In: J Radinrron o,ZCO/OR), Biol Phys.. Vol. Printed in the U.S.A. All rights reserved.
IS, pp.
455-46
I Copyright
0360-3016/90 $3.00 + .oO 0 1990 Pergamon Press plc
??Technical Innovations and Notes
INFYLUENCE OF HIP PROSTHESES ON HIGH ENERGY PHOTON DOSE DISTRIBUTIONS C.
H.
SIBATA,PH.D.,*
H. C. MOTA, J. P.
M.Sc.,*
SAXTON,
The Cleveland
M.D.*
P. D. HIGGINS, AND
K.
Clinic Foundation,
H. SHIN,
Cleveland,
PH.D.,*
D. GAISSER,
M.S.,+
M.D.*
OH 44 195
Radiotherapy treatment of patients having a hip prosthesis is a common problem facing dosimetrists and physicists when the treatment plan requires irradiation of the pelvic area. To quantify the perturbation of these devices, attenuation studies were done with 6 and 18 MV photon beams using various hip prostheses models with varying size and compos?tion. These studies have shown that an attenuation of as much as 50% can be found in a single beam profile under the prosthesis. We have studied the capability of a dose planning system to predict the transmission of these devices aa compared with measurements. Femoral hip prosthesis,
Radiotherapy treatment planning, Dose distribution, Beam perturbation.
INTRODUCTION Radiotherapy tients
having
planning
involving
a total hip prosthesis
of titanium are the most recent addition to the surgical alloy field. Because of their relatively poor wear characteristics, these alloys generally are not used for articulating
the pelvic region of pahas been found
to be
difficult due to the effects of the prosthesis
on the treatment dose distribution. In 1984, Hudson et al. (5) described a high perturbation caused by some solid prostheses in radiotherapy treatment with 8 MV photons and “Co gamma rays. More recently, Biggs and Russel ( 1) described an average dose decrease: of approximately 2% for 25 MV X rays and average increases of approximately 2% for 10 MV X rays and 5% for 60Co through the head of a hollow prosthesis with a diameter of 5 cm and a wall thickness of 3 mm. The extent of the perturbation is dependent on the model and material ofthe prosthesis, and on the energy of the radiation itself. Implants may vary in size and have either hollow or solid fe-moral heads. For orthopedic implants, the material used are primarily limited to three classes of metallic alloys: stainless steel, cobalt-chrome, and titanium alloys (2, 6). Table 1 lists the composition of the major surgical alltoys used. The majority of current hip prostheses are composed of cobalt-chrome alloys ( 10) as they are considered to have the best combination of corrosion and fatigue resistance and mechanical strength (6). Stainless steel and titanium implants may also be found. Some of the earlier stainless steel prostheses are still functioning well in patients. Alloys
Table 1. Stainless steel wrought
Element (a) Composition
of prosthesis
Carbon Manganese Phosphorus Sulfur Silicon Oxygen Cobalt Chromium Nickel Molybdenum Iron Aluminum Vanadium Titanium
0.08 max 2.00 max 0.03 max 0.03 max 0.75 max 17-20 10-14 2-4 Bal(59-70)
(b) Electron Range Average
This work was presented at the 30th Annual Meeting of the American Association of Physicists in Medicine, San Antonio, TX, August 1988. * Department of Radiation Therapy. + Department of Musculoskeletal Sciences.
-
Co-Cr cast
Titanium cast-wrought
alloys (%w) (7) 0.35 max 1.OO max 1.OO max Bal (57.4-65) 27-30 2.50 max 5-7 0.75 max -
0.08 max
0.13 max 0.25 max 5.6-6.5 3.5-4.5 Bal (88.5-91)
density relative to water 6.55-6.6 1 6.58
6.79-6.90 6.84
3.72-3.76 3.74
Reprint requests to: C. H. Sibata, Ph.D. This research was supported in part by the Conselho National de Desenvolvimento Cientifico e Tecnologico (CNPq/Brazil). Accepted for publication 14 June 1989.
