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A181 brain equivalent plastic


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A181 brain equivalent plastic

  1. 1. A conducting plastic simulating brain tissue Jay Burmeister,a) Chandrasekhar Kota, and Richard L. Maughan Gershenson Radiation Oncology Center, Karmanos Cancer Institute, Harper Hospital and Wayne State University, Detroit, Michigan 48201 John J. Spokas Exradin, Inc., 1950 University Lane, Lisle, Illinois 60532 Jeffrey A. Coderre, Ruimei Ma, and Lucian Wielopolski Medical Department, Brookhaven National Laboratory, Upton, New York 11973 Received 12 May 2000; accepted for publication 31 August 2000 A new conducting plastic has been composed which accurately simulates the photon and neutron absorption properties of brain tissue. This tissue-equivalent TE plastic was formulated to match the hydrogen and nitrogen constituents recommended by ICRU Report #44 for brain tissue. Its development was initiated by the inability of muscle tissue-equivalent plastic to closely approxi- mate brain tissue with respect to low-energy neutron interactions. This new plastic is particularly useful as an electrode in TE dosimetry devices for boron neutron capture therapy BNCT , which utilizes low-energy neutrons for radiotherapy of the brain. Absorbed dose measurements in a clinical BNCT beam using a proportional counter constructed from this TE plastic show good agreement with Monte Carlo calculations. © 2000 American Association of Physicists in Medi- cine. S0094-2405 00 02211-2 Key words: Tissue equivalent plastic, neutron dosimetry, BNCT, BNCEFNT I. INTRODUCTION allows it to serve as an adequate tissue substitute for fast neutron therapy applications as well. This similarity in car- Accurate radiation dosimetry measurements rely on the abil- bon and oxygen kerma coefficients diminishes with decreas- ity of the dosimeter to suitably simulate the radiation inter- ing neutron energy. However, at lower neutron energies, the action characteristics of the medium of interest. In the case elemental kerma contributions from carbon and oxygen are of photon dosimetry at clinical energies, the exact elemental masked by the hydrogen contribution. Therefore, with re- composition is not vitally important since photon interactions spect to the carbon and oxygen disparity, A-150 may be primarily depend only on the electron density of the medium. However, neutron interaction characteristics are dependent considered a suitable muscle tissue substitute for all neutron upon neutron cross sections unique to each nucleus. Strictly energies. Another problem with tissue matching does arise at speaking, neutron dosimetry therefore obligates one to match low neutron energies, however. Neutron capture by nitrogen the elemental composition of the detector to the medium of begins to contribute substantially to the kerma at low neutron interest. Unfortunately, the exact elemental compositions of energies, making the equivalence of nitrogen contents be- biological media of interest do not yield a mixture suitable tween tissue and tissue substitutes crucial. for use in a gas cavity-based dosimetry device. True tissue In order to convert a neutron absorbed dose in A-150 to equivalence requires a high oxygen content, allowing the for- absorbed dose in tissue, a kerma coefficient ratio between the mulation of only gel-like compounds. A solid, conducting two materials must be derived over the range of neutron tissue-equivalent plastic TEP may be produced by ex- energies involved. Assuming knowledge of the neutron en- changing the majority of the oxygen component present in ergy spectrum at each measurement point, a spectrum- tissue for carbon. The muscle-tissue-equivalent plastic weighted neutron kerma coefficient may be calculated which A-150, developed by Shonka in 1958, is unquestionably the allows the conversion of the measured neutron dose from most popular such conducting TE plastic and is widely used A-150 to the tissue of interest. For fast neutrons, the kerma in the construction of TE dosimeters.1–3 A-150 TE plastic is coefficient ratio for soft tissue to A-150 is fairly constant well suited for the measurement of absorbed dose to any soft approximating unity and is relatively insensitive to changes tissue for most photon and fast neutron applications. The in the shape of the neutron spectrum.8 However, this is not substitution of carbon in place of oxygen does not have a the case for low-energy neutron beams. The kerma from low- significant effect on photon attenuation coefficients or elec- energy neutrons in tissue is strongly dependent upon quanti- tron stopping powers.4,5 The majority of the kerma in tissue ties of both hydrogen and nitrogen. The thermal neutron cap- from fast neutrons of clinical energies is deposited by recoil ture cross section for nitrogen is relatively large (1.83 protons resulting from neutron interactions with hydrogen. 10 24 cm2 ) and results in the local deposition of a 580 keV A-150 TE plastic is well matched in hydrogen content to proton. Low-energy neutrons have little energy to impart to ICRU #44 muscle tissue. This, along with the relative simi- hydrogen nuclei in elastic collisions. In addition, local en- larity in kerma coefficients between carbon and oxygen,6,7 ergy deposition from 2.2 MeV gamma ray produced in the 2560 Med. Phys. 27 „11…, November 2000 0094-2405Õ2000Õ27„11…Õ2560Õ5Õ$17.00 © 2000 Am. Assoc. Phys. Med. 2560
  2. 2. 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 1 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. III. RESULTS 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
  3. 3. 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 10 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 10 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
