- Email: [email protected]
Study of electron densities of normal and neoplastic human breast tissues by Compton scattering using synchrotron radiation
Applied Radiation and Isotopes 70 (2012) 1351–1354
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Study of electron densities of normal and neoplastic human breast tissues by Compton scattering using synchrotron radiation M. Antoniassi, A.L.C. Conceic- a~ o, M.E. Poletti n ~ Preto, Sao ~ Paulo, Brazil Departamento de Fı´sica—Faculdade de Filosofia Ciˆencias e Letras de Ribeira~ o Preto—Universidade de Sa~ o Paulo, Ribeirao
a r t i c l e i n f o
abstract
Available online 10 January 2012
Electron densities of 33 samples of normal (adipose and fibroglangular) and neoplastic (benign and malignant) human breast tissues were determined through Compton scattering data using a monochromatic synchrotron radiation source and an energy dispersive detector. The area of Compton peaks was used to determine the electron densities of the samples. Adipose tissue exhibits the lowest values of electron density whereas malignant tissue the highest. The relationship with their histology was discussed. Comparison with previous results showed differences smaller than 4%. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Compton scattering Electron density Breast cancer Mammography
1. Introduction The Compton scattered radiation has been widely reported as an important tool for characterization of materials since it allows assessing their electron densities (Cesareo et al., 1992; Sharaf, 2001). Specifically for breast tissue, the Compton scattering technique has been applied by some authors. Al-Bahri and Spyrou (1998) used a monoenergetic source (Am241) and a high purity germanium detector in a scattering angle of 901 to obtain the electron densities of normal and pathological breast samples of nine breast cancer patients. In this study a water sample was used to calibrate the system and the electron densities were determined neglecting all possible corrections (background subtraction, multiple scattering and attenuation effects). Ryan et al. (2005) measured the electron densities of 44 breast samples of four different classifications: adipose, malignant, fibroadenoma and fibrocystic change, using a polienergetic beam from a tungsten target industrial X-rays unit and a scattering angle of 301. The system was calibrated using solutions of known electron density and their method considered background and attenuation corrections. More recently, Antoniassi et al. (2010) determined the electron densities of 109 samples of breast tissues, composed of normal (adipose and fibrous), benign (fibroadenoma) and malignant (carcinoma) neoplasias using the Ka line from a molybdenum industrial X-ray tube and a scattering angle of 901. The system was calibrated using standard reference materials, liquids and solids, with similar properties to breast tissues. In the present work, the electron densities of normal and neoplastic breast tissues were determined through measurements of Compton scattering photons from a monoenergetic synchrotron radiation source. The advantages of the use of this type of source are
high intensity, natural collimation, linear polarization and energy tunability, features that allow time reduction and minimal spectral correction, reducing the experimental uncertainties. The distributions of electron densities were presented and correlated with histological characteristics of each tissue type. The results were compared to previously published data for normal adipose (both theoretical and experimental) and neoplastic breast tissues (only experimental).
2. Materials and method 2.1. Samples The breast tissue samples were obtained from surgical removal of the entire breast (mastectomy) or plastic surgery for breast reduction (mastoplasty) at the Hospital das Clı´nicas da Faculdade de Medicina de Ribeira~ o Preto, Universidade de Sa~ o Paulo, Ribeira~ o Preto, Brazil, whose collection and handling of these fresh samples were performed in compliance with the requirements established by the local ethical committee. Upon removal, the samples were stored at room temperature in formalin (4% formaldehyde in water) in order to preserve the structures within the tissues such as cells walls and nuclei which are needed for accurate histopathological analysis. A total of 33 samples were histopathologically classified, being 15 normal adipose breast tissues and 18 neoplastic tissues comprising 6 fibroadenomas (benign tissues) and 12 invasive ductal carcinomas (malignant tissues). 2.2. Experimental arrangement
n
Corresponding author. Tel.: þ55 16 3602 4442; fax: þ 55 16 3602 4887. E-mail address: [email protected] (M.E. Poletti).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2012.01.001
The experiments were performed at the D12A-XRD-1 beamline in the Laborato´rio Nacional de Luz Sı´ncrotron in Campinas,
1352
M. Antoniassi et al. / Applied Radiation and Isotopes 70 (2012) 1351–1354
Fig. 1. Experimental setup.
