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Evaluation of computed tomography assisted and transit dosimetry treatment planning with thermoluminescent dosimetry measurements
03h0-3010:R0!I:1739-06502.00,10 Capypht T IWO Pcreamon Press Ltd
In, / RoJ~vrion Oncolo~ Bd Phrs.. Vol. 6. pp. 1739.1744 Primal tn Ihe U.S A. All rights rcscnd.
@ Technical Innovations and Notes EVALUATION OF COMPUTED TORIOGRAPHY ASSISTED AND TRANSIT DOSIMETRY TREATMENT PLANNING WITH THERMOLUMINESCENT DOSIMETRY MEASUREMENTS DONALD
E. VELKLEY,
Division of Medical Physics,
PH.D.
AND
DAVID E. CCKNINGHAM.
PH.D.
Department of Radiology. M.S. Hershey Medical Center. Unilsersity. Hershey. Pennsylvania 17033
MELVIN Department
of Radiology.
F.
STROCKBINE,
The York Hospital,
The Pennsylvania
State
M.D.
York. Pennsylvania
17405
Transit dosimetry methods have been compared with computer plans which use computed tomography (CT) determined patient geometry to make corrections for inhomogeneities in radiation therapy treatment planning. These two planning methods have been applied to treatment sites in the head and chest of an anthropomorphic phantom. The results of the treatment plans are compared to delivered doses as measured with thermoluminescent dosimeters (TLD) in tbe phantom. The treatment planning results from two independent commercial computer systems have been found to agree with each other and with transit dosimetry calculations to within 2 %, In the head, the result of both inhomogeneity correction methods agree with measured doses to within 2%. In the chest the calcu!ated doses disagree with measured values by 6%. This results from the inadequacy of the correction methods to account for the inhomogeneity distribution. When this effect is taken into account, the calculated and measured doses agree to within 2% even for the chest irradiations. Treatment
planning, Transit
dosimetry, Inhomogeneity
corrections,
INTRODUCTION
scanner and a treatment planning computer has led us to compare the two methods of determining radiotherapy dosage and evaluate the results with TLD measurements in an anthropomorphic phantom.
The advent of CT scanning combined with versatile computing capabilities has recently altered the techniques for making heterogeneity corrections in radiation therapy treatment planning at many institutions. In order to use the CT information. most centers are magnifying the CT image and entering the structures into the computer using conventional means. Also, some CT systems are under development which will provide on-line treatment planning capabilities4 However, the current generation of treatment planning computers may not be able to make inhomogeneity corrections with the accuracy needed to properly utilize the detailed information provided by CT devices. Prior to CT, transaxial tomography could be used to obtain patient data and transit dosimetry techniques were available to measure the absorption characteristics of individual patients with the therapy beam. Neither of these methods has enjoyed widespread use in the radiotherapy community although transit methods require no more equipment than that normally available in a radiotherapy facility. A transit dosimetry system has been in routine use at York Hospital, York, Pennsylvania, for several years. The recent acquisition of a total-body CT
Reprint
requests
Computers.
METHODS All results reported MV linear accelerator.
AND RjATERIALS here are for treatment
with a 4
Transit method The transit system has been described previously’ and will briefly be discussed here. A 0.6 cm3 ionization chamber with build-up can be affixed in a fuse clip to the back stop of the accelerator so that the central axis of the beam passes through the center of the sensitive volume. During the patient’s initial treatment, the ionization induced in the chamber is integrated with an electrometer. Following therapy, the treatment is repeated under identical conditions except that the patient is removed to generate an unattenuated chamber reading. The ratio of the two chamber readings is related to the effective attenuation coefficients. The effective attenuation coefficient values were determined by phantom measurements; since the chamber is not collimated, the effective attenua-
Accepted
to: Donald E. Velkley, Ph.D. 1739
for publication
23 July 1980.
Radiation Oncology 0 Biology 0 Physics
1740
tion coefficient depends on field size and source-tosurface distance (SSD). Values were tabulated for 80 and 100 cm SSD and for rotational treatments. For calculations of a specified dose at mid-plane, the percentage depth dose (PDD) is given by: PDD = PDD,X((SSD
+ &/(SSD
+ d,))’
(1)
December 1980, Volume 6. Number 12
nated polytetrafluoroethylene (PTFE)$ which were loaded into the phantom in drilled Mix-D plugs of 5 mm in diameter. The TLDs were calibrated in the linear accelerator beam; the results of multiple calibrations had a standard deviation of about 5% for a single measurement. Multiple exposures were made and averaged for measurements so that the relative uncertainty in any reported dose value was less than 2%.