455
I. J. Radiation Oncology 0 Biology 0 Physics
February 1990, Volume 18, Number 2
Fig. 1. (a) Prosthesis models used in the experiment. Number 1 is a 28 mm diameter solid head made of either Co-Cr or Ti. Number 2 is a 32 mm diameter head made of either Ti (solid) or Co-Cr (hollow). Number 3 is a 32 mm diameter detachable head made of Co-Cr. Number 4 is a 46 mm diameter head made of stainless steel (hollow), and Number 5 is a 54 mm diameter head made of Co-Cr (hollow). (b) Cross sections of the five models shown in Figure la, and the planes where profiles were taken.
surfaces (2, 6). Because of the large number of possible geometry and material combinations, it is virtually impossible to provide dosimetric information for each individual implant. This paper reports on the capability of a simple treatment planning computer program to predict the dose distribution under prostheses. The computer accounts for the inhomogeneity by considering the geometric parameters and composition of the implant. The results measured from prostheses of different geometries and materials were compared with those predicted by the computer planning program. It also reports the measured perturbation of 7 different prostheses that represent the commonly-used implants.
METHODS
AND
MATERIALS
Figure la shows five models of hip prostheses used in this experiment. There were two implants for each of the first two models, one made of a titanium alloy and the other made of a cobalt-chrome alloy. A total of seven prostheses were studied. Figure 1b shows the cross sections * Mevatron 67, Siemens, Iselin, NJ. + Therac 20 Saturne, AECL, Canada. * Farmer, PTW, Freiburg, Federal Republic
of these prostheses and the planes where the profiles were taken. The femoral heads of the prostheses ranged in diameter from 28 mm to 54 mm and were composed of examples of all three major alloys. Six MV* and 18 MVt X ray beams were selected since they are the most frequently used beams for treating the pelvic region in our institution. The implants were submerged in water and the head centered at 5 cm or 6.5 cm depth in the central axis of the beam. The profile measurements were made at 10 cm, 15 cm and 20 cm depth. Field sizes of 10 X 10 cm, 10 X 15 cm, and 15 X 15 cm at 100 cm SSD were used. Figure 2 shows a measured dose profile at 10 cm depth, under the femoral head of a prosthesis using an ion chambeti and film@. The size of the chamber has been found to affect the measurements due to averaging in the sharp fall off region (9). Therefore, film dosimetry was used to obtain dose profiles as well as isodose curves under the heads of the prostheses. A calibration curve was done for each set of film measurements to avoid dependence of the processing on film response. The profile data were obtained using a scanner system** controlled by a PC microcomputer. 5 Kodak X-Omat XV-2 Eastman Kodak Co., Rochester, ** Therados RFA-3, Scanditronix, Uppsala, Sweden.
of Germany.
NY.
Influence of hip prostheses0 C. H. SIBATA et al.
OFih 0
Ion
Cbambsr
6MV X-RAYS 0
-2 -10
’ -8
-0
-4
’
’
0
-2
’
’ 2
’
’
’
4
’ 6
’
’ 2
’
1 10
DISTANCE FROM CENTRAL AXIS (cm) Fig. 2. Comparison of a dose profile under hip prosthesis model 5 using a Farmer ion chamber and film dosimetry. The scans were taken at 10 cm depth in water. The prosthesis head was centered in the central axis of a 6 MV photon beam at 5 cm depth. Field size 10 X 10 cm at 100 cm SD.
of cobalt-chrome (type #5 on Fig. 1) for a 6 MV photon beam are shown in Figure 5. All the points were normalized to the maximum dose at 10 cm depth, for comparison purposes. The agreement is good, with maximum discrepancy of about 5%, in the region close to the prosthesis. Figures 6 and 7 show the calculated and measured dose profiles under 28 mm solid femoral heads of type 1 prostheses made of Ti and Co-Cr, respectively, at depth of 10, 15, and 20 cm for (a) 6 MV and (b) 18 MV photon beams. Agreement between the measured and calculated dose profiles was better than 2% for the lighter titanium prostheses at all depths and for both energies used. Similar results were seen for the cobalt-chrome prosthesis with the 6 MV photon beam. The calculated profile under the cobalt-chrome prosthesis, for the 18 MV photon beam, however, tended to be higher than that measured by up to 8%. Note that the exact composition of the metal implants was not determined but rather, an estimate based on published data was used. Although in the laboratory it may be possible to measure the composition of the implant, this is not possible in the patient. In our institution, this information is recorded in the patient files at the time of the prosthesis insertion. The geometric properties of the
100 00
To summarize
the perturbation caused by each prosthesis, and considering that the attenuation varies along the measured profile, an index of perturbation (IP) was defined as:
1p =:
60
40
A2-Al
A2
457
20
’
where Al is the integral of the measured dose distribution under the dimension of the prosthesis along the profile, and A2 is the integral of t.he unperturbed dose distribution under the same dimension (see Fig. 3). This index rep-
resents the average attenuation due to the prosthesis under the section studied. The predicted dose distributions were calculated using the GE RT/Plan (3), which uses the equivalent path length correction method (8) for inhomogeneities.
