2561 Burmeister et al.: A conducting plastic 2561
TABLE I. Elemental composition percent weight of tissue and tissue sub- TABLE II. Material composition percent weight of A-181 brain tissue
stitutes. equivalent plastic.
Material H C N O F Ca Polyethylene (CH2 ) n 58.22
Nylon DuPont Zytel 69 C6H11NO 22.14
Muscle ICRU #44 10.2 14.3 3.4 71.0 ¯ ¯ Carbon black C 16.06
Brain ICRU #44 10.7 14.5 2.2 71.2 ¯ ¯ Calcium ﬂuoride CaF2 3.58
A-150 10.1 77.6 3.5 5.2 1.7 1.8
A-181 10.7 80.3 2.2 3.3 1.7 1.8
Propane-based TE gas 10.0 55.9 4.9 29.2 ¯ ¯
relatively large conversion factor which is quite dependent
on the shape of the epithermal spectrum. If it were a trivial
task to obtain the neutron spectrum at each measurement
H(n, ) 2 H thermal neutron capture reaction is negligible. point, this issue would not present a signiﬁcant problem.
The elemental kerma contribution from nitrogen therefore However, measurements or calculations to obtain the in-
becomes a signiﬁcant contributor to the kerma for low- phantom neutron spectrum are neither elementary nor exact.
energy neutrons in tissue or tissue substitute, becoming the Based on the preceding discussion, a new brain-tissue-
dominant component below approximately 0.1 keV. Since equivalent conducting plastic named A-181 has been com-
the nitrogen components of different tissue types can vary posed for the simulation of brain tissue for low-energy neu-
signiﬁcantly, the kerma coefﬁcient ratio of any one plastic tron applications such as BNCT. The elemental composition
composition to tissue will deviate dramatically from one tis- of A-181 is presented in Table I. The mixture was fabricated
sue to another. A tissue substitute more closely approximat- by Dr. John Spokas at Exradin, Inc., according to the mate-
ing both the hydrogen and nitrogen components of the tissue rial composition presented in Table II. The carbon black and
of interest is therefore more appropriate for low-energy neu- calcium ﬂuoride components are identical to those in A-150
tron applications. TEP in order to minimize differences between A-181 and the
One application of low-energy neutrons in radiotherapy is already well-characterized A-150.
boron neutron capture therapy BNCT , which employs an
epithermal neutron beam to irradiate the tumor site. Epither- II. MATERIALS AND METHODS
mal neutrons are generally deﬁned as those possessing ener-
gies in the range of 0.5 eV to roughly 10 keV. BNCT is Based on results of work by Wuu,10 Maughan,11 and
currently used almost exclusively for brain tumors. Although Kota,12,13 tissue-equivalent proportional counter TEPC mi-
the hydrogen contents of brain tissue and A-150 differ crodosimetry has proven to be an ideal dosimetry method for
slightly, their disparity in kerma coefﬁcients at low neutron BNCT. A miniature tissue-equivalent proportional counter
energies is due primarily to differences in their respective TEPC microdosimetry system designed for use in clinical
nitrogen content. The elemental compositions of A-150, BNCT beams has recently been developed.14,15 This dual
muscle tissue, and brain tissue may be found in Table I. counter technique uses matching TEPCs with 2 mm thick
A-150 TEP contains approximately 3.5% nitrogen by weight walls, one made of A-150 and the other of A-150 loaded
while ICRU #44 speciﬁes only approximately 2.2% for brain with 200 g/g 10B. These detectors provide the photon, neu-
tissue.9 Accordingly, the kerma coefﬁcient ratio of brain to tron, and boron neutron capture doses to A-150. In addition,
A-150, as observed in Fig. 1, begins to deviate signiﬁcantly a miniature TEPC has been constructed with an A-181 BTEP
from unity below approximately 0.1 keV. This necessitates a wall in order to provide more accurate neutron-absorbed
dose measurements to brain tissue for BNCT facilities. As
observed in Table I, the composition of the proportional
counter ﬁll gas does not closely match that of either A-150 or
A-181. However, as previously mentioned, the neutron
kerma coefﬁcients for carbon and oxygen are very similar,
minimizing the effects of differences in these components.
Experiments by DeLuca et al. have shown that differences in
proton stopping power and neutron kerma coefﬁcient be-
tween TE gas and an A-150 plastic-equivalent gas are rela-
tively small.16 Moreover, at the small site sizes simulated
with the cavity, nearly all of the charged particles respon-
sible for energy deposition in the cavity arise in the wall.
This condition eliminates the necessity for a signiﬁcant cor-
rection due to the difference in nitrogen contents between the
TE gas and A-150 or A-181.
