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Absorbed dose measurements using parallel plate polystyrene ionization chambers in polystyrene phantoms
Int. .I. Radiation Oncology Rio/. Phys., Vol. 5, pp. 2031-2038 B Pergamon Press Ltd., 1979. Printed m the U.S.A.
0360-3016/79/l
12031-08$02.00/O
??Original Contribution ABSORBED DOSE MEASUREMENTS USING PARALLEL POLYSTYRENE IONIZATION CHAMBERS IN POLYSTYRENE
PLATE PHANTOMS
J.GARRETT HOLT, A.B., ALFONSO BUFFA,B.S.,DAVID J.PERRY,M.S., I-CHANG MA,M.S. andJosEpH C. McDoNALD,M.S. Department of Medical Physics, Memorial Hospital, 1275 York Avenue, New York, NY 10021 Ionometric methods are commonly used for the routine calibration of high energy photon and electron beams. Since the basis for all ionometric dose measurements for high energy radiation is the Bragg-Gray relation, the dosimeter and phantom used clearly must achieve close approximation to ideal Bragg-Gray conditions. A polystyrene parallel plate ionization chamber used by the authors in polystyrene phantoms is described which closely fulfills the Bragg-Gray conditions. Intercomparison of dose measurements made in a polystyrene phantom with this chamber and a tissue-equivalent (A-150) calorimeter are described for cobalt-60 gamma rays and 10 MV X-rays, as well as for 11 MeV electrons and 20 McV electrons. The agreement between the two methods was within 1.3% or better. From this it is concluded that the use of a polystyrene parallel plate ionization chamber in a polystyrene phantom provides an accurate, reliable and simple method for the calibration of high energy photon and electron beams. Dosimetry, Ionization chambers, Absorbed dose intercomparison..
Calibration
of high energy X-ray and electron beams,
INTRODUCTION
consistency in dosimetry methodology, problems such as the inconsistency between C, and Ch,8,17,*3 have become apparent; it is becoming recognized increasingly that these protocols are in need of revision. At this time, several scientific task groups here and abroad are actively working on the development of new protocols for the dosimetry of high energy electron and X-ray beams. In this work an ionometric method using a parallel plate polystyrene ionization chamber in a polystyrene phantom will be described; it avoids most of the difficulties encountered by the use of commercial thimble chambers in water phantoms as is recommended in all present protocols. Intercomparisons of dose measurements made with such a parallel plate (pancake) ionization chamber and a tissue-equivalent (A-150)21 calorimeter16 have been made and demonstrate very close agreement in the determination of dose to tissue for the photon and electron modalities investigated. The pancake chamber described in this paper was used for nearly a decade (before any of the high energy protocols were established) and the method has withstood the test of time. Since the basis for all ionometric dose measurements for high energy radiation is the Bragg-Gray
The widespread use of high energy accelerators capable of producing high energy electron and photon beams has emphasized the problems encountered in the calibration of these beams for application in radiation therapy. In the past, high energy electron and photon beams were used therapeutically only by a few major institutions which had adequate physics staffs capable of investigating the fundamental problems encountered in the dosimetry of high energy particles and photons. However, today high energy accelerators frequently are found in small institutions and community hospitals. It is clear that the Faraday cup, the calorimeter and the Fricke dosimeters, which provided the basis for absolute dosimetry, are not suitable for the everyday calibration of therapy machines. This was recognized in the development of the proto~01s~~~~~~“~‘ for ~~~” the dosimetry of high energy electron and photon beams, which were oriented toward the routine application in the field. All protocols recommended the use of ionometric methods of dosimetry using commercially available ionization chambers for the calibration of therapy machines. While these protocols served to establish
Dr. Steven M. Selzer of the National Bureau of Standards computed the electron stopping powers for A-150. We wish to acknowledge also the contribution of instrument maker Mr. Karl Pfaff.
Reprint requests to: J. Garrett Holt, A.B. Accepted for publication 21 August 1979. Acknowledgement-This work was supported in part by the Department of Energy Contract EY-76-S02-3522 and by National Cancer Institute (NCI) Grant CA 08748-12. 203 1
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November-December 1979, Vol. 5, No. 11 and No. 12
relation,6a22 the dosimeter that is used clearly must achieve close approximation to ideal Bragg-Gray conditions. A heterogeneous chamber constructed of different materials which do not match the phantom is subject to many uncertainties concerning the origin of the cavity ionization and the subsequent conversion of ionization to dose. Before discussing the characteristics of a pancake chamber, it is useful to consider some of the criteria which an ion chamber that is designed for the calibration of high energy therapy machines should meet. The following list of criteria is aimed at achieving Bragg-Gray cavity approximation as well as compliance with some practical considerations. Criteria for ion chambers for calibration machines
of therapy
1. Homogeneous construction; chamber walls and phantom of same material. 2. Negligible perturbation of particle and photon energy fluence. 3. Well-defined collection volume; collection electrode both guarded and electrostatically shielded. 4. Well-defined point of measurement. 5. Ability to measure at d,,, in both electron and photon beams. 6. High collection efficiency in pulsed beams. 7. Structural and electrical stability. 8. Permit accurate conversion of cavity ionization to dose in water or tissue. Spherical symmetry of response was not included in this list, because this is a requirement which is unachievable even by spherical ionization chambers. It is clear that any well designed ionization chamber must achieve a reasonable compromise of requirements. It is not possible to meet all the criteria listed above with a thimble or spherical ionization chamber design, such as those available commercially. It will be shown that the criteria can be met with a pancake chamber constructed of polystyrene with which measurements are to be made at d,,, in a polystyrene phantom. METHODS
AND MATERIALS
Description of pancake chamber The pancake chamber used is built into a 25x25x 1 cm slab of polystyrene, and is designed to be incorporated into a polystyrene phantom with the same lateral dimensions. The chamber is designed to make measurements in the phantom only and has the following characteristics. 1. Material: polystyrene (C,H&. Electron density: 3.238~ 10z5 electrons kg-‘. Physical density: 1.047~ lo3 kg rn-“.
Fig. 1. Pancake chamber embedded in the 25x25~ 1 cm slab of polystyrene. The top and bottom electrodes are separated by a distance of 2 mm and are firmly held in place by means of nylon screws. The nominal collection volume of the chamber is 1.0 cm3. The pancake chamber is designed to be incorporated into the polystyrene phantom.
2. 3. 4. 5.
Nominal volume: 1.0 cm3. Spacing: 2.0 mm. Point of measurement: upper electrode. Spatial resolution (depth): better than 1 mm. 6. Leakage current: less than 10 femtoamp. 7. Guard: collection electrode is effectively guarded and electrostatically shielded in all directions. 8. Electron and photon perturbation: negligible.
