 6 diode readings taken over depth were normalized the central ray.

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Radiation

Oncology

0 Biology 0 Physics

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5 IO 15 20 25 LENGTH ADDED TO SIDE OF STANDARD PHANTOM (CM)

with a 25 x 25 X 25 cm3 polystyrene phantom and additional material was added to one side. It appears

that the dose rate approaches an asymptotic level at a total length of 65 cm. Therefore, -the midline dose rates obtained by calibration in a 25 x 25 x 25 cm3 phantom would have to be increased by about 4% in typical patients. PHANTOM CONSTRUCTION An adult size phantom 165 cm in length was developed for this pr0ject.t The phantom head and

Research Corp.

6

195

Fig. 4. Effect of phantom length: Absorbed dose as a function of phantom length measured at the surface and depths of 5, 10,20,25 cm on the center line of a 25 cm cubic phantom and normalized to 100% of the asymptotic values. The height of the error bars represents twice the probable error of the data.

tAlderson

5, Number

trunk are constructed of tissue equivalent material (density = 0.985 gmlcc, Z, = 7.30) in slabs 2.5 cm thick. Nylon rods served to align individual segments and hold the phantom together. The phantom was provided with a skeleton, simulated lungs and with jointed limbs. To accommodate the large number of thermoluminescent dosimetry (TLD) probes required for the dosimetry, a series of slots 1.5 X 1.5 mm in cross section on 25 mm centers were machined across each slab surface (Fig. 5). Areas of the slabs containing lung sections could not be measured in this way as the thin protective coating covering each slab was not equivalent to lung tisse.* Accordingly, it was necessary to drill small holes through the slabs to locate the TLD probes completely within the lung tissue equivalent material. To locate probes in the limbs, holes were drilled approximately radially to the geometric center of the limb (Fig. 5). A child sized phantom 86cm in length was cast from a store manikin. Initially, hollow plastic replicas were made of the head, trunk and 4 limbs using a thermoplastic vacuum forming system. Steel rods 1.5 mm in diameter were inserted through the plastic cast to accommodate placement of TLD’s in the finished phantom. Simulated lungs molded from lung tissue equivalent material were cast into the plastic mold. A skeleton was not used. The casts were then filled with a gelatinous solution of 90% water (weight) and 10% Ge1trate.S The various sections of the phantom were taped together for the dosimetry measurements.

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June 1979, Volume

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The dosimetry of @‘Cototal body irradiation 0 W. C. LAM et al.

DOSIMETRY SYSTEM An automated TLD measuring system,t extensively

for the phantom

measurements.

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20% results if an 8 cm thick arm is moved in front of the trunk.

was used The basic

probe consisted of a length of thin walled Teflon tubing, 1.5 mm in external diameter, loaded with 10 Lithium Fluoride rods measuring 1 x 6 mm and spaced on 25 mm centers. For control purposes sets of 5 probes were calibrated simultaneously in a 35 cm diameter tissue equivalent phantom. Four of the probes were used for measurement in the body phantom while the fifth (control probe) was irradiated with a known dose. The 5 probes were then processed simultaneously in the measuring system. The control probe served to eliminate short term fluctuations in offset and gain sensitivity in the systems reader. A consistency verification was also obtained by irradiating and processing all probes to a known dose (100 rad) after the phantom measurement. The average dose readout for a nominal 100 rad exposure was 99.5 rad with 93% of all dosimeters lying within t5% of the mean and essentially 100% of the dosimeters within 2 10%.