where: d, - one half the physical thickness as measured with
body calipers d, - one half the effective thickness as determined by
transit dose measurements and PDD, - percentage depth dose for the depth d,. The inverse-square correction is necessary to account for the distance dependent component of the depth-dose curve. Since no information is obtained with this method as to the distribution of the inhomogeneity, in general the results are only correct for parallel-opposed treatment fields. The discussion here of the transit method will only consider central-axis dose calculations for parallelopposed fields. CT scanner and computer techniques
The CT scans discussed here werp all obtained with the same device.* The transparency of the image was enlarged to life size by a photographic enlarger and the desired structures were outlined on paper. Inhomogeneity corrections were made for aerated cavities and for bone. All other structures were treated as water-equivalent. This is a good approximation in the phantom used here which is cast from a uniform material around a skeleton with air cavities and simulated lungs. The density (p,) of an aerated cavity with an average CT number of CT, was determined as
P.3 =
1000 - CT, 1000
RESULTS The phantom was irradiated with the linear accelerator X ray beam using standard, parallel-opposed fields for whole brain with 14 cm x 16 cm fields at 80 cm SSD and for chest treatment with 10 cm x 10 cm fields at 80 cm SSD. In each case, physical measurements of the phantom thickness (d,) were used to calculate a desired tumor dose; during the irradiation transit measurements were made to determine the effective central-axis thickness (d,). TLDs were inserted during the irradiations to measure the dose delivered. Figure 1 is a contact radiograph of section 17 of the phantom through which the central axis of the treatment fields passed. Figure 2 shows the CT scan of the same section. Direct measurements have shown that there is distortion in the CT image. Enlargement of the CT image to match the AP phantom dimension left about a 1 cm discrepancy in the lateral dimension. This is evident in CT scans of round objects which appear elliptical and in a rectangular grid display which has obvious barrel distortion. Considerable effort in adjustment has improved but not removed all of the distortion. The image was magnified to match the dimension along the treatment axis for the cases discussed here which used parallel-opposed
(2)
since air has a CT number of - 1000 and water has a value of zero. This has been shown to be a good estimate from CT scans of cork blocks and other low density materials. Two independent computers were used for treatment planning and are denoted here as system A and system B.** Commercially available software was used for all the calculations reported here with these two computers. TLD measurements
TLD measurements were made for treatments in the head and chest of an anthropomorphic phantom.? The TLD devices were 1 mm x 6 mm rods of LiF impreg*Ohio Nuclear FS25, Ohio Nuclear Corp., Solon, Ohio. **System A: PC-12, Artronix B: Capintec Therarad. Capintec,
Corp., St. Louis, MO. System Inc., Montvale. N.J.
Fig. 1. Contact radiograph of section of anthropomorphic phantom chest through which the central-axis of the irradiation fields passed.
tRando Conn. STeRon.
Phantom,
Alderson
Research
Labs.,
Stamford,
Evaluation of CT 0 D. E. VELKLEY er al.
1741
PHANTOM PARALLEL 10x10
CHEST
OPPOSED FIFLDS @ 80cm SSD
GIVEN DOSE = 180
Fig. 2. CT scan of section of anthropomorphic radiographically in Figure 1.
phantom
shown
fields. For some multiple field and rotational planning the distortion could lead to inaccuracy. This demonstrates the importance of verifying the geometrical accuracy of the CT image if it is to be used for treatment planning. Figure 3 is a computer generated treatment plan of the chest irradiation with no inhomogeneity corrections and Figure 4 is the same plan with corrections applied for the lung density. In all cases the two computers gave dose distributions which generally agreed with each other to better than 2%. Table 1 gives the doses at mid-plane as determined from the treatment plans and from the transit measurements. The doses calculated from the physically measured dimensions and the average dose measured with TLD are also given. The dose based on physical measurement was about 20% less than the measured value. The dose determined by transit methods and the inhomogeneity-corrected treatment
PHANTOM PARALLEL 10x10
CHEST
DENSITY
= 100
LUNG
DENSITY
= 0 30
,/
---. Fig. 4. Same plan as shown in Figure 3 except that corrections were made for a lung density of 0.30 using the manufacturer supplied software.