z cl
u g
3 z
0 100
00 60
40
20
RESULTS
AND DISCUSSION
Figures 4a and 4b show the dose distribution measured under the femoral heads of four prostheses in a plane at 10 cm depth, for 6 MV and 18 MV x-ray beams, respectively. Table 2 sum-marizes the perturbation for the seven prostheses studied, for the set-up described in Figure 4. The calculated and measured beam dose distributions under the central plane of a 58 mm hollow head prosthesis
0 -10
-2
-6
-4
-2
0
2
4
2
2
10
DISTANCE FROM CENTRAL AXIS (cm) Fig. 3. Definition of the index of perturbation (Eq. 1). Al is the area under the profile of the perturbed beam, limited by the projected dimension of the prosthesis along the scan direction, and represents the integral dose transmited. A2 is the area for the unperturbed beam, and represents the integral dose delivered to the same region.
458
I. J. Radiation Oncology 0 Biology 0 Physics
February 1990, Volume 18, Number 2
DISTANCE FROM CENTRAL AXIS (cm)
DISTANCEFROMCENTRALAXIS (cm) Fig. 4. Dose distribution profiles measured under the femoral heads of four prostheses in a plane at 10 cm depth, for 6 MV (a) and 18 MV (b) x-ray beams, respectively. Sections A, B and C as shown in figure lb. The prostheses were submerged in water at 5 cm depth. Field size 15 X 15 cm at 100 cm SSD.
prosthesis can be determined by portal films since CT is generally not useful because of the artifacts produced. The largest discrepancies noted between the predicted and actual dose distribution for the seven hip prostheses
studied occurred when using the higher energy beam and for the materials having a higher electron density relative to water. It was found that by using an artificially higher electron density, one could compensate for the overdose
459
Influence of hip prostheses 0 C. H. SIBATAet al. Table 2. Summary
of measured
Max att. No. xost.
Alloy
1
Co-Cr
1
Ti
2
Co-Cr
2
Ti
3
Co-Cr
4
S. Steel
5
Co-Cr
Section A B A B A B A B A B C ; C A B
Our results are in disagreement with the results by Hazuka et al. (4). They have found that it is necessary to apply a correction factor to the prosthesis electron density for a 4 MV photon beam and no correction for a 10 MV photon beam, in order to predict the transmission through the prosthesis for points at the central axis of the beam. In our case, we have good agreement with both low and high-energy photon beams, except for the case where an 18 MV photon beam and high electron density material are used. Furthermore, we have studied the whole dose distribution under the prosthesis. This discrepancy may be due to the different methods of inhomogeneity correction. Hazuka ef al. (4) used a treatment planning system with the ratio of TMRs correction method. In ou; case, we used a treatment planning system with the equivalent path length correction method.
perturbation IP
6MV
18MV
6MV
18MV
.45 .45 .26 .26 .44 .36 .28 .28 .42 .40 .42 .44 .24 .46 .50 .42
.35 .35 .17 .17 .32 .25 .20 .20 .30 .30 .31 .41 .14 .38 .37 .28
.29 .29 .16 .21 .28 .27 .18 .24 .21 .32 .36 .27 .08 .34 .20 .32
.22 .28 .ll .15 .18 .20 .14 .17 .14 .26 .28 .15 .05 .29 .15 .24
CONCLUSION We have expanded the information available in the literature to the most commonly-used prosthesis alloys and models, and to other photon beam energies. The perturbation caused by the hip prosthesis is significant for all cases studied and the treatment plan should consider this effect. A maximum attenuation of 50% was found for a solid Co-Cr alloy implant. Since this occurs in relatively small regions under the prosthesis, the index of perturbation (IP) was defined and calculated for the prostheses studied. This index, which represents the average attenuation of the prosthesis, varies from 5 to 36%
shown in Figure 1. Measurements taken at 10 cm depth. Set-up described in Figure 4. IP as defined in equation I. Note: Prostheses
sections as
predicted by the planning program. Considering, however, that these discrepancies depend on the point of measurement (off-axis distance and depth) and that the electron density for a given alloy may vary, no attempt was done to calculate a correction factor to adjust the electron density.
measured
calculated
,.,’
15Cl
____
_ -__
00 -
-.--________
. __.__
_ __
--.-
--
60/
a:
!