  4. 4. 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- No. DE-FG02-96ER62217. 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 1 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 constitutes a significant fraction of the total absorbed dose in tissue, air and polystyrene,’’ Progress in Nuclear Energy, Series XII, BNCT. Absorbed doses calculated from TEPC measure- Health Phys. 1, 160–166 1959 . 3 J. J. Spokas, ‘‘Composition variability and equivalence of Shonka TE ments at a depth of 1 cm in phantom in the BMRR beam plastic,’’ Radiation Research Society, Miami Beach, 11–15 May 1975. 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- burg, MD, 1995 . 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, respectively. As indicated in the previous section, the differ- MD, 1984 . 6 ence in elemental composition between brain and muscle R. S. Caswell, J. J. Coyne, and M. L. Randolph, ‘‘Kerma factors for neutron energies below 30 MeV,’’ Radiat. Res. 83, 217–254 1980 . tissues results in a difference of approximately 25% in the 7 M. B. Chadwick, H. H. Barschall, R. S. Caswell, P. M. DeLuca, G. M. neutron absorbed dose. Hale, D. T. L. Jones, R. E. MacFarlane, J. P. Meulders, H. Schuhmacher, Another prerequisite for accurate boron neutron capture U. J. Schrewe, A. Wambersie, and P. G. Young, ‘‘A consistent set of dosimetry is that the thermal neutron distribution should be neutron kerma coefficients from thermal to 150 MeV for biologically important materials,’’ Med. Phys. 26, 974–991 1999 . the same in the detector as at that point in tissue in the 8 M. Awschalom, I. Rosenberg, and A. Mravca, ‘‘Kermas for various sub- absence of the detector. The total thermal neutron absorption stances averaged over the energy spectra of fast neutron therapy beams: cross sections are very nearly the same between the tissues A study in uncertainties,’’ Med. Phys. 10, 395–409 1983 . 9 and tissue substitutes described in this work. The only ICRU Report #44, ‘‘Tissue substitutes in radiation dosimetry and mea- surement’’ International Commission on Radiation Units and Measure- nucleus with a significant cross section which is present in ments, Bethesda, MD, 1989 . 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– the total thermal neutron absorption cross section of brain 359 1992 . 11 tissue. For a thin-walled detector such as that described in R. L. Maughan, C. Kota, and M. Yudelev, ‘‘A microdosimetric study of the dose enhancement in a fast neutron beam due to boron neutron cap- this work, the thermal neutron distribution at the point of ture,’’ Phys. Med. Biol. 37, 1957–1961 1993 . measurement will be determined primarily by the phantom 12 C. Kota, ‘‘Microdosimetric considerations in the use of the boron neutron material thus the absence of 35Cl in the detector wall may be capture reaction in radiation therapy,’’ Ph.D. thesis, Wayne State Univer- neglected. sity, 1996. 13 C. Kota, R. L. Maughan, D. Tattam, and T. D. Beynon, ‘‘The use of low-pressure tissue equivalent proportional counters for the dosimetry of neutron beams used in BNCT and BNCEFNT,’’ Med. Phys. 27, 535–548 2000 . 14 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 . 15 low-energy neutron dosimetry applications. It has proven J. Burmeister, ‘‘Specification of the physical and biologically effective absorbed dose in radiation therapies utilizing the boron neutron capture useful as a proportional counter cathode for microdosimetry reaction,’’ Ph.D. thesis, Wayne State University, 1999. for BNCT, offering accurate brain tissue-equivalent absorbed 16 P. M. DeLuca, Jr., F. H. Attix, D. W. Pearson, M. C. Schell, and M. doses and providing further validation for the Monte-Carlo- Awschalom, ‘‘Application of A-150 plastic-equivalent gas in A-150 plas- based treatment planning systems used in BNCT. A-181 also tic ionization chambers for Co-60 rays and 14.8-MeV neutrons,’’ Med. Phys. 9, 378–384 1982 . promises to be useful for ionization chamber dosimetry in 17 D. W. Nigg, F. J. Wheeler, D. E. Wessol, J. Capala, and M. Chadha, mixed fields used to irradiate the brain, allowing more accu- ‘‘Computational dosimetry and treatment planning for boron neutron rate specification of the neutron absorbed dose. capture therapy,’’ J. Neuro-Oncol. 33, 93–104 1997 . 18 ICRU Report #36, ‘‘Microdosimetry’’ International Commission on Ra- diation Units and Measurements, Bethesda, MD, 1983 . 19 J. B. Smathers and V. A. Otte, ‘‘Composition of A-150 tissue-equivalent plastic,’’ Med. Phys. 4, 74–77 1977 . ACKNOWLEDGMENTS 20 Y. D. Harker, R. A. Anderl, G. K. Becker, and L. G. Miller, ‘‘Spectral characterization of the epithermal neutron beam at the Brookhaven Medi- The authors would like to acknowledge the staff at the cal Research Reactor,’’ J. Nucl. Sci. Eng. 110, 355–368 1992 . 21 Brookhaven Medical Research Reactor for their help in fa- R. D. Rogus, O. K. Harling, and J. C. Yanch, ‘‘Mixed field dosimetry of Medical Physics, Vol. 27, No. 11, November 2000
  5. 5. 2564 Burmeister et al.: A conducting plastic 2564 epithermal neutron beams for boron neutron capture therapy at the Med. Phys. 22, 321–329 1995 . 23 MITR-II research reactor,’’ Med. Phys. 21, 1611–1625 1994 . C. P. J. Raaijmakers, P. R. D. Watkins, E. L. Nottelman, H. W. Verha- 22 C. P. J. Raaijmakers, M. W. Konijnenberg, H. W. Verhagen, and B. J. gen, J. T. M. Jansen, J. Zoetelief, and B. J. Mijnheer, ‘‘The neutron Mijnheer, ‘‘Determination of dose components in phantoms irradiated sensitivity of dosimeters applied to boron neutron capture therapy,’’ Med. with an epithermal neutron beam for boron neutron capture therapy,’’ Phys. 23, 1581–1589 1996 . Medical Physics, Vol. 27, No. 11, November 2000