computer dose agreed with each other to better than 2%: however, they were about 6% higher than the measured value. Batho has shown that the proper correction for 6oCo therapy depends not only on the inhomogeneity density and thickness but also on its distribution.’ This was not taken into account in the transit calculations described above and evidently not in the computer calculations either. In the Batho method as outlined by Young and Gaylord,* a correction to the depth dose is given by F = [ TAR(A,d,)/TAR(A,d,)]‘“-”
(3)
where TAR = tissue-air ratio ‘A = field area d, = distance from point to proximal aspect of inhomogeneity d, = distance from point to distal aspect of inhomogeneity and pI = electron density of inhomogeneity. This method was applied here for the transit calculations and since the measurement point was at the center of a mix-D plug which was imbedded in the lung, d, was
OPPOSED FIELDS @ 80cm SSD
GIVEN DOSE = 180 TISSUE
TISSUE
DENSITY
= 1.00
Table 1. Calculated and measured doses (rad) at mid-plane and on the central-axis for chest irradiation of the phantom Calculated Measured
Fig. 3. Result of treatment plan in chest done with computer.* All tissues were assumed to have unit density. *Artronix.
Based on dp
Based on d, and equation #l
200
256
CT = Computed dosimetry.
tomography;
CT scan computer 2.58 TLD
TLD 239
= Thermoluminescent
1742
Radiation Oncology 0 Biology 0 Physics
PHANTOM
December 1980, Volume 6, Number 12
HEAD
?LI 10x16 GIVEN TISSUE
Fig. 5. Treatment density. ?? Capintec.
approximated measurements value found
@I 80cm
SSD
DOSE = 100 RAD DENSITY
= 1.00
plan of whole-brain
irradiation
done with computer. * All tissues were assumed to have unit
as one centimeter, d2 was determined from of the CT scan and pc was taken as the from equation 2 above. Since scatter of
megavoltage photons is primarily in the forward direction, the error in dose calculated for the small mass imbedded in the lung by this method should be small even though there is also some reduction in lateral-and backscattered radiation. When the correction factor so deter-
PHANTOM
10x16 GIVEN TISSUE BONE
@ 80cm DOSE
mined was applied to the transit calculations in the chest, the calculated dose and measured dose agreed to within 1%. Figures 5 and 6 are the treatment plans for the brain irradiations and Table 2 is a comparison of the calculated and measured central-axis doses. Here, where the inhomogeneity is relatively thin and a relatively large distance from the measurement point, good agreement is obtained
HEAD
SSD
= 100 RAD
DENSITY DENSITY
= 1.00 = 1.67
,
Fig. 6. Same plan as shown in Figure 5 except corrections for bone density were made. The outline of the bone was taken from the CT scan.
Evaluation of CT 0 D. E. VELKLEYet al.
Table 2. Calculated and measured doses (rad) at mid-plane and on the central-axis for the irradiation of the brain of AP phantom Calculated Measured Based on dp 149
Based on d, and equation
142
#I
CT scan computer
138
TLD 140
by all the calculation methods with the measured dose value without need to apply the Batho correction method. The application of the Batho method assumes that the point of dose calculation is not within the inhomogeneity. For the calculaticns performed here for the lung case, the calculated dose is not that which the lung tissue would receive but the dose in unit density tissue behind the lung. Electronic equilibrium associated with soft tissue is lost in traversal of the lung and must be reestablished before the calculated dose represents the delivered value. Thus in the case described here the thickness of the plug is sufficient to reestablish electronic equilibrium and the calculated and measured doses are at a point near the center of the plug. At the interface between the plug and lung, the dose will differ by an amount which depends on the radiation energy.’ The detailed dosimetry at interfaces has not been represented by any of the computation methods discussed here but may be important for high energy radiation. These interface effects require more study before they can be used in routine calculation procedures. DISCUSSION The commercial computer systems used in this work have been found to make inhomogeneity corrections which are consistent with each other. These results are also in agreement with doses calculated from transit measurements when corrections are made using an effective attenuation method. In brain irradiation of the phantom, the calculated doses agree with TLD measurements to within 2%. In the case of lung irradiation a more sophisticated method of making inhomogeneity corrections is required as pointed out by previous authors.1s3,6 The Batho correction is easily applied manually in the transit calculations by using CT information to determine the correction parameters. With this method, the calculated central-axis dose agrees with the measured value to within 2% at mid-plane for parallel-opposed fields, whereas the discrepancy with the non-Bath0 corrected result is about 6%. The 6% error in the lung case is about the largest error to be expected for calculations which do not use the Batho method (discounting the