The electrical components of the chamber are terminated in a triax connector and the chamber is intended to be used with mini-noise triaxial cable. The pancake chamber is illustrated in Fig. 1 and a scaled drawing illustrating the connections and dimensions is shown in Fig. 2. The polystyrene surfaces are made conducting by means of a polished coat of graphite obtained from a colloidal suspension. The guard and collecting electrode are separated by means of a machined scratch which has a width of approximately l/l0 millimeter. The chamber is effectively shielded electrostatically in all directions by covering the external surface of the lower plate with a layer of conducting graphite which is also connected to the guard plate. This electrostatic shielding is essential to eliminate the effect on the collection electrode of the charge buildup in the polystyrene during particle irradiation. l* In addition,
Absorbed dose measurements
-
,,I”-
0 J.G.
HOLT ct cd
2033
Collector (25.23 @i Collector wire 0.08 Al H V wire 0: 08 A I lwiih Anon lnsulo/orl
Brass sleeve, soldered to connecfor guardring
Fig. 2. Pancake chamber. All materials are polystyrene except for the polished coating of graphite and the wire connections. All indicated dimensions are in millimeters. The small gap in the conducting surface of the lower plate separates the collection electrode from the guard. For electrostatic shielding the conducting surface coating is extended to exterior surfaces of the chamber.
the guard was extended to include the central pin of the triax connector, by slightly modifying the connector. The chamber normally is operated with a collection potential of +300 V which is usually adequate to assure a collection efficiency of >99.5%. The chamber may be operated with a collection potential of up to 900 volts which is the upper limit of the connector. The following section will present data to show that the pancake chamber described meets the criteria previously listed. Examination of the radiation properties of the pancake chamber In order to investigate the properties of the pancake chamber, an extrapolation chamber was constructed with identical physical dimensions as the pancake chamber described, except that the spacing between the electrodes could be varied by the insertion of spacer rings of different thickness. This permitted a precise determination of the space between the parallel electrodes, and the thickness of the spacer rings was ascertained by means of a micrometer measurement before and after each radiation measurement. The cylindrical air gap produced in the phantom when the electrode spacing is decreased is filled in with a machined circular disk. Figure 3 shows the extrapolation chamber together with the spacers and disks. Radiation measurements made with the extrapolation chamber with different thickness of spacers inserted between the electrodes permitted the extrapolation of charge per unit volume, Q/V, to zero volume for equal exposures. The results of these measurements are shown in Fig. 4. It is clear from Fig. 4 that the cavity ionization obtained with a 2 mm spacing is indistinguishable from the extrapolated value of the cavity ionization for zero spacing (zero volume). For these measurements, the upper electrode was always kept at the
same depth in the phantom as well as the same distance from the source. This experiment demon-
strated that there was no appreciable perturbation of the photon energy fluence for cobalt-60 gamma rays. Hence, extrapolation of Q/V to zero volume is not necessary for the pancake chamber when its point of measurement is the upper electrode. A circular hole was cut into a sheet of polystyrene of 2 mm thickness with the same radial dimensions (collector plus guard) as the pancake chamber, in order to examine the perturbation of the electron fluence by the cavity of the pancake chamber. The upper boundary of this cylindrical cavity was placed at a depth of 2.5 cm in a polystyrene phantom and a piece of film was inserted in the phantom at the lower boundary of this cylindrical disk cavity. The test phantom and film subsequently was exposed to 11 MeV electrons. The developed film was scanned with a microdensitometer. The results of this experiment are shown in Fig. 5. As was expected there was a circular zone of decreased density extending a few millimeters radially outside the boundary of the cavity, and a zone of increased density extending about 3 mm towards the center from the radial boundary of the cavity. The perturbation was of the order of +2% and did not extend inside the cavity beyond the area of the guard. Therefore it was demonstrated that the guard electrode successfully prevents any perturbation of the electron fluence within the collection volume. The collection volume was measured mechanically, electrostatically and radiologically. Before the assembly, the mean collection area and the separation between the collector and guard were measured with a traveling microscope and the plate separation was measured with a micrometer. After assembly the plate separation was re-computed from a capacitance measurement. The radiation volume was computed
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Radiation Oncology 0 Biology 0 Physics
November-December 1979, Vol. 5, No. 11 and No. 12
Fig. 3. Extrapolation chamber with ring spacers and disk inserts that has identical dimensions as the pancake chamber and may be embedded in the polystyrene slab. Separation between electrodes can be varied by the substitution of different ring spacers. Disks are used to fill in any air gaps created by the variable spacing.
Extrapolation 6oCo
Pancake
Measurements
Chamber
in Polystyrene
1.02
c
Q v.
(Q/V)o
/J
5
1.01 -
A A
A
Perturbation Effect at the Edge of Thin Disk Cavity I‘
1.00 t
I
I
1 PLATE
I
2 SEPgRATION
I
I
3
4
I I I
Edge of Cavity
(mm.)
Fig. 4. The cavity ionization vs plate separation to permit the extrapolation to the value of Q/V for zero spacing (volume). Within the scatter of the measurement points, the value of the collected charge per unit chamber volume, Q/V, for a plate separation of 2 mm is indistinguishable from the extrapolated value of (Q/V),. This demonstrates that for the pancake chamber the extrapolation to zero volume is not required. From this it may be concluded also that there is no significant perturbation of the photon fluence for 6oCo gamma rays.
-I
I
I
-
Collector
‘*O*
, 00
0.98
*
Plate
Fig. 5. A film density scan across the lower boundary of a thin disk cavity of same dimension as the pancake chamber. Film was taken with 11 MeV electrons at d,,, in polystyrene. Perturbation inside the cavity is of the order of 2% and does not extend radially beyond 60% of the dimension of the guard. Electron fluence within the collection volume is not perturbed.
Absorbed dose measurements
2035
0 J.G. HOLT if al.
TMR Measurements with Extrapolation Pancake Chamber in Polystyrene
TMR
Measurements Pancake
5mm spacer 0 /mm spacer
oo +I++--+
IO0
??
I
II MeV Electrons
09
Extrapolation
in Polystyrene
D
. D
90
80
with
Chamber
8
80
??
5mm
spacer
0
I mm
spacer
-:-
Intersection of 5 and Imm points
II MeV
Electrons
:
S .‘5 .s
70
70
60
.g ‘, .N -5 E G k a
_o T;
50
S j)
40
60 50 40 30
30
1
20
20
a
10
a
10 I
I
I
I
I
I
IO
20
30
40
50
60
Depth to geometrical
center
(mm in Polystyrene)
Fig. 6. Tissue-maximum-ratios (TMR) for 11 MeV electrons measured with the extrapolation chamber in polystyrene. The measurements were made with 1 and 5 mm spacing between the plates. The collection voltages were 2300 V for the 1 mm spacing and ?700 V for the 5 mm spacing. The points represent the average of the reading taken with negative and positive polarity. The data are plotted vs depth to the geometrical center of the chamber. The two curves are separated by about 112 the difference
in electrode spacing between the two sets of measurements. from the exposure sensitivity of the chamber in a calibrated Co-60 beam. The mechanical and electrical volume agreed within 2/10 of l%, and the radiation volume, V,, was about 1% greater. This comparison in effect constitutes an independent confirmation of the National Bureau of Standards (NBS) calibration of our transfer standard. The good agreement of the pancake chamber volume as determined by the three methods indicates that the collection volume is well defined. For consistency with NBS the radiation volume, V,, is used for all measurements of dose. The method of determining V, is described in detail in a subsequent section. The integrity of the electrostatic shielding was verified electrostatically by bringing a charged rod in
I
I
I
I
10
20
30
40
I 50
I
60
Depth to upper electrode (mm
in Polystyrene)
Fig. 7. The same data as shown in Fig. 6 re-plotted vs depth to the upper electrode. The two curves have been brought into near coincidence, which is exact at d,,,. This demonstrates that the upper electrode can be designated as the location of the point of measurement.
very close proximity to the outer surfaces of the chamber. No effect could be observed with an electrometer connected to the chamber. To verify that the upper electrode is the point of tissue-maximum ratios measurement at d,,,, (TMR’s) were taken in polystyrene with the extrapolation chamber at electron energies of 11 and 20 MeV. Figure 6 illustrates the TMR’s in polystyrene taken at 11 MeV for electrode spacings of 1 and 5 mm. The data are plotted against the depth defined by the geometrical center of the chamber. Although the difference in charge collected was only of the order of a few l/l0 of 1% for each point, the average of positive and negative polarity readings were taken. The resulting two TMR curves are clearly separated by the difference in spacing between the two measurements. Figure 7 displays the same data re-plotted with the upper electrode defining the depth of measurement. It can be seen that the two curves have been brought into close coincidence, which is exact at
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Radiation Oncology 0 Biology 0 Physics
and diverges somewhat at mid-range. It is clear from Fig. 7 that the uncertainty in the point of measurement is less than 1 mm, provided the upper electrode is taken as the point of measurement. A similar experiment performed at 20 MeV produced very similar results. From these two experiments it can be concluded that the point of measurement for electrons of the pancake chamber is well established at the inner conducting surface of the upper elec-
d max
trode .