Adult phantom Dose distributions in the head and trunk were obtained by measuring the AP and PA distributions

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RESULTS OF PHANTOM MEASUREMENTS The phantom was set-up for irradiation to simulate the patient configuration as shown in Fig. 1. The phantom was positioned such that the central ray was at the pelvic region as indicated in Figs. 6 and 7. The legs and arms were slightly bent. Because the phantom was quite rigid, the arms were not resting against the body as would be the case for a patient. The significance of the positioning of the limbs is indicated by noting that an estimated shielding effect of

25

50

75

Dlstonce

100 Along

Body

125 (cm)

Fig. 6. (A) Dose distribution along adult phantom. The symbol + indicates average dose in a transverse section for head and abdomen, * is the average in the sagittal plane through the midline, A is the average in the para-sagittal plane at 7.5 cm from midline through the right lung, 0 represents sections through left limbs, x represents sections through right limbs. The continuous line represents the beam exposure profile at the midline of the phantom. (B) Phantom thickness measured through the thickest part of section.

1 CENTRAL

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Fig. 7. Child phantom. (A) Dose distribution along child phantom. The continuous curve represents exposure profile at the midline of the phantom. (B) Distribution of phantom thickness.

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Radiation

Oncology

??Biology

0 Physics

separately and performing a pointwise summation. Limb measurements were made with the TLD probes in position for both the AP and PA irradiations so that the total dose was obtained directly. As expected, the dose uniformity for those sections not containing lung tissue was quite good, i.e., in the order of +5% of prescribed mid-line dose. Measurement results in the thorax region were exceedingly irregular because of the nonuniformities introduced by the lungs and rib cage. The lung region shows an increase of about 10% relative to prescribed mid-line dose. Figure 6A presents the distribution of the average dose over a transverse section plotted as a function of the section location. The data is plotted relative to 1OOrad delivered on the central axis to the patient’s mid-line. The solid line in Pig. 6A represents the beam profile normalized on the same scale. It is noted that the average dose in the head region is diminished by 10% because of the combination of reduced beam intensity and the relative lack of scatter material at the top of the head. Fig. 6B shows the profile of phantom thickness (maximum) as a function of length. The variation in average dose corresponds quite well with beam profile and patient thickness data. Absorbed dose in the rigid phantom arms varied considerably depending on their orientation with respect to beam direction. The data suggests a more homogeneous distribution would result if the patient rested his arms against his body during treatment. Child phantom The infant phantom was irradiated AP-PA to 100 rad mid-line. The TLD probes remained in the phantom for both the AP and PA irradiations. Sections measured included one through the head at eye level, 3 sections through the thorax and 1 at midabdomen. TLD probes were inserted inferior-superior

June 1979, Volume

5, Number

6

to measure the legs. Since there was no skeleton, the dose distribution is very uniform across each section. The lung volume indicated an increase of about 10% relative to prescribed midline dose. Figure 7A shows the distribution of the average dose of each section along the phantom. The curved line represents the beam exposure profile. A dose reduction of 10% at the head is evident as with the adult phantom. Figure 7B shows the variation in phantom transverse thickness as a function of length and the correlation with dose is noted. ABSORBED DOSE CALCULATION Absolute calibration was performed with the Baldwin-Farmer ion chamber at 5 cm depth in a 10 cm thick polystyrene phantom located 340 cm from the source. Figure 3 shows the dose rate relative to the absolute calibration as a function of thickness. Depending on patient height and thickness, the calibrated dose rate must be adjusted using the results of Figs. 3 and 4. The average of sagittal thicknesses taken at middle chest, upper abdomen and pelvis were used to approximate the phantom thickness. A dose rate correction as a result of length for the infant and adult phantom of 3 and 5% respectively was required according to Fig. 4. It should be pointed out that the correction for thickness other than 25 cm is only approximately correct because the backscattering thickness is not exact. However, this introduces only a small error. The measured dose in the abdominal region was equal to the computed dose using the above corrections to within 2%. CONCLUSION is presumed that a homogeneous dose distribution in TBI treatments will provide for a more or less uniform destruction of malignant cells over the It

Table 2. Computed dose distribution vs measured dose distribution for adult phantom

A Beam profile % of maximum Equivalent thickness (cm) Average dose from PDD normalized to 100% at D Predicted average dose including beam profile Measured average dose tPositions

Position along phantom? B (Mediastinum, lung) C

D

E

F

100

97.5

90

89

95

95

98.5

19

21

10.2

20

23

15

10

108

103.5

125

106

100

116

126

96

98

119

104

100

113

113

86

95

110

105

loo

106

100

along the phantom

were indicated in Fig. 6.