I743
interface effects discussed above). Numerous calculations have shown that transit dose measurements together with physical measurements and standard cross-sectional anatomy charts to estimate the inhomogeneity distribution for Batho calculations enable the error in calculated dose at midplane to be reduced from 6-10s to 2-3s in the case of lung. In regions away from lung in which the inhomogeneity corrections are smaller, the uncertainties in calculated dose are also smaller. A significant disadvantage of the transit dose method is that it only allows calculations of dose at isolated points. A treatment planning computer (system C*) has recently become commercially available; this system claims to use a Batho-type method for making inhomogeneity corrections. This computer was not available for our use in making comparisons. However, company personnel produced plans using patient data and radiation parameters for the treatment unit which we supplied. This was the same patient data described above which was used in the other computer calculations. The results of these computations using their standard software gave mid-plane doses which agreed with our measured doses to better than 2% even in the lung. In addition, software is under development for use with the system B computer with which inhomogeneity corrections are made using methods similar to that employed in the system C unit. Corporation development personnel made calculations with their new software using our data as input and generated treatment plans which generally agreed very well with the results from system C. As of this writing the improved system B software is not currently available but probably will be supplied in the near future to their users. It should be re-emphasized that in the lung the computer calculations agree with the measured dose in the center of a tissue equivalent plug. i.e., after tissue equilibrium is re-established. The effects at the lung-tissue (or lungtumor) interface are not described and may be clinically significant. As Geise and McCullough discussed, the ability of CT devices to locate internal structures accurately is more important than the detailed map of CT numbers or electron densities.2 The CT image is liable to have distortions which could negate the advantages of a CT unit. If the patient is not in the same position for the CT scan as for radiotherapy, the detailed localization of internal structures which are not in the correct positions could cause unacceptable errors in the radiotherapy treatment plan. In contrast, transit dosimetry measurements are obtained with the radiotherapy unit coincidentally with the treatment of the patient. The dosage calculations are amenable to any chosen correction method either manually or within a computer or programmable calculator. The transit method remains as a viable alternative to CT *System
C: TP-I 1, Atomic
Ottowa, Canada.
Energy
of Canada
Limited,
1744
Radiation Oncology 0 Biology 0 Physics
for the generation of inhomogeneity corrections for institutions without CT or for patients in which the cost of CT in time and money is unwarranted. The cost of the transit dosimetry system is small and it has been found to be capable of reliable results in a very busy clinical environment. The use of CT in radiotherapy treatment planning will probably become a routine tool in many institutions. As
December
1980, Volume 6, Number
12
noted above, most planning computer software must be updated to provide more accurate inhomogeneity corrections in some cases to take full advantage of the CT information. Even at the most sophisticated centers, a transit system should be considered as a dosimetry check to verify the correctness of a treatment plan by measurements of the patient with the treatment unit.
REFERENCES 1. Batho, H.F.: Lung corrections in Cobalt 60 beam therapy. J. assisted tomography in radiation therapy treatment 2.
3.
4.
5.
Can. Assoc. Radiol. 15: 79-83, 1964. Gcise, R.A., McCullough, E.C.: The use of CT scanners in megavoltage photon-beam therapy planning. Radiology 124: 133-141,1977. McDonald, S.C., Keller, B.E., Rubin, P.: Method for calculating dose when lung tissue lies in the treatment field. Med. Phys. k 210-216,1976. Prasad, SC., Glasgow, G.P., Purdy, J.A.: An operational computed tomography radiotherapy planning system (Abstr.). Med. Phys. 5: 339, 1978. Purdy, J.A., Velkley, D.E., Ter-Pogossian, M.: Computer
planning-Effect of inhomogeneities. Phys. Can. 32: 26.1, 1976. 6. Sontog. M.R., Battista, J.J., Bronskull, M.J., Cunningham, J.R.: Implications of computed tomography for inhomogeneity corrections in photon beam dose calculations. Radiology 124: 143-149,1977. 7. Wirt, J.: Transit dosimetry in determining tissue-equivalent thickness. App. Radiol. 6: 71-75, 1977. 8. Young, M.E.J., Gaylord, J.D.: Experimental tests of corrections for tissue inhomogeneities in radiotherapy. Br. J. Radiol. 43: 349-355. 1970.