FIG. 1. Kerma coefﬁcient ratios for brain and muscle tissues to A-150 TEP
and for brain tissue to A-181 BTEP. Kerma coefﬁcients were obtained from Measurements using the A-181 TEPC and an identical
data presented by Caswell et al. Ref. 5 and Chadwick et al. Ref. 7 . A-150 TEPC have been performed in the Brookhaven Medi-
Medical Physics, Vol. 27, No. 11, November 2000
2562 Burmeister et al.: A conducting plastic 2562
yd(y) versus y format as prescribed by ICRU #36.18 The
ordinate, yd(y), is unitless and the distribution is normalized
to unity. By deﬁnition of the normalization, an increase in
one portion of the spectrum must be accompanied by a de-
crease in the remainder of the spectrum, making it difﬁcult to
directly compare individual features of the lineal energy
spectra. The plots shown in Figs. 2 and 3 represent fractional
dose weighted by lineal energy, y, as a function of lineal
energy. This yD(y) presentation is unnormalized and is
equivalent to y 2 •N(y)/ y, where N(y) is the number of
events of lineal energy y. The units of dose on the ordinate of
the plots are arbitrary since one is only concerned with the
relative heights of the event size spectra. These spectra fa-
cilitate the observation of changes in charged particle ﬂuence
FIG. 2. Fractional dose weighted by lineal energy, y, as a function of y in the inside the sensitive volume for different detectors in the
BMRR epithermal beam measured in a 14 14 14 cm3 acrylic cube phan- same radiation ﬁeld. The decrease in neutron dose in the
tom at a depth of 2 cm. Measurements were made with the A-150 TEP and
A-181 BTEP TEPCs using a simulated site diameter of 1 m.
A-181 TEPC is evident in Figs. 2 and 3 from approximately
10 keV/ m to the proton edge of approximately 150 keV/
m. The balance of the spectrum remains essentially un-
cal Research Reactor BMRR clinical BNCT beam at the changed. A small increase in the heavy ion region is ob-
Brookhaven National Laboratory. These measurements uti- served in both A-181 spectra relative to the A-150 spectra.
lized a 14 14 14 cm3 acrylic phantom. The Monte Carlo This is most likely due to differences in the amount of trace
treatment planning system MCTPS used for BNCT treat- B present in the different batches of plastic. Attributing the
ments at the BMRR17 furnishes both the fast neutron dose heavy ion signal entirely to the BNC reaction would yield a
and the thermal neutron ﬂux, providing a means for assess- B content of 0.8 0.1 ppm for the A-150 plastic and 1.1
ing the effects of changes in nitrogen content. Results from 0.1 ppm for the A-181 plastic. The magnitude and variance
the MCTPS have been used to calculate neutron absorbed in these 10B contents is consistent with results from trace
doses for equilibrium volumes of brain tissue and muscle element analysis of A-150 samples given by Smathers
tissue at various points in the 14 14 14 cm3 acrylic cube et al.19 This analysis of two A-150 samples yielded 10B con-
phantom. This allows a comparison of the calculated brain tents of approximately 0.7 ppm and 0.4 ppm.
tissue to muscle tissue neutron dose ratio with the results of Despite their respective equivalence in hydrogen and ni-
A-181 BTEP and A-150 TEP proportional counter measure- trogen contents to muscle and brain tissues, both A-150 and
ments. Lineal energy spectra measured with these TEPCs for A-181 require spectrum-weighted kerma coefﬁcient ratios.
a simulated site diameter of 1 m in tissue are exhibited in The epithermal neutron beam spectrum for the BMRR was
Figs. 2 and 3. These spectra represent absorbed dose per unit extracted from calculations by Harker et al.20 The calculated
logarithmic interval of lineal energy density, y, against loga- kerma coefﬁcient ratios based on this epithermal neutron
rithm of y. Standard lineal energy spectra are presented in the spectrum were 0.972 and 0.982 for muscle tissue to A-150
and brain tissue to A-181, respectively. The proximity to
unity of the kerma coefﬁcient ratio of brain tissue to A-181 is
evidence of the excellent representation of brain tissue by
A-181 for this neutron spectrum. Measured neutron absorbed
dose ratios for A-181 to A-150 are 0.778 0.043 and 0.741
0.041 at depths of 2 and 7 cm, respectively. These mea-
sured ratios show good agreement with results from Monte
Carlo calculations. Calculated neutron absorbed dose ratios
for brain tissue to muscle tissue from the MCTPS are 0.744
and 0.720 at depths of 2 and 7 cm, respectively. The incident
fast neutron component is rapidly attenuated in tissue while
the thermal neutron component diminishes less rapidly with
depth. This leads to an increase in the nitrogen capture dose
relative to the fast neutron dose with increasing depth. This
phenomenon is observed as a decrease in the calculated neu-
tron absorbed dose ratio for brain tissue to muscle tissue
from 2 to 7 cm. This trend appears to be present in the
FIG. 3. Fractional dose weighted by lineal energy, y, as a function of y in the
neutron absorbed dose ratios for A-181 to A-150 at 2 and 7
BMRR epithermal beam measured in a 14 14 14 cm3 acrylic cube phan-
tom at a depth of 7 cm. Measurements were made with the A-150 TEP and cm, although such a small difference is difﬁcult to resolve
A-181 BTEP TEPCs using a simulated site diameter of 1 m. given the size of the uncertainty in these values.