The dosimetry of “OCototal

Table

3. Computed dose distribution vs measured distribution for infant phantom

A Beam profile % of maximum Thickness (cm) Average dose from PDD normalized to 100 rad at C Predicted average dose (rad) Measured average dose (rad) *Positions

Positions along phantomt B C D

dose

E

95.5

98.5

100

99

97

15.5 95

12 101

13 loo

8 108

7 109

91

99.5

loo

107

106

89

104

100

100

98

along the phantom

were indicated

body irradiation

in Fig. 7.

body. It is recognized, however, that the desired biological end-point is to maximize both healthy tissue sparing and malignant cell destruction. Hence, a multitude of biological parameters must be considered in bone marrow transplantation. Varying tissue sensitivity, whole organ dose, total dose, dose rate, fractionation schedule, and the interaction of the radiation with various drug regimen are all critical factors and require investigation. The experience of conventional radiotherapy suggests that a 2 10% homogeneity is a reasonable constraint to assure consistency in the study of these other parameters. Also, it is necessary that one should know the detailed dose distribution at least at the organ level in order to correlate these other parameters. Now, for example, studies of the white lung syndrome with entire

0

W. C. LAM et al.

911

variability in total dose, dose rate and drugs can be carried out with confidence. The measurements presented indicate that patients treated with AP-PA configurations would satisfy the -+10% homogeneity restriction over most of the body. As is noted the top of the head falls outside this range on the low side. However, as many TBI patients also receive CNS irradiation prior to TB13 the effect is somewhat compensated for. It is further noted that this configuration results in an increased dose to the lungs of approximately 10% relative to the midline dose. This data is at variance with the report of Miller, et aL5 In this reference only 2 data points were reported in the lung region and it was not clear whether lung tissue equivalent material was used. One could expect a higher dose in the lungs (Table 2) because of the reduced equivalent thickness. The predicted dose distribution’ taking the beam profile, percentage depth dose and phantom thickness into account but assuming a homogeneous medium is tabulated in Table 2 for several locations (i.e., Fig. 6, planes A, B, C, D, E, F). Table 3 presents a similar tabulation for the child phantom, where the locations of points A, B, C, D, E are indicated in Fig. 7. There is qualitative agreement between the predicted dose and the measured dose. However, this simple model is inadequate to explain the measured dose in the head and limbs since it does not take into account the effects of bones in the skull and the lack of scattering because of the smaller head size and leg size. A further effort is underway so that these effects can be incorporated into dose calculations.

REFERENCES Aget, H., Van Dyk, J., Leung, P.M.K.: Utilization of a high energy photon beam for whole body irradiation. Med. Phys. 123: 747-751, 1977. Archer, B.R., Glaze, S., North, L.B., Bushong, S.C.: Dosimeter placement in the rando phantom. Med. Phys. 4: 315-318, 1977. Hustu, H.O., Aur, R.J.A., Verzosa, MS., Simone, J.V., Pinkel, D.: Prevention of central nervous system leukemia by irradiation. Cancer 32: 585-596, 1973.

Mayneord, W.V., Lamerton, L.F.: A survey of depth dose data. Br. J. Radiol. 14: 255-264, 1944. Miller, R.J., Langon, E.A., Tesler, A.S.: Total body irradiation utilizing a single @‘Co source. Znt. J. Radial. Oncol. Biol. Phys. 1: 549-552, 1976. Thomas, E.D., Herman, Jr., E.C., Greenough, III, W.B., Hager, E.B., Cannon, J.H., Sahler, O.D., Ferrebee, J.W.: Irradiation and marrow infusion in leukemia. Arch. Zntern. Med. 107: 829-845, 1961.