In, / RoJ~vrion Oncolo~ Bd Phrs.. Vol. 6. pp. 1739.1744 Primal tn Ihe U.S A. All rights rcscnd.
@ Technical Innovations and Notes EVALUATION OF COMPUTED TORIOGRAPHY ASSISTED AND TRANSIT DOSIMETRY TREATMENT PLANNING WITH THERMOLUMINESCENT DOSIMETRY MEASUREMENTS DONALD
E. VELKLEY,
Division of Medical Physics,
PH.D.
AND
DAVID E. CCKNINGHAM.
PH.D.
Department of Radiology. M.S. Hershey Medical Center. Unilsersity. Hershey. Pennsylvania 17033
MELVIN Department
of Radiology.
F.
STROCKBINE,
The York Hospital,
The Pennsylvania
State
M.D.
York. Pennsylvania
17405
Transit dosimetry methods have been compared with computer plans which use computed tomography (CT) determined patient geometry to make corrections for inhomogeneities in radiation therapy treatment planning. These two planning methods have been applied to treatment sites in the head and chest of an anthropomorphic phantom. The results of the treatment plans are compared to delivered doses as measured with thermoluminescent dosimeters (TLD) in tbe phantom. The treatment planning results from two independent commercial computer systems have been found to agree with each other and with transit dosimetry calculations to within 2 %, In the head, the result of both inhomogeneity correction methods agree with measured doses to within 2%. In the chest the calcu!ated doses disagree with measured values by 6%. This results from the inadequacy of the correction methods to account for the inhomogeneity distribution. When this effect is taken into account, the calculated and measured doses agree to within 2% even for the chest irradiations. Treatment
planning, Transit
dosimetry, Inhomogeneity
corrections,
INTRODUCTION
scanner and a treatment planning computer has led us to compare the two methods of determining radiotherapy dosage and evaluate the results with TLD measurements in an anthropomorphic phantom.
The advent of CT scanning combined with versatile computing capabilities has recently altered the techniques for making heterogeneity corrections in radiation therapy treatment planning at many institutions. In order to use the CT information. most centers are magnifying the CT image and entering the structures into the computer using conventional means. Also, some CT systems are under development which will provide on-line treatment planning capabilities4 However, the current generation of treatment planning computers may not be able to make inhomogeneity corrections with the accuracy needed to properly utilize the detailed information provided by CT devices. Prior to CT, transaxial tomography could be used to obtain patient data and transit dosimetry techniques were available to measure the absorption characteristics of individual patients with the therapy beam. Neither of these methods has enjoyed widespread use in the radiotherapy community although transit methods require no more equipment than that normally available in a radiotherapy facility. A transit dosimetry system has been in routine use at York Hospital, York, Pennsylvania, for several years. The recent acquisition of a total-body CT
Reprint
requests
Computers.
METHODS All results reported MV linear accelerator.
AND RjATERIALS here are for treatment
with a 4
Transit method The transit system has been described previously’ and will briefly be discussed here. A 0.6 cm3 ionization chamber with build-up can be affixed in a fuse clip to the back stop of the accelerator so that the central axis of the beam passes through the center of the sensitive volume. During the patient’s initial treatment, the ionization induced in the chamber is integrated with an electrometer. Following therapy, the treatment is repeated under identical conditions except that the patient is removed to generate an unattenuated chamber reading. The ratio of the two chamber readings is related to the effective attenuation coefficients. The effective attenuation coefficient values were determined by phantom measurements; since the chamber is not collimated, the effective attenua-
Accepted
to: Donald E. Velkley, Ph.D. 1739
for publication
23 July 1980.
Radiation Oncology 0 Biology 0 Physics
1740
tion coefficient depends on field size and source-tosurface distance (SSD). Values were tabulated for 80 and 100 cm SSD and for rotational treatments. For calculations of a specified dose at mid-plane, the percentage depth dose (PDD) is given by: PDD = PDD,X((SSD
+ &/(SSD
+ d,))’
(1)
December 1980, Volume 6. Number 12
nated polytetrafluoroethylene (PTFE)$ which were loaded into the phantom in drilled Mix-D plugs of 5 mm in diameter. The TLDs were calibrated in the linear accelerator beam; the results of multiple calibrations had a standard deviation of about 5% for a single measurement. Multiple exposures were made and averaged for measurements so that the relative uncertainty in any reported dose value was less than 2%.