Medical Physics, Vol. 27, No. 11, November 2000
2563 Burmeister et al.: A conducting plastic 2563
IV. DISCUSSION cilitating the measurements presented here. This work was
supported in part by the U. S. Department of Energy, Grant
Large uncertainties on the order of 15%–20% are as-
cribed to the measurement of the neutron absorbed dose in
epithermal neutron beams using A-150 ionization a
Address for correspondence: Jay Burmeister, Harper Hospital, Gershen-
chambers.21–23 These uncertainties are, in large part, due to son R. O. C., 3990 John R, Detroit, MI 48201. Electronic mail:
the differences in the elemental composition of the ionization firstname.lastname@example.org
chamber and brain tissue. The use of ionization chambers F. R. Shonka, J. E. Rose, and G. Failla, 2nd United Nations International
Conference on the Peaceful Uses of the Atom; Health and Safety; Do-
fabricated from A-181 may be expected to alleviate a signiﬁ- simetry and Standards 1958 , Vol. 21, p. 753.
cant fraction of this uncertainty. The neutron absorbed dose 2
F. R. Shonka, J. E. Rose, and G. Failla, ‘‘Conducting plastic equivalent to
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BNCT. Absorbed doses calculated from TEPC measure- Health Phys. 1, 160–166 1959 .
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reveal that the neutron component delivers 13% and 31% of 4
J. H. Hubbell and S. M. Seltzer, ‘‘Tables of X-ray mass attenuation
the total RBE-weighted absorbed dose to tumor and normal coefﬁcients and mass energy absorption coefﬁcients 1 keV–20 MeV for
tissue, respectively.15 These calculations assume a 10B load- elements Z 1 to 92 and 48 additional substances of dosimetric interest,’’
ing of 30 ppm in the tumor, a tumor to normal tissue boron U.S. Dept of Commerce, NIST Report #NISTIR 5632 NIST, Gaithers-
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concentration ratio of 4 to 1, and RBE values of 3.8, 3.2, and 5
ICRU Report #37, ‘‘Stopping powers for electrons and positrons’’ In-
1.0 for the BNC, fast neutron, and photon dose components, ternational Commission on Radiation Units and Measurements, Bethesda,
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M. B. Chadwick, H. H. Barschall, R. S. Caswell, P. M. DeLuca, G. M.
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M. Awschalom, I. Rosenberg, and A. Mravca, ‘‘Kermas for various sub-
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brain tissue and not in A-181 is 35Cl, with a thermal neutron 10
C. S. Wuu, H. I. Amols, P. Kliauga, L. E. Reinstein, and S. Saraf, ‘‘Mi-
cross section of 44.1 barns.9 35Cl constitutes roughly 7% of crodosimetry for boron neutron capture therapy,’’ Radiat. Res. 130, 355–
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low-pressure tissue equivalent proportional counters for the dosimetry of
neutron beams used in BNCT and BNCEFNT,’’ Med. Phys. 27, 535–548
V. CONCLUSION J. Burmeister, C. Kota, and R. L. Maughan, ‘‘Paired miniature tissue-
equivalent proportional counters for dosimetry in high ﬂux epithermal
In summary, a conducting brain-tissue-equivalent plastic neutron capture therapy beams,’’ Nucl. Instrum. Methods Phys. Res. A
has been developed which displays useful characteristics for 422, 606–610 1999 .
low-energy neutron dosimetry applications. It has proven J. Burmeister, ‘‘Speciﬁcation of the physical and biologically effective
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doses and providing further validation for the Monte-Carlo- Awschalom, ‘‘Application of A-150 plastic-equivalent gas in A-150 plas-
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2564 Burmeister et al.: A conducting plastic 2564
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Medical Physics, Vol. 27, No. 11, November 2000