The collection efficiency of the pancake chamber was measured for cobalt-60 gamma rays, 11 MeV electrons and 20 MeV electrons, according to the methods outlined by Holt et ~1.~ With a polarization potential of +300 V, the collection efficiency for 11 MeV electrons at 0.005 Gyisecond (300 rad/min) was 99.6%, at 20 MeV and 0.015 Gyisecond (90 rad/min), the collection efficiency was 99.8%, and for cobalt-60 gamma rays, at a rate of 0.03 Gyisecond (200 radimin), the collection efficiency was 99.9%. This was considered adequate for most applications. However, it is possible to triple the electric field strength for special applications if necessary. The accuracy of the conversion of cavity ionization to absorbed dose in tissue was tested against a tissue-equivalent absorbed dose microcalorimeter and will be described in the following section. Absorbed dose intercomparison The absorbed dose to tissue as computed from an ionometric measurement in a polystyrene phantom using the pancake chamber described was compared to the dose at the same depth in tissue as computed from a calorimetric measurement using a tissueequivalent (A-150) calorimeter. The radiation modalities used in the intercomparison were cobalt-60 gamma rays, 10 MV x-rays, 11 MeV electrons and 20 MeV electrons. In each case, the center of the wafer of the calorimeter was placed at an equivalent depth in polystyrene expressed in number of electrons cm-2 at the upper electrode of the pancake chamber. At each measurement a 10x10 cm field was used in the plane of the point of measurement. Measurements were made at a depth of maximum ionization except for cobalt-60 gamma rays, where the measurements were made at a depth of 5.5 cm in polystyrene (1.865~ 10z4 electrons cmP2). The calorimeter was also placed in the polystyrene phantom. Since A-150 is tissue-equivalent only for cobalt-60 gamma rays, the dose to tissue was computed from the dose to A-150 using the appropriate mass energy absorption coefficients or stopping power ratios. The pancake chamber was initially calibrated in a cobalt-60 therapy beam using a 10x 10 cm field at 80 cm source-axis distance (SAD). The free-air expo-
November-December
1979, Vol. 5, No. 11 and No. 12
sure was first calibrated using a National Bureau of Standards (NBS) calibrated Shonka-Wyckoff chamber. The pancake chamber embedded at d,,, in the polystyrene phantom was then placed in the same field with upper electrodes positioned at 80 cm SAD. The charge collected (Q) for an exposure to free air at the same point (XJ was determined. From the quotient of charge to exposure (Q/Xd, either the Volume, V,, of the pancake chamber or the air mass contained by the volume (pV,) was computed according to eq. (1).
x,.--w .(LL’“pY e
.$!.($olY-
0
.A,,,.B-
p au, Co
p(T,P)V,
e
P air, E(Co)
(1)
where X, is the exposure in free air obtained with a 10x 10 cm field at 80 cm SAD. _w LSthe average energy expended per unit charge of ionization in e air. (cL,“)PplY
p
is the quotient of the mass energy absorption coefficient of polystyrene to air averaged over the Co-60 gamma ray spectrum.
au, Co
is a constant relating collision air kerma to the dose within a small mass of polystyrene of just sufficient extent to provide maximum electronic buildup in air. The value of A,,, was determined by us for polystyrene or water as 0.990.’ A,, appears in eq. (1) because of the exposure measurement in air. B is the backscatter size.
factor in polystyrene
for a 1OXlOcm field
Q is the collected charge. p(T,P) is the density of air in the chamber volume at temperature
T
and pressure P (e.g., p(T,P) =p(To,P,,) . 2. $)). V,. is the collection volume of the pancake chamber which is the quantity computed by means of eq. (1). (3) PolYp au, E(Co)
is the quotient of the collision mass stopping powers of polystyrene to air averaged over the electron fluence entering the cavity.
The dose to muscle was calculated from measurements made with the pancake chamber in the polystyrene phantom according to eqs. (2) and (3). _ Photons:Dmu&,h
= OS!. V,. p(T,P)
e
(S)Ppfy p au,E (X)
Electrons:Dmusc~e.E=~.~. (&)potY V,.. p(T,P) e p air,E(Z)
(&)muscle (2) P POlY, A
(3) musck p
poly,E(Z)
(3)
where
Q ‘c
is the cavity mass ionization. The calibrated volume,
dTyP) V,. is obtained from a Co-60 calibration and eq. (1).
(E) P$Y P
rur,E(z)
is the quotient of the restricted collision mass stopping power ratios of polystyrene to air averaged over the electron energy fluence at the depth of measurement (Z).” The minimum cut-off energy (a = 15 keV) has been omitted in this notation.
Absorbed dose measurements
Table 1. Physical constants
(_S, muscle
is the quotient of the collision mass stopping powers P ~oly,WZ) of muscle to polystyrene averaged over the electron energy fluence spectrum at the depth of the measurement (Z).
_ (&)muscle
p poly k~~the quotient of the mass energy absorption coeffi’ clents of muscle to polystyrene averaged over the photon energy spectrum for photon beam energy A.
If the appropriate SI units are used in eqs. (l), (2) and (3), the dose will be given in Gy (e.g., exposure as 2.58x
2037
0 J.G. HOLT ttal.
lo-” C Kg-’ R-l, Tz33.7
J C-l and - Q V,.P
inC
Kg-‘). Equations (2) and (3) assume that there is no perturbation of the electron or photon fluence resulting from the cavity or the chamber material, which is the case for the pancake chamber, providing the upper electrode is taken as the point of measurement. The appropriate values of the parameters used in eqs. (l), (2) and (3), as well as the electron density and stopping powers used for the calorimeter measurements, are summarized in Table 1. RESULTS
Table 2 lists the results of the absorbed dose intercomparison for the four radiation modalities investigated. As can be seen, the agreement between the two methods is excellent. The precision of the ionometric measurements was within 0.2%, but the uncertainty in the measurement is difficult to estimate. NBS states the absolute uncertainty in the exposure rate calibration as ?2.0%, although Loftus and WeaverI have reported an uncertainty of less than + 1%. The uncertainties in the ratios of the stopping powers and absorption coefficient are not too well known since these calculations depend on the electron and photon spectra at the point of measurement. The uncertainty of +2.5% assigned to the ionometric measurement represents a conservative estimate. As for the calorimeter, the greatest uncertainty encountered is the determination of the thermal defect. The endothermic radiochemical effects in tissue equivalent plastics were initially described theoretically and measured experimentally in our laboratory. a1 A value of (4 22%) has been employed for the thermal defect in these experiments. Nevertheless, the uncertainty in the estimate of the experimental errors should not detract from the good agreement obtained in the determination of the absorbed dose to tissue by the two methods. It can be concluded that the calibration of the high energy therapy beam (at least for the four modalities tested) by means of a pancake chamber embedded in the polystyrene phantom has been demonstrated to be as accurate a method as calorimetry at present.