/ {
,;
/ i’:: 110 i, ;, ;__ ..
---~___+_+~
____
i
i
_.......
.,“.
”
,; ?: : i:,
,:
_____________T;_
!):
j :: :: , 0 ,,._..-.” ij i ,..... .“.“’
::
‘. :
_:+___. j
:.
i:
10
: ::: j :: : 1:
/’ i: :: : .Y
. . . . .’
::
:::
1
:::: : : 1; ,; ; i ,. ::
Fig. 5. Comparison of transverse cross-section isodose curves with measurements and calculations done with the GE RT/Plan using a 6 MV photon beam, (15 X 15) cm field size, 100 cm SSD for a hollow hip prosthesis made of Co-Cr alloy (model 5) centered at 5 cm depth in water.
I. J. Radiation Oncology 0 Biology 0 Physics
460
February 1990, Volume 18, Number 2
100 SO 80
I
-0
-6
-4
-2
0
2
4
0
-8
0
-8
I
I
-4
.
I
I
I
-2
I
I
0
I
I
2
I
4
Ia
1
0
8
DISTANCE FROM CENTRAL AXIS (cm)
DISTANCE FROM CENTRAL AXIS (cm)
Fig. 6. Comparison between calculated and measured dose profiles at depths of 10, 15 and 20 cm, for a solid hip prosthesis model 1 made of Ti, 28 mm head diameter, for 6 MV (a) and 18 MV (b) photon beams. The field size measured; A was (10 X 15) cm, 100 cm SSD and the prosthesis was centered at 6.5 cm depth in water. ~ calculated.
i 100
b! z
-
80
-
60
-
70
cm
cm
1
cm
-
-
cm . z z
60
-
w 2
50
-
z3
40
;
cm
-
20
-
10
-
-
60
-
cm g F:
50
1
z3
40
-
30
-
20
r
c
ot*‘s”‘a”“‘c”ll -0
70
:
z 30
-6
-4
DISTANCE
-2
FROM
0
2
CENTRAL
4
6
AXIS (cm)
0
-
-0
-6
-4
-2
0
2
4
6
DISTANCE FROM CENTRAL AXIS (cm)
Fig. 7. Comparison between calculated and measured dose profiles at depths of 10, 15 and 20 cm, for a solid hip prosthesis model 1 made of Co-Cr, 28 mm head diameter, for 6 MV (a) and I8 MV (b) photon beams. The field size was (10 X 15) cm, 100 cm SSD and the prosthesis was centered at 6.5 cm depth in water. measured; A calculated.
6
:
Influence of hip prostheses 0 C. H. SIBATAet al.
for the prostheses and energies studied. This index and the maximum attenuation should provide enough clinical information to aid the clinician to assess a treatment plan, even when no capability for inhomogeneity correction is available.
461
It was also found that the algorithm used in the GE RT/Plan may be used to predict the dose distribution under a femoral hip prosthesis for 6 and 18 MV photon beams within acceptable uncertainty, assuming the geometry and metallic alloy are known.
REFERENCES 1. B&s, P. J.; Russel, M. D. Effect of a femoral head prosthesis on megavoltage beam radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 14:581-5868; 1988.
2. Cohen, J. Metal implants:
historical background and biological response to implantation. In: Rubin, L. R., ed. Biomaterials in reconstructive surgery. St. Louis: C.V. Mosby; 1983.
3. GE RT/plan
physics manual. Slough, Electric Medical Systems; 1984.
England:
General
4. Hazuka, M. B.; Ibbott, G. S.; Kinzie, J. J. Hip prostheses during pelvic irradiation: effects and corrections. Int. J. Radiat. Oncol. Biol. Phys. 14:131 l-1317; 1988. M. Radio5. Hudson, F. H.; Crawley, M. T.; Samarasekera, therapy treatment planning for patients fitted with prostheses. Brit. J. Radiol. 57:603-608; 1984.