where: d, - one half the physical thickness as measured with
body calipers d, - one half the effective thickness as determined by
transit dose measurements and PDD, - percentage depth dose for the depth d,. The inverse-square correction is necessary to account for the distance dependent component of the depth-dose curve. Since no information is obtained with this method as to the distribution of the inhomogeneity, in general the results are only correct for parallel-opposed treatment fields. The discussion here of the transit method will only consider central-axis dose calculations for parallelopposed fields. CT scanner and computer techniques
The CT scans discussed here werp all obtained with the same device.* The transparency of the image was enlarged to life size by a photographic enlarger and the desired structures were outlined on paper. Inhomogeneity corrections were made for aerated cavities and for bone. All other structures were treated as water-equivalent. This is a good approximation in the phantom used here which is cast from a uniform material around a skeleton with air cavities and simulated lungs. The density (p,) of an aerated cavity with an average CT number of CT, was determined as
P.3 =
1000 - CT, 1000
RESULTS The phantom was irradiated with the linear accelerator X ray beam using standard, parallel-opposed fields for whole brain with 14 cm x 16 cm fields at 80 cm SSD and for chest treatment with 10 cm x 10 cm fields at 80 cm SSD. In each case, physical measurements of the phantom thickness (d,) were used to calculate a desired tumor dose; during the irradiation transit measurements were made to determine the effective central-axis thickness (d,). TLDs were inserted during the irradiations to measure the dose delivered. Figure 1 is a contact radiograph of section 17 of the phantom through which the central axis of the treatment fields passed. Figure 2 shows the CT scan of the same section. Direct measurements have shown that there is distortion in the CT image. Enlargement of the CT image to match the AP phantom dimension left about a 1 cm discrepancy in the lateral dimension. This is evident in CT scans of round objects which appear elliptical and in a rectangular grid display which has obvious barrel distortion. Considerable effort in adjustment has improved but not removed all of the distortion. The image was magnified to match the dimension along the treatment axis for the cases discussed here which used parallel-opposed
(2)
since air has a CT number of - 1000 and water has a value of zero. This has been shown to be a good estimate from CT scans of cork blocks and other low density materials. Two independent computers were used for treatment planning and are denoted here as system A and system B.** Commercially available software was used for all the calculations reported here with these two computers. TLD measurements
TLD measurements were made for treatments in the head and chest of an anthropomorphic phantom.? The TLD devices were 1 mm x 6 mm rods of LiF impreg*Ohio Nuclear FS25, Ohio Nuclear Corp., Solon, Ohio. **System A: PC-12, Artronix B: Capintec Therarad. Capintec,
Corp., St. Louis, MO. System Inc., Montvale. N.J.
Fig. 1. Contact radiograph of section of anthropomorphic phantom chest through which the central-axis of the irradiation fields passed.
tRando Conn. STeRon.
Phantom,
Alderson
Research
Labs.,
Stamford,
Evaluation of CT 0 D. E. VELKLEY er al.
1741
PHANTOM PARALLEL 10x10
CHEST
OPPOSED FIFLDS @ 80cm SSD
GIVEN DOSE = 180
Fig. 2. CT scan of section of anthropomorphic radiographically in Figure 1.
phantom
shown
fields. For some multiple field and rotational planning the distortion could lead to inaccuracy. This demonstrates the importance of verifying the geometrical accuracy of the CT image if it is to be used for treatment planning. Figure 3 is a computer generated treatment plan of the chest irradiation with no inhomogeneity corrections and Figure 4 is the same plan with corrections applied for the lung density. In all cases the two computers gave dose distributions which generally agreed with each other to better than 2%. Table 1 gives the doses at mid-plane as determined from the treatment plans and from the transit measurements. The doses calculated from the physically measured dimensions and the average dose measured with TLD are also given. The dose based on physical measurement was about 20% less than the measured value. The dose determined by transit methods and the inhomogeneity-corrected treatment
PHANTOM PARALLEL 10x10
CHEST
DENSITY
= 100
LUNG
DENSITY
= 0 30
,/
---. Fig. 4. Same plan as shown in Figure 3 except that corrections were made for a lung density of 0.30 using the manufacturer supplied software.