used for computations A-150
Polystyrene
1.125 x 10SKg rnmR 3.307 X 109W mm”
1.047 x 103Kg m-” 3.238 x 102% m-:j
P PC
B(OS,lOx 10) A
1.036
X,
=
0.990
*le
= 33.7 J C’ 10 MVX
““Co
_
‘1
(1L,e)POlY
2.58 x 10mxC Kg-’ R-’
11 MeV
1.078
t
(j&muscle 11 P P0lY.h
1.019
1.030
+
&&)muscle 11 p A-150-r
1.001
1.010
t
1.121
1.092
:
p
air,h
18, 2
(3) pplr P air,(E)
(L)PplY 4 p alr,E (Z) (3) muscle p poly,E(Z)
I
1.0148
0.953
I
t
1.0252
1.0232
t
:
1.005
1.004
I
1
18,3
(3) muscle_ $ p A-lSO,E(Z)
20 MeV
The sources used for the computations of the constants are indicated by the Reference numbers. tNot applicable. *Personal communication, S.M. Seltzer, August, 1978. rable 2. Comparison
of pancake chamber with calorimeter dose to muscle (Gy)
Pancake
Pancake chamber
chamber+ Co-60 10 MVX 11 MeV 20 MeV
3.897 2.961 3.030 1.649
? * * +
Calorimeter*
2.5% 2.5% 2.5% 2.5%
3.867 2.999 2.9% 1.665
” & ? ?
2.5% 2.5% 2.5% 2.5%
Calorimeter 1.008 0.987 1.011 0.990
iMeasured in polystyrene and computed to muscle. *Measured in A-150 plastic and computed to muscle. DISCUSSION
It has been demonstrated that when the pancake chambe; is used in a polystyrene phantom to calibrate high energy therapy machines, it exhibits very close Bragg-Gray cavity behavior, and fulfills the criteria listed. The method described is capable of yielding results of sufficient accuracy for clinical application. The radiation calibration of the pancake chamber has the additional advantage of being traceable to NBS. While the constants used in equations (l), (2) and (3) may be adjusted slightly as better data become available, it is this straightforward method that is recommended.
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Radiation Oncology 0 Biology 0 Physics
The use of a pancake-type chamber, homogeneously constructed in a phantom of the same material, inherently avoids many of the difficulties to which thimble or spherical ionization chambers are subjected, especially when they are used in water phantoms. The main objection that the equivalent depth in tissue is not accurately established when a measurement is made in a solid phantom can be met successfully by making the measurement at the depth of maximum ionization in the solid phantom and computing the dose to tissue at its characteristic depth of maximum ionization. However, this requirement is not rigid, since it has been shown recently that an equivalent depth in phantom of different composition can be scaled.5 The depth of maximum ionization is the depth to which all isodoses are normalized, and also the depth where the dose gradient is at a minimum. Making a measurement at d,,, has the advantage that any uncertainty in the point of measurement has a negligible
November-December 1979, Vol. 5, No. 11 and No. 12
effect on the measurement. Aside from the cost, polystyrene has much to recommend it as a material for phantom measurements. It is nearly water- or tissue-equivalent since the electron area1 densities are very close; (P’Pe)PolYwater= 1.016. In addition, making a measurement in a solid phantom rather than water avoids the necessity of waterproofing the ion chamber, and permits the use of a homogeneous chamber and phantom combination. The depth of measurement is easily established and any set-up is exactly reproducible. Ten years experience in the calibration of therapy machines has established this method to be accurate, practical, and easy to use for the routine calibration of therapy equipment. The consistent accuracy and simplicity of this method make it quite suitable for general calibrations of high energy therapy beams, and we recommend this method for general use.
REFERENCES 1. American Association of Physicists in Medicine: Protocol for the dosimetry of X- and gamma-ray beams with maximum energies between 0.6 and 50 MeV. (SCRAD) Phys. Med. Biol. 16: 379, 1971. 2. Berger, M.J. and Seltzer, S.M.: Table of energy losses and ranges of electrons and positrons. NASA-N65 12506. National Technical Information Service, U. S. Department of Commerce. 1964, pp. 1-127. 3. Berger, M.J. and Seltzer, S.M.: Additional stopping power and range tables for protons, mesons and electrons. N67-14099, 37038. National Technical Information Service, U.S. Department of Commerce. 1966. 4. Berger, M.J., Seltzer, S.M., Domen, S.R. and Lampower ratios for electron perti, P.J.: Stopping dosimetry with ionization chambers. In Biomedical Dosimetty, IAEA, SM-193139. 1975, pp. 589-609. 5. Casson, H.: Correction of measurements in plastic phantoms to obtain dose in water. Med. Phys. 5: 321, 1978. 6. Gray, L.H.: An ionization method for the absolute dosimetry of gamma-ray energy. Proc. R. Sot. A156, 1936, p. 578. 7. Holt, J.G., Fleischman, R.C., Perry, D.J. and Buffa, A.: An examination of the factors A, and A,,, for cylindrical ion chambers used in cobalt-60 beams. (To be published in Medical Physics, July/August, 1979.) 8. Holt, J.G. and Kessaris, N.D.: Discrepancy between CA and C,. Phys. Med. Biol. 22: 538, 1977. 9. Holt, J.G., Stanton, R.E. and Sell, R.E.: Ionization collection efftciencies of some ionization chambers in pulsed and continuous radiation beams. Med. Phys. 5(2): 107-l 10, 1978. 10. Hospital Physicists Association: A code of practice for dosimetry of 2 to 35 MV X-ray and caesium-137 and cobalt-60 gamma-ray beams. Phys. Med. Biol. 14: 1, 1969. 11. Hubbell, J.H.: Photon mass attenuation and mass energy-absorption coefftcients for H, C, N, 0, Ar, and Seven Mixtures from 0.1 keV to 20 MeV. Rad. Res. 70: 58-81.
1977.
12. International Commission on Radiation Units and Measurements: Radiation Dosimetry: X-ray and gamma rays with maximum photon energies between 0.6 and 50 MeV. Report 14, Washington, DC, l-30, 1969. 13. International Commission on Radiation Units and Measurements: Radiation Dosimetry: Electrons with initial energies between 1 and 50 MeV. Report 21, Washington, DC, l-64, 1972. 14. Kessaris, N.D.: Penetration of high energy electron beams in water. Phys. Rev. 145: 164, 1966. 15. Loftus, T.T. and Weaver, J.T.: Standardization of cobalt-60 and cesium-137 gamma ray beams in terms of exposure. J. of Research of National Bureau of Standards, 4(78A): 465, 1974. 16. McDonald, J.C., Laughlin,
Portable
tissue-equivalent
80, 1976. 17. Nahum, A.E.
J.S. and Freeman, R.E.: calorimeter. Med. Phys. 3:
and Greening, on the discrepancy between
J.R.: Further comments CA and CE. Phys. Med.