Brazil (Cusatis et al., 1998). The experimental setup is exhibited in Fig. 1. A double-crystal Si(111) monochromator was used to provide ˚ with an energy an X-ray beam energy of 14 keV (l ¼0.8856 A, bandwidth of Dl/l 10 4) and a beam size of (5.0 mm 1.0 mm) on the sample. During measurements, the samples were positioned inside an acrylic sample holder (15 mm 10 mm 4 mm) and covered by thin kapton foils (0.125 mm thickness) at both sides. The sample holder was positioned into the Huber diffractometer assembled on a rotative table and a scattering angle of 1501 (corresponding to a momentum transfer x ¼1.09 A˚ 1) was fixed during the experiment. In order to minimize the angular spread of the scattering beam, a collimation system was placed in front of the detector, being composed of a cylindrical collimator housing of 30 mm-length and 14 mm-external radius made of stainless steel and two Tungsten (W) collimator disks 2 mm-thick with a 3 mm hole placed at the beginning and the end of the collimator housing. The scattering photons (coherent and incoherent) were detected in reflection mode using a detector of Si-Pin XR-100CR by AmpTek (145 eV of energetic resolution in 5.9 keV) coupled with a multichannel analyzer and controlled by a main computer. The sample to detector distance was fixed at 210 mm and the measurement counting time (around 300 s for adipose tissues and 550 s for neoplastic tissues) was chosen in order to provide a statistical uncertainty in peak area of less than 1% for the tissues samples. An ionization chamber was used for monitoring the incident beam in order to permit the normalization of the measurements due to the diminution of the electron current in the storage ring. 2.3. Data collection For each sample, three datasets were collected from different regions of the sample to minimize the influence of a possible sample inhomogeneity. After measured the scattered intensity by each sample, additional measurements were performed in order to quantify the contributions from every other spurious scattering sources (air, kapton foil and sample holder) from the original data. The corrected data, Ninc, was obtained from Ninc ¼ Ntot Si T i Ni , where Ntot is the total photon counting measured, Ni represents each spurious scattering counting, and Ti is the appropriate transmission factor to be applied to each Ni, as described by Poletti et al. (2002c). 2.4. Basis of the experimental model The number of photons single incoherently (Compton) scattered (Ninc ) from the material at a scattering angle y, can be written in the
form (Kane, 1992): ds ds Ninc ðyÞ ¼ N 0 nat DOdet eVA ¼ N 0 nat Sðx,ZÞ DOdet eVA, dO inc dO KN ð1Þ where N0 is the number of incident photons per unit area; nat is the number of atoms by volume; ðds=dOÞ inc corresponds to the mean differential inelastic cross-section for the range of scattering angle which can be written as ðds=dOÞ inc ¼ Sðx,ZÞðds=dOÞ KN where S(x,Z) is the incoherent scattering function (Hubbell et al., 1975) which 1 depends on the momentum transfer, x, defined as x ¼ l sinðy=2Þ (where l is the wavelength of the incident photon) and the atomic number Z; ðds=dOÞ KN is the well-known Klein-Nishina differential scattering cross section averaged for the range of scattering angle considered; the term DOdet is the mean solid angle subtended by the detector; e is the detector efficiency at the scattering energy; V is the scatterer volume and A is the