computer dose agreed with each other to better than 2%: however, they were about 6% higher than the measured value. Batho has shown that the proper correction for 6oCo therapy depends not only on the inhomogeneity density and thickness but also on its distribution.’ This was not taken into account in the transit calculations described above and evidently not in the computer calculations either. In the Batho method as outlined by Young and Gaylord,* a correction to the depth dose is given by F = [ TAR(A,d,)/TAR(A,d,)]‘“-”
(3)
where TAR = tissue-air ratio ‘A = field area d, = distance from point to proximal aspect of inhomogeneity d, = distance from point to distal aspect of inhomogeneity and pI = electron density of inhomogeneity. This method was applied here for the transit calculations and since the measurement point was at the center of a mix-D plug which was imbedded in the lung, d, was
OPPOSED FIELDS @ 80cm SSD
GIVEN DOSE = 180 TISSUE
TISSUE
DENSITY
= 1.00
Table 1. Calculated and measured doses (rad) at mid-plane and on the central-axis for chest irradiation of the phantom Calculated Measured
Fig. 3. Result of treatment plan in chest done with computer.* All tissues were assumed to have unit density. *Artronix.
Based on dp
Based on d, and equation #l
200
256
CT = Computed dosimetry.
tomography;
CT scan computer 2.58 TLD
TLD 239
= Thermoluminescent
1742
Radiation Oncology 0 Biology 0 Physics
PHANTOM
December 1980, Volume 6, Number 12
HEAD
?LI 10x16 GIVEN TISSUE
Fig. 5. Treatment density. ?? Capintec.
approximated measurements value found
@I 80cm
SSD
DOSE = 100 RAD DENSITY
= 1.00
plan of whole-brain
irradiation
done with computer. * All tissues were assumed to have unit
as one centimeter, d2 was determined from of the CT scan and pc was taken as the from equation 2 above. Since scatter of
megavoltage photons is primarily in the forward direction, the error in dose calculated for the small mass imbedded in the lung by this method should be small even though there is also some reduction in lateral-and backscattered radiation. When the correction factor so deter-
PHANTOM
10x16 GIVEN TISSUE BONE
@ 80cm DOSE
mined was applied to the transit calculations in the chest, the calculated dose and measured dose agreed to within 1%. Figures 5 and 6 are the treatment plans for the brain irradiations and Table 2 is a comparison of the calculated and measured central-axis doses. Here, where the inhomogeneity is relatively thin and a relatively large distance from the measurement point, good agreement is obtained
HEAD
SSD
= 100 RAD
DENSITY DENSITY
= 1.00 = 1.67
,
Fig. 6. Same plan as shown in Figure 5 except corrections for bone density were made. The outline of the bone was taken from the CT scan.
Evaluation of CT 0 D. E. VELKLEYet al.
Table 2. Calculated and measured doses (rad) at mid-plane and on the central-axis for the irradiation of the brain of AP phantom Calculated Measured Based on dp 149
Based on d, and equation
142
#I
CT scan computer
138
TLD 140
by all the calculation methods with the measured dose value without need to apply the Batho correction method. The application of the Batho method assumes that the point of dose calculation is not within the inhomogeneity. For the calculaticns performed here for the lung case, the calculated dose is not that which the lung tissue would receive but the dose in unit density tissue behind the lung. Electronic equilibrium associated with soft tissue is lost in traversal of the lung and must be reestablished before the calculated dose represents the delivered value. Thus in the case described here the thickness of the plug is sufficient to reestablish electronic equilibrium and the calculated and measured doses are at a point near the center of the plug. At the interface between the plug and lung, the dose will differ by an amount which depends on the radiation energy.’ The detailed dosimetry at interfaces has not been represented by any of the computation methods discussed here but may be important for high energy radiation. These interface effects require more study before they can be used in routine calculation procedures. DISCUSSION The commercial computer systems used in this work have been found to make inhomogeneity corrections which are consistent with each other. These results are also in agreement with doses calculated from transit measurements when corrections are made using an effective attenuation method. In brain irradiation of the phantom, the calculated doses agree with TLD measurements to within 2%. In the case of lung irradiation a more sophisticated method of making inhomogeneity corrections is required as pointed out by previous authors.1s3,6 The Batho correction is easily applied manually in the transit calculations by using CT information to determine the correction parameters. With this method, the calculated central-axis dose agrees with the measured value to within 2% at mid-plane for parallel-opposed fields, whereas the discrepancy with the non-Bath0 corrected result is about 6%. The 6% error in the lung case is about the largest error to be expected for calculations which do not use the Batho method (discounting the