Biol. 22: 1020, 1977. 18. Nelms, A.T.: Graphs of the Compton
energy-angle relationship and the Klein-Nishina formula from 10 keV to 500 MeV. National Bureau of Standards Circular 542, Superintendent of Documents, U.S. Government Printing Office, Washington, DC, 1953, p. 89. 19. Nordic Association of Clinical Physics: Procedures in radiation therapy dosimetry with 50 MeV electrons and roentgen and gamma rays with maximum photon energies between 1 and 50 MeV. Acta Radiol. (Ther.) 11: 603, 1972. 20. Pinkerton, A.P.: Comparison of calorimetric and other methods for the determination of absorbed dose. Ann. New York Acad. Sci. 161: 63, 1969. 21. Smathers, J.B., Otte, V.A., Smith,
A.R., Almond, P.R., Attix, F.H., Spokas, J.J., Quam, W.M. and Composition of A-150 tissueGoodman, L.J.: equivalent plastic. Med. Phys. 4: 74, 1977. 22. Spencer, L.V. and Attix, F.H.: A theory of cavity ionization. Rad. Res. 3: 239, 1955. 23. Williams, P.C.: Discrepancy between CA and C,. Phys. Med. Biol. 22: 535. 1977.
0360-3016/79/l
12031-08$02.00/O
??Original Contribution ABSORBED DOSE MEASUREMENTS USING PARALLEL POLYSTYRENE IONIZATION CHAMBERS IN POLYSTYRENE
PLATE PHANTOMS
J.GARRETT HOLT, A.B., ALFONSO BUFFA,B.S.,DAVID J.PERRY,M.S., I-CHANG MA,M.S. andJosEpH C. McDoNALD,M.S. Department of Medical Physics, Memorial Hospital, 1275 York Avenue, New York, NY 10021 Ionometric methods are commonly used for the routine calibration of high energy photon and electron beams. Since the basis for all ionometric dose measurements for high energy radiation is the Bragg-Gray relation, the dosimeter and phantom used clearly must achieve close approximation to ideal Bragg-Gray conditions. A polystyrene parallel plate ionization chamber used by the authors in polystyrene phantoms is described which closely fulfills the Bragg-Gray conditions. Intercomparison of dose measurements made in a polystyrene phantom with this chamber and a tissue-equivalent (A-150) calorimeter are described for cobalt-60 gamma rays and 10 MV X-rays, as well as for 11 MeV electrons and 20 McV electrons. The agreement between the two methods was within 1.3% or better. From this it is concluded that the use of a polystyrene parallel plate ionization chamber in a polystyrene phantom provides an accurate, reliable and simple method for the calibration of high energy photon and electron beams. Dosimetry, Ionization chambers, Absorbed dose intercomparison..
Calibration
of high energy X-ray and electron beams,
INTRODUCTION
consistency in dosimetry methodology, problems such as the inconsistency between C, and Ch,8,17,*3 have become apparent; it is becoming recognized increasingly that these protocols are in need of revision. At this time, several scientific task groups here and abroad are actively working on the development of new protocols for the dosimetry of high energy electron and X-ray beams. In this work an ionometric method using a parallel plate polystyrene ionization chamber in a polystyrene phantom will be described; it avoids most of the difficulties encountered by the use of commercial thimble chambers in water phantoms as is recommended in all present protocols. Intercomparisons of dose measurements made with such a parallel plate (pancake) ionization chamber and a tissue-equivalent (A-150)21 calorimeter16 have been made and demonstrate very close agreement in the determination of dose to tissue for the photon and electron modalities investigated. The pancake chamber described in this paper was used for nearly a decade (before any of the high energy protocols were established) and the method has withstood the test of time. Since the basis for all ionometric dose measurements for high energy radiation is the Bragg-Gray
The widespread use of high energy accelerators capable of producing high energy electron and photon beams has emphasized the problems encountered in the calibration of these beams for application in radiation therapy. In the past, high energy electron and photon beams were used therapeutically only by a few major institutions which had adequate physics staffs capable of investigating the fundamental problems encountered in the dosimetry of high energy particles and photons. However, today high energy accelerators frequently are found in small institutions and community hospitals. It is clear that the Faraday cup, the calorimeter and the Fricke dosimeters, which provided the basis for absolute dosimetry, are not suitable for the everyday calibration of therapy machines. This was recognized in the development of the proto~01s~~~~~~“~‘ for ~~~” the dosimetry of high energy electron and photon beams, which were oriented toward the routine application in the field. All protocols recommended the use of ionometric methods of dosimetry using commercially available ionization chambers for the calibration of therapy machines. While these protocols served to establish
Dr. Steven M. Selzer of the National Bureau of Standards computed the electron stopping powers for A-150. We wish to acknowledge also the contribution of instrument maker Mr. Karl Pfaff.
Reprint requests to: J. Garrett Holt, A.B. Accepted for publication 21 August 1979. Acknowledgement-This work was supported in part by the Department of Energy Contract EY-76-S02-3522 and by National Cancer Institute (NCI) Grant CA 08748-12. 203 1
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Radiation Oncology 0 Biology ??Physics
November-December 1979, Vol. 5, No. 11 and No. 12
relation,6a22 the dosimeter that is used clearly must achieve close approximation to ideal Bragg-Gray conditions. A heterogeneous chamber constructed of different materials which do not match the phantom is subject to many uncertainties concerning the origin of the cavity ionization and the subsequent conversion of ionization to dose. Before discussing the characteristics of a pancake chamber, it is useful to consider some of the criteria which an ion chamber that is designed for the calibration of high energy therapy machines should meet. The following list of criteria is aimed at achieving Bragg-Gray cavity approximation as well as compliance with some practical considerations. Criteria for ion chambers for calibration machines
of therapy
1. Homogeneous construction; chamber walls and phantom of same material. 2. Negligible perturbation of particle and photon energy fluence. 3. Well-defined collection volume; collection electrode both guarded and electrostatically shielded. 4. Well-defined point of measurement. 5. Ability to measure at d,,, in both electron and photon beams. 6. High collection efficiency in pulsed beams. 7. Structural and electrical stability. 8. Permit accurate conversion of cavity ionization to dose in water or tissue. Spherical symmetry of response was not included in this list, because this is a requirement which is unachievable even by spherical ionization chambers. It is clear that any well designed ionization chamber must achieve a reasonable compromise of requirements. It is not possible to meet all the criteria listed above with a thimble or spherical ionization chamber design, such as those available commercially. It will be shown that the criteria can be met with a pancake chamber constructed of polystyrene with which measurements are to be made at d,,, in a polystyrene phantom. METHODS
AND MATERIALS
Description of pancake chamber The pancake chamber used is built into a 25x25x 1 cm slab of polystyrene, and is designed to be incorporated into a polystyrene phantom with the same lateral dimensions. The chamber is designed to make measurements in the phantom only and has the following characteristics. 1. Material: polystyrene (C,H&. Electron density: 3.238~ 10z5 electrons kg-‘. Physical density: 1.047~ lo3 kg rn-“.
Fig. 1. Pancake chamber embedded in the 25x25~ 1 cm slab of polystyrene. The top and bottom electrodes are separated by a distance of 2 mm and are firmly held in place by means of nylon screws. The nominal collection volume of the chamber is 1.0 cm3. The pancake chamber is designed to be incorporated into the polystyrene phantom.
2. 3. 4. 5.
Nominal volume: 1.0 cm3. Spacing: 2.0 mm. Point of measurement: upper electrode. Spatial resolution (depth): better than 1 mm. 6. Leakage current: less than 10 femtoamp. 7. Guard: collection electrode is effectively guarded and electrostatically shielded in all directions. 8. Electron and photon perturbation: negligible.