E. P. Metals and alloys. In: 6. Keller, J. C.; Lautenschlager, Von Recum, A. F., ed. Handbook of biomaterials evaluation: scientific, technical, and clinical testing of implant materials. New York: Macmillan Pub. Co.; 1986. 7. Mears, D. C. Materials and orthopaedic surgery. Baltimore: Williams and Wilkins Co.; 1979. 8. Parker, R. P.; Hobday, P. A.; Cassell, K. J. The direct use of CT numbers in radiotherapy dosage calculations for inhomogeneous media. Phys. Med. Biol. 26:825-833; 1979. 9. Sibata, C. H.; Mota, H. C.; Higgins, P. D.; Beddar, A. S. Detector size influence in photon beam profile measurements (Abstr.) ESTRO Meeting Proceedings, Den Haag, The Netherlands; 1988:4 18. 10. Yamamuro, T. Recent advances in clinical use of artificial joint of the hip and knee. In: Rubin, L. R., ed. Biomaterials in reconstructive surgery. St. Louis: C.V. Mosby; 1983.
IS, pp.
455-46
I Copyright
0360-3016/90 $3.00 + .oO 0 1990 Pergamon Press plc
??Technical Innovations and Notes
INFYLUENCE OF HIP PROSTHESES ON HIGH ENERGY PHOTON DOSE DISTRIBUTIONS C.
H.
SIBATA,PH.D.,*
H. C. MOTA, J. P.
M.Sc.,*
SAXTON,
The Cleveland
M.D.*
P. D. HIGGINS, AND
K.
Clinic Foundation,
H. SHIN,
Cleveland,
PH.D.,*
D. GAISSER,
M.S.,+
M.D.*
OH 44 195
Radiotherapy treatment of patients having a hip prosthesis is a common problem facing dosimetrists and physicists when the treatment plan requires irradiation of the pelvic area. To quantify the perturbation of these devices, attenuation studies were done with 6 and 18 MV photon beams using various hip prostheses models with varying size and compos?tion. These studies have shown that an attenuation of as much as 50% can be found in a single beam profile under the prosthesis. We have studied the capability of a dose planning system to predict the transmission of these devices aa compared with measurements. Femoral hip prosthesis,
Radiotherapy treatment planning, Dose distribution, Beam perturbation.
INTRODUCTION Radiotherapy tients
having
planning
involving
a total hip prosthesis
of titanium are the most recent addition to the surgical alloy field. Because of their relatively poor wear characteristics, these alloys generally are not used for articulating
the pelvic region of pahas been found
to be
difficult due to the effects of the prosthesis
on the treatment dose distribution. In 1984, Hudson et al. (5) described a high perturbation caused by some solid prostheses in radiotherapy treatment with 8 MV photons and “Co gamma rays. More recently, Biggs and Russel ( 1) described an average dose decrease: of approximately 2% for 25 MV X rays and average increases of approximately 2% for 10 MV X rays and 5% for 60Co through the head of a hollow prosthesis with a diameter of 5 cm and a wall thickness of 3 mm. The extent of the perturbation is dependent on the model and material ofthe prosthesis, and on the energy of the radiation itself. Implants may vary in size and have either hollow or solid fe-moral heads. For orthopedic implants, the material used are primarily limited to three classes of metallic alloys: stainless steel, cobalt-chrome, and titanium alloys (2, 6). Table 1 lists the composition of the major surgical alltoys used. The majority of current hip prostheses are composed of cobalt-chrome alloys ( 10) as they are considered to have the best combination of corrosion and fatigue resistance and mechanical strength (6). Stainless steel and titanium implants may also be found. Some of the earlier stainless steel prostheses are still functioning well in patients. Alloys
Table 1. Stainless steel wrought
Element (a) Composition
of prosthesis
Carbon Manganese Phosphorus Sulfur Silicon Oxygen Cobalt Chromium Nickel Molybdenum Iron Aluminum Vanadium Titanium
0.08 max 2.00 max 0.03 max 0.03 max 0.75 max 17-20 10-14 2-4 Bal(59-70)
(b) Electron Range Average
This work was presented at the 30th Annual Meeting of the American Association of Physicists in Medicine, San Antonio, TX, August 1988. * Department of Radiation Therapy. + Department of Musculoskeletal Sciences.
-
Co-Cr cast
Titanium cast-wrought
alloys (%w) (7) 0.35 max 1.OO max 1.OO max Bal (57.4-65) 27-30 2.50 max 5-7 0.75 max -
0.08 max
0.13 max 0.25 max 5.6-6.5 3.5-4.5 Bal (88.5-91)
density relative to water 6.55-6.6 1 6.58
6.79-6.90 6.84
3.72-3.76 3.74
Reprint requests to: C. H. Sibata, Ph.D. This research was supported in part by the Conselho National de Desenvolvimento Cientifico e Tecnologico (CNPq/Brazil). Accepted for publication 14 June 1989.