I743
interface effects discussed above). Numerous calculations have shown that transit dose measurements together with physical measurements and standard cross-sectional anatomy charts to estimate the inhomogeneity distribution for Batho calculations enable the error in calculated dose at midplane to be reduced from 6-10s to 2-3s in the case of lung. In regions away from lung in which the inhomogeneity corrections are smaller, the uncertainties in calculated dose are also smaller. A significant disadvantage of the transit dose method is that it only allows calculations of dose at isolated points. A treatment planning computer (system C*) has recently become commercially available; this system claims to use a Batho-type method for making inhomogeneity corrections. This computer was not available for our use in making comparisons. However, company personnel produced plans using patient data and radiation parameters for the treatment unit which we supplied. This was the same patient data described above which was used in the other computer calculations. The results of these computations using their standard software gave mid-plane doses which agreed with our measured doses to better than 2% even in the lung. In addition, software is under development for use with the system B computer with which inhomogeneity corrections are made using methods similar to that employed in the system C unit. Corporation development personnel made calculations with their new software using our data as input and generated treatment plans which generally agreed very well with the results from system C. As of this writing the improved system B software is not currently available but probably will be supplied in the near future to their users. It should be re-emphasized that in the lung the computer calculations agree with the measured dose in the center of a tissue equivalent plug. i.e., after tissue equilibrium is re-established. The effects at the lung-tissue (or lungtumor) interface are not described and may be clinically significant. As Geise and McCullough discussed, the ability of CT devices to locate internal structures accurately is more important than the detailed map of CT numbers or electron densities.2 The CT image is liable to have distortions which could negate the advantages of a CT unit. If the patient is not in the same position for the CT scan as for radiotherapy, the detailed localization of internal structures which are not in the correct positions could cause unacceptable errors in the radiotherapy treatment plan. In contrast, transit dosimetry measurements are obtained with the radiotherapy unit coincidentally with the treatment of the patient. The dosage calculations are amenable to any chosen correction method either manually or within a computer or programmable calculator. The transit method remains as a viable alternative to CT *System
C: TP-I 1, Atomic
Ottowa, Canada.
Energy
of Canada
Limited,
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Radiation Oncology 0 Biology 0 Physics
for the generation of inhomogeneity corrections for institutions without CT or for patients in which the cost of CT in time and money is unwarranted. The cost of the transit dosimetry system is small and it has been found to be capable of reliable results in a very busy clinical environment. The use of CT in radiotherapy treatment planning will probably become a routine tool in many institutions. As
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1980, Volume 6, Number
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noted above, most planning computer software must be updated to provide more accurate inhomogeneity corrections in some cases to take full advantage of the CT information. Even at the most sophisticated centers, a transit system should be considered as a dosimetry check to verify the correctness of a treatment plan by measurements of the patient with the treatment unit.
REFERENCES 1. Batho, H.F.: Lung corrections in Cobalt 60 beam therapy. J. assisted tomography in radiation therapy treatment 2.
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Can. Assoc. Radiol. 15: 79-83, 1964. Gcise, R.A., McCullough, E.C.: The use of CT scanners in megavoltage photon-beam therapy planning. Radiology 124: 133-141,1977. McDonald, S.C., Keller, B.E., Rubin, P.: Method for calculating dose when lung tissue lies in the treatment field. Med. Phys. k 210-216,1976. Prasad, SC., Glasgow, G.P., Purdy, J.A.: An operational computed tomography radiotherapy planning system (Abstr.). Med. Phys. 5: 339, 1978. Purdy, J.A., Velkley, D.E., Ter-Pogossian, M.: Computer
planning-Effect of inhomogeneities. Phys. Can. 32: 26.1, 1976. 6. Sontog. M.R., Battista, J.J., Bronskull, M.J., Cunningham, J.R.: Implications of computed tomography for inhomogeneity corrections in photon beam dose calculations. Radiology 124: 143-149,1977. 7. Wirt, J.: Transit dosimetry in determining tissue-equivalent thickness. App. Radiol. 6: 71-75, 1977. 8. Young, M.E.J., Gaylord, J.D.: Experimental tests of corrections for tissue inhomogeneities in radiotherapy. Br. J. Radiol. 43: 349-355. 1970.