The electrical components of the chamber are terminated in a triax connector and the chamber is intended to be used with mini-noise triaxial cable. The pancake chamber is illustrated in Fig. 1 and a scaled drawing illustrating the connections and dimensions is shown in Fig. 2. The polystyrene surfaces are made conducting by means of a polished coat of graphite obtained from a colloidal suspension. The guard and collecting electrode are separated by means of a machined scratch which has a width of approximately l/l0 millimeter. The chamber is effectively shielded electrostatically in all directions by covering the external surface of the lower plate with a layer of conducting graphite which is also connected to the guard plate. This electrostatic shielding is essential to eliminate the effect on the collection electrode of the charge buildup in the polystyrene during particle irradiation. l* In addition,
Absorbed dose measurements
-
,,I”-
0 J.G.
HOLT ct cd
2033
Collector (25.23 @i Collector wire 0.08 Al H V wire 0: 08 A I lwiih Anon lnsulo/orl
Brass sleeve, soldered to connecfor guardring
Fig. 2. Pancake chamber. All materials are polystyrene except for the polished coating of graphite and the wire connections. All indicated dimensions are in millimeters. The small gap in the conducting surface of the lower plate separates the collection electrode from the guard. For electrostatic shielding the conducting surface coating is extended to exterior surfaces of the chamber.
the guard was extended to include the central pin of the triax connector, by slightly modifying the connector. The chamber normally is operated with a collection potential of +300 V which is usually adequate to assure a collection efficiency of >99.5%. The chamber may be operated with a collection potential of up to 900 volts which is the upper limit of the connector. The following section will present data to show that the pancake chamber described meets the criteria previously listed. Examination of the radiation properties of the pancake chamber In order to investigate the properties of the pancake chamber, an extrapolation chamber was constructed with identical physical dimensions as the pancake chamber described, except that the spacing between the electrodes could be varied by the insertion of spacer rings of different thickness. This permitted a precise determination of the space between the parallel electrodes, and the thickness of the spacer rings was ascertained by means of a micrometer measurement before and after each radiation measurement. The cylindrical air gap produced in the phantom when the electrode spacing is decreased is filled in with a machined circular disk. Figure 3 shows the extrapolation chamber together with the spacers and disks. Radiation measurements made with the extrapolation chamber with different thickness of spacers inserted between the electrodes permitted the extrapolation of charge per unit volume, Q/V, to zero volume for equal exposures. The results of these measurements are shown in Fig. 4. It is clear from Fig. 4 that the cavity ionization obtained with a 2 mm spacing is indistinguishable from the extrapolated value of the cavity ionization for zero spacing (zero volume). For these measurements, the upper electrode was always kept at the
same depth in the phantom as well as the same distance from the source. This experiment demon-
strated that there was no appreciable perturbation of the photon energy fluence for cobalt-60 gamma rays. Hence, extrapolation of Q/V to zero volume is not necessary for the pancake chamber when its point of measurement is the upper electrode. A circular hole was cut into a sheet of polystyrene of 2 mm thickness with the same radial dimensions (collector plus guard) as the pancake chamber, in order to examine the perturbation of the electron fluence by the cavity of the pancake chamber. The upper boundary of this cylindrical cavity was placed at a depth of 2.5 cm in a polystyrene phantom and a piece of film was inserted in the phantom at the lower boundary of this cylindrical disk cavity. The test phantom and film subsequently was exposed to 11 MeV electrons. The developed film was scanned with a microdensitometer. The results of this experiment are shown in Fig. 5. As was expected there was a circular zone of decreased density extending a few millimeters radially outside the boundary of the cavity, and a zone of increased density extending about 3 mm towards the center from the radial boundary of the cavity. The perturbation was of the order of +2% and did not extend inside the cavity beyond the area of the guard. Therefore it was demonstrated that the guard electrode successfully prevents any perturbation of the electron fluence within the collection volume. The collection volume was measured mechanically, electrostatically and radiologically. Before the assembly, the mean collection area and the separation between the collector and guard were measured with a traveling microscope and the plate separation was measured with a micrometer. After assembly the plate separation was re-computed from a capacitance measurement. The radiation volume was computed
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Radiation Oncology 0 Biology 0 Physics
November-December 1979, Vol. 5, No. 11 and No. 12
Fig. 3. Extrapolation chamber with ring spacers and disk inserts that has identical dimensions as the pancake chamber and may be embedded in the polystyrene slab. Separation between electrodes can be varied by the substitution of different ring spacers. Disks are used to fill in any air gaps created by the variable spacing.
Extrapolation 6oCo
Pancake
Measurements
Chamber
in Polystyrene
1.02
c
Q v.
(Q/V)o
/J
5
1.01 -
A A
A
Perturbation Effect at the Edge of Thin Disk Cavity I‘
1.00 t
I
I
1 PLATE
I
2 SEPgRATION
I
I
3
4
I I I
Edge of Cavity
(mm.)
Fig. 4. The cavity ionization vs plate separation to permit the extrapolation to the value of Q/V for zero spacing (volume). Within the scatter of the measurement points, the value of the collected charge per unit chamber volume, Q/V, for a plate separation of 2 mm is indistinguishable from the extrapolated value of (Q/V),. This demonstrates that for the pancake chamber the extrapolation to zero volume is not required. From this it may be concluded also that there is no significant perturbation of the photon fluence for 6oCo gamma rays.
-I
I
I
-
Collector
‘*O*
, 00
0.98
*
Plate
Fig. 5. A film density scan across the lower boundary of a thin disk cavity of same dimension as the pancake chamber. Film was taken with 11 MeV electrons at d,,, in polystyrene. Perturbation inside the cavity is of the order of 2% and does not extend radially beyond 60% of the dimension of the guard. Electron fluence within the collection volume is not perturbed.
Absorbed dose measurements
2035
0 J.G. HOLT if al.
TMR Measurements with Extrapolation Pancake Chamber in Polystyrene
TMR
Measurements Pancake
5mm spacer 0 /mm spacer
oo +I++--+
IO0
??
I
II MeV Electrons
09
Extrapolation
in Polystyrene
D
. D
90
80
with
Chamber
8
80
??
5mm
spacer
0
I mm
spacer
-:-
Intersection of 5 and Imm points
II MeV
Electrons
:
S .‘5 .s
70
70
60
.g ‘, .N -5 E G k a
_o T;
50
S j)
40
60 50 40 30
30
1
20
20
a
10
a
10 I
I
I
I
I
I
IO
20
30
40
50
60
Depth to geometrical
center
(mm in Polystyrene)
Fig. 6. Tissue-maximum-ratios (TMR) for 11 MeV electrons measured with the extrapolation chamber in polystyrene. The measurements were made with 1 and 5 mm spacing between the plates. The collection voltages were 2300 V for the 1 mm spacing and ?700 V for the 5 mm spacing. The points represent the average of the reading taken with negative and positive polarity. The data are plotted vs depth to the geometrical center of the chamber. The two curves are separated by about 112 the difference
in electrode spacing between the two sets of measurements. from the exposure sensitivity of the chamber in a calibrated Co-60 beam. The mechanical and electrical volume agreed within 2/10 of l%, and the radiation volume, V,, was about 1% greater. This comparison in effect constitutes an independent confirmation of the National Bureau of Standards (NBS) calibration of our transfer standard. The good agreement of the pancake chamber volume as determined by the three methods indicates that the collection volume is well defined. For consistency with NBS the radiation volume, V,, is used for all measurements of dose. The method of determining V, is described in detail in a subsequent section. The integrity of the electrostatic shielding was verified electrostatically by bringing a charged rod in
I
I
I
I
10
20
30
40
I 50
I
60
Depth to upper electrode (mm
in Polystyrene)
Fig. 7. The same data as shown in Fig. 6 re-plotted vs depth to the upper electrode. The two curves have been brought into near coincidence, which is exact at d,,,. This demonstrates that the upper electrode can be designated as the location of the point of measurement.