455
I. J. Radiation Oncology 0 Biology 0 Physics
February 1990, Volume 18, Number 2
Fig. 1. (a) Prosthesis models used in the experiment. Number 1 is a 28 mm diameter solid head made of either Co-Cr or Ti. Number 2 is a 32 mm diameter head made of either Ti (solid) or Co-Cr (hollow). Number 3 is a 32 mm diameter detachable head made of Co-Cr. Number 4 is a 46 mm diameter head made of stainless steel (hollow), and Number 5 is a 54 mm diameter head made of Co-Cr (hollow). (b) Cross sections of the five models shown in Figure la, and the planes where profiles were taken.
surfaces (2, 6). Because of the large number of possible geometry and material combinations, it is virtually impossible to provide dosimetric information for each individual implant. This paper reports on the capability of a simple treatment planning computer program to predict the dose distribution under prostheses. The computer accounts for the inhomogeneity by considering the geometric parameters and composition of the implant. The results measured from prostheses of different geometries and materials were compared with those predicted by the computer planning program. It also reports the measured perturbation of 7 different prostheses that represent the commonly-used implants.
METHODS
AND
MATERIALS
Figure la shows five models of hip prostheses used in this experiment. There were two implants for each of the first two models, one made of a titanium alloy and the other made of a cobalt-chrome alloy. A total of seven prostheses were studied. Figure 1b shows the cross sections * Mevatron 67, Siemens, Iselin, NJ. + Therac 20 Saturne, AECL, Canada. * Farmer, PTW, Freiburg, Federal Republic
of these prostheses and the planes where the profiles were taken. The femoral heads of the prostheses ranged in diameter from 28 mm to 54 mm and were composed of examples of all three major alloys. Six MV* and 18 MVt X ray beams were selected since they are the most frequently used beams for treating the pelvic region in our institution. The implants were submerged in water and the head centered at 5 cm or 6.5 cm depth in the central axis of the beam. The profile measurements were made at 10 cm, 15 cm and 20 cm depth. Field sizes of 10 X 10 cm, 10 X 15 cm, and 15 X 15 cm at 100 cm SSD were used. Figure 2 shows a measured dose profile at 10 cm depth, under the femoral head of a prosthesis using an ion chambeti and film@. The size of the chamber has been found to affect the measurements due to averaging in the sharp fall off region (9). Therefore, film dosimetry was used to obtain dose profiles as well as isodose curves under the heads of the prostheses. A calibration curve was done for each set of film measurements to avoid dependence of the processing on film response. The profile data were obtained using a scanner system** controlled by a PC microcomputer. 5 Kodak X-Omat XV-2 Eastman Kodak Co., Rochester, ** Therados RFA-3, Scanditronix, Uppsala, Sweden.
of Germany.
NY.
Influence of hip prostheses0 C. H. SIBATA et al.
OFih 0
Ion
Cbambsr
6MV X-RAYS 0
-2 -10
’ -8
-0
-4
’
’
0
-2
’
’ 2
’
’
’
4
’ 6
’
’ 2
’
1 10
DISTANCE FROM CENTRAL AXIS (cm) Fig. 2. Comparison of a dose profile under hip prosthesis model 5 using a Farmer ion chamber and film dosimetry. The scans were taken at 10 cm depth in water. The prosthesis head was centered in the central axis of a 6 MV photon beam at 5 cm depth. Field size 10 X 10 cm at 100 cm SD.
of cobalt-chrome (type #5 on Fig. 1) for a 6 MV photon beam are shown in Figure 5. All the points were normalized to the maximum dose at 10 cm depth, for comparison purposes. The agreement is good, with maximum discrepancy of about 5%, in the region close to the prosthesis. Figures 6 and 7 show the calculated and measured dose profiles under 28 mm solid femoral heads of type 1 prostheses made of Ti and Co-Cr, respectively, at depth of 10, 15, and 20 cm for (a) 6 MV and (b) 18 MV photon beams. Agreement between the measured and calculated dose profiles was better than 2% for the lighter titanium prostheses at all depths and for both energies used. Similar results were seen for the cobalt-chrome prosthesis with the 6 MV photon beam. The calculated profile under the cobalt-chrome prosthesis, for the 18 MV photon beam, however, tended to be higher than that measured by up to 8%. Note that the exact composition of the metal implants was not determined but rather, an estimate based on published data was used. Although in the laboratory it may be possible to measure the composition of the implant, this is not possible in the patient. In our institution, this information is recorded in the patient files at the time of the prosthesis insertion. The geometric properties of the
100 00
To summarize
the perturbation caused by each prosthesis, and considering that the attenuation varies along the measured profile, an index of perturbation (IP) was defined as:
1p =:
60
40
A2-Al
A2
457
20
’
where Al is the integral of the measured dose distribution under the dimension of the prosthesis along the profile, and A2 is the integral of t.he unperturbed dose distribution under the same dimension (see Fig. 3). This index rep-
resents the average attenuation due to the prosthesis under the section studied. The predicted dose distributions were calculated using the GE RT/Plan (3), which uses the equivalent path length correction method (8) for inhomogeneities.