attenuation factor given in a R general form as: A ¼ ð1=VÞ V eðm1 l1 þ m2 l2 Þ dV, where eðm1 l1 þ m2 l2 Þ is the attenuation factor through the distances l1 from surface to the elemental scattering volume (dV) in the sample and l2 from this element to the surface of the sample in the direction of the detector, being m1 and m2 the linear attenuation coefficients for incident and scattered energy respectively. Writing nat ¼ re =Z, we can rewrite Eq. (1) as: S ds Ninc ðyÞ ¼ N 0 re DOdet eVA ð2Þ Z dO KN It is convenient to eliminate N0, DOdet and e and simultaneously express (S/Z) and A in terms of suitable ratios. For this purpose, it is needed to use a reference material for this simplification. In this experience, water was chosen for presenting similar scattering properties to breast tissues (Al-Bahri and Spyrou, 1998; Poletti et al., 2002b) and because its electron density is known. Thus, from the ratio between the number of incoherent scattered photons by tissue sample (Ntissue inc ) and by water (Nw inc ), using the Eq. (2) and further manipulation, yields the ratio of the electron density of the tissue, rtissue e , to the electron density of water, rw e , as:
rtissue N tissue ðS=ZÞw Aw e ¼ inc , w tissue tissue re Nw A inc ðS=ZÞ
ð3Þ
where the (S/Z) values for compounds and mixtures (water and tissues) were obtained using the independent atomic model (IAM) approximation (Poletti et al., 2002b): P ðo =M i ÞSi ðxÞ ð4Þ ðS=ZÞ ¼ P i i i ðoi =M i ÞZ i ðxÞ
M. Antoniassi et al. / Applied Radiation and Isotopes 70 (2012) 1351–1354
1353
It is worth noting that the accuracy of this method is limited by the accuracy of the quantities ½S=Zw =½S=Ztissue and Aw =Atissue . The ½S=Zw =½S=Ztissue ratio, at x ¼1.09 A˚ 1, was calculated using mean chemical composition of adipose (Woodard and White, 1986) and neoplastic (Poletti et al., 2002a) breast tissues. For adipose tissue the ½S=Zw =½S=Zadipose ratio was 0.980,while the ½S=Zw =½S=Zneoplastic ratio, for neoplastic tissue, was 0.994. For both tissue type the uncertainty of ½S=Zw =½S=Ztissue values around the adopted value (mean value) was evaluated from the variation range of the chemical composition present in the literature (Woodard and White, 1986; Poletti et al., 2002a) and it was lesser than 0.5%. The attenuation factors Aw and Atissue were calculated using an in-house-developed Monte Carlo code considering just geometrical conditions (reflection geometry), without introducing any other physical constraint when sampling the photon paths during the scattering process. The linear attenuation coefficients for the incident and scattering energies used as sources in the code were obtained experimentally for each breast tissue and for water during the experiment. The experimental uncertainties associated with the values of electron densities have been estimated by error propagation in Eq. (3), assuming all variables uncorrelated. Then, the statistical uncertainties related to Ninc (Eq. (3)) was from 1% to 1.6%, while the statistical uncertainties arising from simulation code (including the experimental uncertainties in the linear attenuation coefficients) was between 1% and 1.5% and the uncertainty arising from calibration process was estimated in 2.1%, yielding a final uncertainty ranging from 4.1% to 5.0%.
3. Results and discussion
Fig. 2. (a) Average processed spectra in arbitrary unity (a.u.) for each tissue type and (b) corrected following Eq. (3).