very close proximity to the outer surfaces of the chamber. No effect could be observed with an electrometer connected to the chamber. To verify that the upper electrode is the point of tissue-maximum ratios measurement at d,,,, (TMR’s) were taken in polystyrene with the extrapolation chamber at electron energies of 11 and 20 MeV. Figure 6 illustrates the TMR’s in polystyrene taken at 11 MeV for electrode spacings of 1 and 5 mm. The data are plotted against the depth defined by the geometrical center of the chamber. Although the difference in charge collected was only of the order of a few l/l0 of 1% for each point, the average of positive and negative polarity readings were taken. The resulting two TMR curves are clearly separated by the difference in spacing between the two measurements. Figure 7 displays the same data re-plotted with the upper electrode defining the depth of measurement. It can be seen that the two curves have been brought into close coincidence, which is exact at
2036
Radiation Oncology 0 Biology 0 Physics
and diverges somewhat at mid-range. It is clear from Fig. 7 that the uncertainty in the point of measurement is less than 1 mm, provided the upper electrode is taken as the point of measurement. A similar experiment performed at 20 MeV produced very similar results. From these two experiments it can be concluded that the point of measurement for electrons of the pancake chamber is well established at the inner conducting surface of the upper elec-
d max
trode .
The collection efficiency of the pancake chamber was measured for cobalt-60 gamma rays, 11 MeV electrons and 20 MeV electrons, according to the methods outlined by Holt et ~1.~ With a polarization potential of +300 V, the collection efficiency for 11 MeV electrons at 0.005 Gyisecond (300 rad/min) was 99.6%, at 20 MeV and 0.015 Gyisecond (90 rad/min), the collection efficiency was 99.8%, and for cobalt-60 gamma rays, at a rate of 0.03 Gyisecond (200 radimin), the collection efficiency was 99.9%. This was considered adequate for most applications. However, it is possible to triple the electric field strength for special applications if necessary. The accuracy of the conversion of cavity ionization to absorbed dose in tissue was tested against a tissue-equivalent absorbed dose microcalorimeter and will be described in the following section. Absorbed dose intercomparison The absorbed dose to tissue as computed from an ionometric measurement in a polystyrene phantom using the pancake chamber described was compared to the dose at the same depth in tissue as computed from a calorimetric measurement using a tissueequivalent (A-150) calorimeter. The radiation modalities used in the intercomparison were cobalt-60 gamma rays, 10 MV x-rays, 11 MeV electrons and 20 MeV electrons. In each case, the center of the wafer of the calorimeter was placed at an equivalent depth in polystyrene expressed in number of electrons cm-2 at the upper electrode of the pancake chamber. At each measurement a 10x10 cm field was used in the plane of the point of measurement. Measurements were made at a depth of maximum ionization except for cobalt-60 gamma rays, where the measurements were made at a depth of 5.5 cm in polystyrene (1.865~ 10z4 electrons cmP2). The calorimeter was also placed in the polystyrene phantom. Since A-150 is tissue-equivalent only for cobalt-60 gamma rays, the dose to tissue was computed from the dose to A-150 using the appropriate mass energy absorption coefficients or stopping power ratios. The pancake chamber was initially calibrated in a cobalt-60 therapy beam using a 10x 10 cm field at 80 cm source-axis distance (SAD). The free-air expo-
November-December
1979, Vol. 5, No. 11 and No. 12
sure was first calibrated using a National Bureau of Standards (NBS) calibrated Shonka-Wyckoff chamber. The pancake chamber embedded at d,,, in the polystyrene phantom was then placed in the same field with upper electrodes positioned at 80 cm SAD. The charge collected (Q) for an exposure to free air at the same point (XJ was determined. From the quotient of charge to exposure (Q/Xd, either the Volume, V,, of the pancake chamber or the air mass contained by the volume (pV,) was computed according to eq. (1).
x,.--w .(LL’“pY e
.$!.($olY-
0
.A,,,.B-
p au, Co
p(T,P)V,
e
P air, E(Co)
(1)
where X, is the exposure in free air obtained with a 10x 10 cm field at 80 cm SAD. _w LSthe average energy expended per unit charge of ionization in e air. (cL,“)PplY
p
is the quotient of the mass energy absorption coefficient of polystyrene to air averaged over the Co-60 gamma ray spectrum.
au, Co
is a constant relating collision air kerma to the dose within a small mass of polystyrene of just sufficient extent to provide maximum electronic buildup in air. The value of A,,, was determined by us for polystyrene or water as 0.990.’ A,, appears in eq. (1) because of the exposure measurement in air. B is the backscatter size.
factor in polystyrene
for a 1OXlOcm field
Q is the collected charge. p(T,P) is the density of air in the chamber volume at temperature
T
and pressure P (e.g., p(T,P) =p(To,P,,) . 2. $)). V,. is the collection volume of the pancake chamber which is the quantity computed by means of eq. (1). (3) PolYp au, E(Co)
is the quotient of the collision mass stopping powers of polystyrene to air averaged over the electron fluence entering the cavity.
The dose to muscle was calculated from measurements made with the pancake chamber in the polystyrene phantom according to eqs. (2) and (3). _ Photons:Dmu&,h
= OS!. V,. p(T,P)
e
(S)Ppfy p au,E (X)
Electrons:Dmusc~e.E=~.~. (&)potY V,.. p(T,P) e p air,E(Z)
(&)muscle (2) P POlY, A
(3) musck p
poly,E(Z)
(3)
where
Q ‘c
is the cavity mass ionization. The calibrated volume,
dTyP) V,. is obtained from a Co-60 calibration and eq. (1).
(E) P$Y P
rur,E(z)
is the quotient of the restricted collision mass stopping power ratios of polystyrene to air averaged over the electron energy fluence at the depth of measurement (Z).” The minimum cut-off energy (a = 15 keV) has been omitted in this notation.
Absorbed dose measurements
Table 1. Physical constants
(_S, muscle
is the quotient of the collision mass stopping powers P ~oly,WZ) of muscle to polystyrene averaged over the electron energy fluence spectrum at the depth of the measurement (Z).
_ (&)muscle
p poly k~~the quotient of the mass energy absorption coeffi’ clents of muscle to polystyrene averaged over the photon energy spectrum for photon beam energy A.