z cl
u g
3 z
0 100
00 60
40
20
RESULTS
AND DISCUSSION
Figures 4a and 4b show the dose distribution measured under the femoral heads of four prostheses in a plane at 10 cm depth, for 6 MV and 18 MV x-ray beams, respectively. Table 2 sum-marizes the perturbation for the seven prostheses studied, for the set-up described in Figure 4. The calculated and measured beam dose distributions under the central plane of a 58 mm hollow head prosthesis
0 -10
-2
-6
-4
-2
0
2
4
2
2
10
DISTANCE FROM CENTRAL AXIS (cm) Fig. 3. Definition of the index of perturbation (Eq. 1). Al is the area under the profile of the perturbed beam, limited by the projected dimension of the prosthesis along the scan direction, and represents the integral dose transmited. A2 is the area for the unperturbed beam, and represents the integral dose delivered to the same region.
458
I. J. Radiation Oncology 0 Biology 0 Physics
February 1990, Volume 18, Number 2
DISTANCE FROM CENTRAL AXIS (cm)
DISTANCEFROMCENTRALAXIS (cm) Fig. 4. Dose distribution profiles measured under the femoral heads of four prostheses in a plane at 10 cm depth, for 6 MV (a) and 18 MV (b) x-ray beams, respectively. Sections A, B and C as shown in figure lb. The prostheses were submerged in water at 5 cm depth. Field size 15 X 15 cm at 100 cm SSD.
prosthesis can be determined by portal films since CT is generally not useful because of the artifacts produced. The largest discrepancies noted between the predicted and actual dose distribution for the seven hip prostheses
studied occurred when using the higher energy beam and for the materials having a higher electron density relative to water. It was found that by using an artificially higher electron density, one could compensate for the overdose
459
Influence of hip prostheses 0 C. H. SIBATAet al. Table 2. Summary
of measured
Max att. No. xost.
Alloy
1
Co-Cr
1
Ti
2
Co-Cr
2
Ti
3
Co-Cr
4
S. Steel
5
Co-Cr
Section A B A B A B A B A B C ; C A B
Our results are in disagreement with the results by Hazuka et al. (4). They have found that it is necessary to apply a correction factor to the prosthesis electron density for a 4 MV photon beam and no correction for a 10 MV photon beam, in order to predict the transmission through the prosthesis for points at the central axis of the beam. In our case, we have good agreement with both low and high-energy photon beams, except for the case where an 18 MV photon beam and high electron density material are used. Furthermore, we have studied the whole dose distribution under the prosthesis. This discrepancy may be due to the different methods of inhomogeneity correction. Hazuka ef al. (4) used a treatment planning system with the ratio of TMRs correction method. In ou; case, we used a treatment planning system with the equivalent path length correction method.
perturbation IP
6MV
18MV
6MV
18MV
.45 .45 .26 .26 .44 .36 .28 .28 .42 .40 .42 .44 .24 .46 .50 .42
.35 .35 .17 .17 .32 .25 .20 .20 .30 .30 .31 .41 .14 .38 .37 .28
.29 .29 .16 .21 .28 .27 .18 .24 .21 .32 .36 .27 .08 .34 .20 .32
.22 .28 .ll .15 .18 .20 .14 .17 .14 .26 .28 .15 .05 .29 .15 .24
CONCLUSION We have expanded the information available in the literature to the most commonly-used prosthesis alloys and models, and to other photon beam energies. The perturbation caused by the hip prosthesis is significant for all cases studied and the treatment plan should consider this effect. A maximum attenuation of 50% was found for a solid Co-Cr alloy implant. Since this occurs in relatively small regions under the prosthesis, the index of perturbation (IP) was defined and calculated for the prostheses studied. This index, which represents the average attenuation of the prosthesis, varies from 5 to 36%
shown in Figure 1. Measurements taken at 10 cm depth. Set-up described in Figure 4. IP as defined in equation I. Note: Prostheses
sections as
predicted by the planning program. Considering, however, that these discrepancies depend on the point of measurement (off-axis distance and depth) and that the electron density for a given alloy may vary, no attempt was done to calculate a correction factor to adjust the electron density.
measured
calculated
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Fig. 5. Comparison of transverse cross-section isodose curves with measurements and calculations done with the GE RT/Plan using a 6 MV photon beam, (15 X 15) cm field size, 100 cm SSD for a hollow hip prosthesis made of Co-Cr alloy (model 5) centered at 5 cm depth in water.
I. J. Radiation Oncology 0 Biology 0 Physics
460
February 1990, Volume 18, Number 2
100 SO 80
I
-0
-6
-4
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0
2
4
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-8
0
-8
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I
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I
I
2
I
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DISTANCE FROM CENTRAL AXIS (cm)
DISTANCE FROM CENTRAL AXIS (cm)
Fig. 6. Comparison between calculated and measured dose profiles at depths of 10, 15 and 20 cm, for a solid hip prosthesis model 1 made of Ti, 28 mm head diameter, for 6 MV (a) and 18 MV (b) photon beams. The field size measured; A was (10 X 15) cm, 100 cm SSD and the prosthesis was centered at 6.5 cm depth in water. ~ calculated.
i 100
b! z
-
80
-
60
-
70
cm
cm
1
cm
-
-
cm . z z
60
-
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-
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;
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-
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Fig. 7. Comparison between calculated and measured dose profiles at depths of 10, 15 and 20 cm, for a solid hip prosthesis model 1 made of Co-Cr, 28 mm head diameter, for 6 MV (a) and I8 MV (b) photon beams. The field size was (10 X 15) cm, 100 cm SSD and the prosthesis was centered at 6.5 cm depth in water. measured; A calculated.
6
:
Influence of hip prostheses 0 C. H. SIBATAet al.
for the prostheses and energies studied. This index and the maximum attenuation should provide enough clinical information to aid the clinician to assess a treatment plan, even when no capability for inhomogeneity correction is available.
461
It was also found that the algorithm used in the GE RT/Plan may be used to predict the dose distribution under a femoral hip prosthesis for 6 and 18 MV photon beams within acceptable uncertainty, assuming the geometry and metallic alloy are known.
REFERENCES 1. B&s, P. J.; Russel, M. D. Effect of a femoral head prosthesis on megavoltage beam radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 14:581-5868; 1988.
2. Cohen, J. Metal implants:
historical background and biological response to implantation. In: Rubin, L. R., ed. Biomaterials in reconstructive surgery. St. Louis: C.V. Mosby; 1983.
3. GE RT/plan
physics manual. Slough, Electric Medical Systems; 1984.
England:
General
4. Hazuka, M. B.; Ibbott, G. S.; Kinzie, J. J. Hip prostheses during pelvic irradiation: effects and corrections. Int. J. Radiat. Oncol. Biol. Phys. 14:131 l-1317; 1988. M. Radio5. Hudson, F. H.; Crawley, M. T.; Samarasekera, therapy treatment planning for patients fitted with prostheses. Brit. J. Radiol. 57:603-608; 1984.
E. P. Metals and alloys. In: 6. Keller, J. C.; Lautenschlager, Von Recum, A. F., ed. Handbook of biomaterials evaluation: scientific, technical, and clinical testing of implant materials. New York: Macmillan Pub. Co.; 1986. 7. Mears, D. C. Materials and orthopaedic surgery. Baltimore: Williams and Wilkins Co.; 1979. 8. Parker, R. P.; Hobday, P. A.; Cassell, K. J. The direct use of CT numbers in radiotherapy dosage calculations for inhomogeneous media. Phys. Med. Biol. 26:825-833; 1979. 9. Sibata, C. H.; Mota, H. C.; Higgins, P. D.; Beddar, A. S. Detector size influence in photon beam profile measurements (Abstr.) ESTRO Meeting Proceedings, Den Haag, The Netherlands; 1988:4 18. 10. Yamamuro, T. Recent advances in clinical use of artificial joint of the hip and knee. In: Rubin, L. R., ed. Biomaterials in reconstructive surgery. St. Louis: C.V. Mosby; 1983.