If the appropriate SI units are used in eqs. (l), (2) and (3), the dose will be given in Gy (e.g., exposure as 2.58x
2037
0 J.G. HOLT ttal.
lo-” C Kg-’ R-l, Tz33.7
J C-l and - Q V,.P
inC
Kg-‘). Equations (2) and (3) assume that there is no perturbation of the electron or photon fluence resulting from the cavity or the chamber material, which is the case for the pancake chamber, providing the upper electrode is taken as the point of measurement. The appropriate values of the parameters used in eqs. (l), (2) and (3), as well as the electron density and stopping powers used for the calorimeter measurements, are summarized in Table 1. RESULTS
Table 2 lists the results of the absorbed dose intercomparison for the four radiation modalities investigated. As can be seen, the agreement between the two methods is excellent. The precision of the ionometric measurements was within 0.2%, but the uncertainty in the measurement is difficult to estimate. NBS states the absolute uncertainty in the exposure rate calibration as ?2.0%, although Loftus and WeaverI have reported an uncertainty of less than + 1%. The uncertainties in the ratios of the stopping powers and absorption coefficient are not too well known since these calculations depend on the electron and photon spectra at the point of measurement. The uncertainty of +2.5% assigned to the ionometric measurement represents a conservative estimate. As for the calorimeter, the greatest uncertainty encountered is the determination of the thermal defect. The endothermic radiochemical effects in tissue equivalent plastics were initially described theoretically and measured experimentally in our laboratory. a1 A value of (4 22%) has been employed for the thermal defect in these experiments. Nevertheless, the uncertainty in the estimate of the experimental errors should not detract from the good agreement obtained in the determination of the absorbed dose to tissue by the two methods. It can be concluded that the calibration of the high energy therapy beam (at least for the four modalities tested) by means of a pancake chamber embedded in the polystyrene phantom has been demonstrated to be as accurate a method as calorimetry at present.
used for computations A-150
Polystyrene
1.125 x 10SKg rnmR 3.307 X 109W mm”
1.047 x 103Kg m-” 3.238 x 102% m-:j
P PC
B(OS,lOx 10) A
1.036
X,
=
0.990
*le
= 33.7 J C’ 10 MVX
““Co
_
‘1
(1L,e)POlY
2.58 x 10mxC Kg-’ R-’
11 MeV
1.078
t
(j&muscle 11 P P0lY.h
1.019
1.030
+
&&)muscle 11 p A-150-r
1.001
1.010
t
1.121
1.092
:
p
air,h
18, 2
(3) pplr P air,(E)
(L)PplY 4 p alr,E (Z) (3) muscle p poly,E(Z)
I
1.0148
0.953
I
t
1.0252
1.0232
t
:
1.005
1.004
I
1
18,3
(3) muscle_ $ p A-lSO,E(Z)
20 MeV
The sources used for the computations of the constants are indicated by the Reference numbers. tNot applicable. *Personal communication, S.M. Seltzer, August, 1978. rable 2. Comparison
of pancake chamber with calorimeter dose to muscle (Gy)
Pancake
Pancake chamber
chamber+ Co-60 10 MVX 11 MeV 20 MeV
3.897 2.961 3.030 1.649
? * * +
Calorimeter*
2.5% 2.5% 2.5% 2.5%
3.867 2.999 2.9% 1.665
” & ? ?
2.5% 2.5% 2.5% 2.5%
Calorimeter 1.008 0.987 1.011 0.990
iMeasured in polystyrene and computed to muscle. *Measured in A-150 plastic and computed to muscle. DISCUSSION
It has been demonstrated that when the pancake chambe; is used in a polystyrene phantom to calibrate high energy therapy machines, it exhibits very close Bragg-Gray cavity behavior, and fulfills the criteria listed. The method described is capable of yielding results of sufficient accuracy for clinical application. The radiation calibration of the pancake chamber has the additional advantage of being traceable to NBS. While the constants used in equations (l), (2) and (3) may be adjusted slightly as better data become available, it is this straightforward method that is recommended.
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Radiation Oncology 0 Biology 0 Physics
The use of a pancake-type chamber, homogeneously constructed in a phantom of the same material, inherently avoids many of the difficulties to which thimble or spherical ionization chambers are subjected, especially when they are used in water phantoms. The main objection that the equivalent depth in tissue is not accurately established when a measurement is made in a solid phantom can be met successfully by making the measurement at the depth of maximum ionization in the solid phantom and computing the dose to tissue at its characteristic depth of maximum ionization. However, this requirement is not rigid, since it has been shown recently that an equivalent depth in phantom of different composition can be scaled.5 The depth of maximum ionization is the depth to which all isodoses are normalized, and also the depth where the dose gradient is at a minimum. Making a measurement at d,,, has the advantage that any uncertainty in the point of measurement has a negligible
November-December 1979, Vol. 5, No. 11 and No. 12
effect on the measurement. Aside from the cost, polystyrene has much to recommend it as a material for phantom measurements. It is nearly water- or tissue-equivalent since the electron area1 densities are very close; (P’Pe)PolYwater= 1.016. In addition, making a measurement in a solid phantom rather than water avoids the necessity of waterproofing the ion chamber, and permits the use of a homogeneous chamber and phantom combination. The depth of measurement is easily established and any set-up is exactly reproducible. Ten years experience in the calibration of therapy machines has established this method to be accurate, practical, and easy to use for the routine calibration of therapy equipment. The consistent accuracy and simplicity of this method make it quite suitable for general calibrations of high energy therapy beams, and we recommend this method for general use.
REFERENCES 1. American Association of Physicists in Medicine: Protocol for the dosimetry of X- and gamma-ray beams with maximum energies between 0.6 and 50 MeV. (SCRAD) Phys. Med. Biol. 16: 379, 1971. 2. Berger, M.J. and Seltzer, S.M.: Table of energy losses and ranges of electrons and positrons. NASA-N65 12506. National Technical Information Service, U. S. Department of Commerce. 1964, pp. 1-127. 3. Berger, M.J. and Seltzer, S.M.: Additional stopping power and range tables for protons, mesons and electrons. N67-14099, 37038. National Technical Information Service, U.S. Department of Commerce. 1966. 4. Berger, M.J., Seltzer, S.M., Domen, S.R. and Lampower ratios for electron perti, P.J.: Stopping dosimetry with ionization chambers. In Biomedical Dosimetty, IAEA, SM-193139. 1975, pp. 589-609. 5. Casson, H.: Correction of measurements in plastic phantoms to obtain dose in water. Med. Phys. 5: 321, 1978. 6. Gray, L.H.: An ionization method for the absolute dosimetry of gamma-ray energy. Proc. R. Sot. A156, 1936, p. 578. 7. Holt, J.G., Fleischman, R.C., Perry, D.J. and Buffa, A.: An examination of the factors A, and A,,, for cylindrical ion chambers used in cobalt-60 beams. (To be published in Medical Physics, July/August, 1979.) 8. Holt, J.G. and Kessaris, N.D.: Discrepancy between CA and C,. Phys. Med. Biol. 22: 538, 1977. 9. Holt, J.G., Stanton, R.E. and Sell, R.E.: Ionization collection efftciencies of some ionization chambers in pulsed and continuous radiation beams. Med. Phys. 5(2): 107-l 10, 1978. 10. Hospital Physicists Association: A code of practice for dosimetry of 2 to 35 MV X-ray and caesium-137 and cobalt-60 gamma-ray beams. Phys. Med. Biol. 14: 1, 1969. 11. Hubbell, J.H.: Photon mass attenuation and mass energy-absorption coefftcients for H, C, N, 0, Ar, and Seven Mixtures from 0.1 keV to 20 MeV. Rad. Res. 70: 58-81.
1977.
12. International Commission on Radiation Units and Measurements: Radiation Dosimetry: X-ray and gamma rays with maximum photon energies between 0.6 and 50 MeV. Report 14, Washington, DC, l-30, 1969. 13. International Commission on Radiation Units and Measurements: Radiation Dosimetry: Electrons with initial energies between 1 and 50 MeV. Report 21, Washington, DC, l-64, 1972. 14. Kessaris, N.D.: Penetration of high energy electron beams in water. Phys. Rev. 145: 164, 1966. 15. Loftus, T.T. and Weaver, J.T.: Standardization of cobalt-60 and cesium-137 gamma ray beams in terms of exposure. J. of Research of National Bureau of Standards, 4(78A): 465, 1974. 16. McDonald, J.C., Laughlin,
Portable
tissue-equivalent
80, 1976. 17. Nahum, A.E.
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