Fig. 2a shows the average processed spectra of normal (adipose) and neoplastic (benign and malignant) human breast tissues and water. For each spectrum, two predominant features can be observed, as previously mentioned: coherent scattering photopeak (14 keV) and incoherent scattering photo-peak (13.3 keV). Considering that these spectra have information about attenuation, associated with the linear attenuation coefficient, and electron density of the tissues, it is clearly observed that neoplastic breast tissues (benign and malignant) and water spectra are similar, while normal adipose tissue spectrum shows higher intensity mainly due to the smaller attenuation of radiation inside of this tissue (Johns and Yaffe, 1987; Tomal et al., 2010). The differences of linear attenuation coefficient (m) and consequently of attenuation factor (A) of adipose (m adipose ¼ 1:23 cm1 , A adipose ¼ 0:45) and neoplastic (m neoplastic ¼ 2:09 cm1 , A neoplastic ¼ 0:26) tissues are significant at the experimental energy of 14 keV. In this way the attenuation w
adipose
w
neoplastic
¼ 0:60 and A =A ¼ 1:03Þ correction Aw =Atissue (A =A for each sample, must be taken in account to determine the electron densities of the tissues. Fig. 2b shows these spectra relative to water (including all correction factors present in Eq. (3)), in which the intensity of the incoherent photo-peak is due basically to the electron density of these tissues. Fig. 3 shows a box and whisker plot of the distribution of electron density obtained in this work. The thick line inside the boxes shows the median of each tissue type. The inter-quartile range is shown by the box and encloses the 25th to the 75th percentile. The whiskers contain all values within 1.5 box lengths. From this figure it is possible to observe that electron density distributions are broad, exhibiting overlapping values between the different types of breast tissues. The variations of electron densities obtained for each tissue type are expected, since these samples belonged to different
Fig. 3. Box plot of the electron density results for each tissue type.
patients, then it can be attributed either to external parameters (associated to the sampling of different patients) such as diet, medication, environment, age, hormonal status, genetics, or to internal parameters associated to particular histological characteristic of each tissue type. The results also show that adipose breast tissue present the smallest electron density values when compared with neoplastic breast tissues. It is well-known that normal adipose tissue is composed principally by cells specialized in storing lipids (triacylglycerols and cholesterol ester), called adipocytes, rich in carbon, presenting a low density, while neoplastic tissues (benign
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M. Antoniassi et al. / Applied Radiation and Isotopes 70 (2012) 1351–1354
Table 1 Summary of electron density values for breast tissue present in literature and in this work. Tissue
Normal Adipose Benign Malignant
Experimental
Theoretical
Al-Bahri and Spyrou 1998
Ryan et al. 2005
Antoniassi et al. 2010
This work
ICRU report 44
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
3.46 NM 3.56
3.24 (0.14) 3.31 (0.14) 3.53 (0.15)
3.24 (0.16) 3.47 (0.12) 3.60 (0.17)
3.21 (0.14) 3.39 (0.17) 3.48 (0.15)
3.18 NA NA
NM¼ not measured. NA¼ not available.
and malignant) are characterized, basically, by an excessive proliferation of cells constricted in the stroma, formed by conjunctive tissue (rich in oxygen) of higher density. In the particular case of malignant tissue, the cells of carcinoma form solid cords which infiltrate the breast tissue and confine the tissue growth. An increase of metabolic activity related to glucose consumption (Ryan et al., 2005) and an extensive fibroblast proliferation and production of collagen fibers, of elevated electron density, (desmoplastic reaction) increases its density, which explains the macroscopic hard consistency of the tumor and its elevated electron density. In other hand, fibroadenomas are less fibrous and the increase of its electron density is only related to cell proliferation. Table 1 compares the electron density values obtained in this work (average electron density and standard deviation SD) for each tissue type with experimental values previously reported by Al-Bahri and Spyrou (1998), Ryan et al. (2005) and Antoniassi et al. (2010) and theoretical data evaluated from the ICRU report 44 (1989), where the electron densities were obtained using elemental composition data of the breast tissues. Finally, our experimental average values are in good agreement with results obtained by Ryan et al. (2005) and Antoniassi et al. (2010) for all investigated tissue types and by Al-Bahri and Spyrou (1998) for the malignant tissues. A reasonable agreement (differences smaller than 1%) was also obtained comparing our normal adipose breast tissues values with theoretical predictions evaluated from the ICRU report 44 (1989).
4. Conclusion In this work the electron densities of normal and neoplastic human breast tissues were determined through measurements of Compton scattering using monoenergetic synchrotron radiation. The methodology was discussed being extensible to other tissues types and other monoenergetic beams, such as radioactive g-ray source beam or even a monochromatized X-ray tube beam, which would make the technique more accessible to a possible clinical application. The values of electron densities obtained in this work are in good agreement with previously published data. It is expected that the obtained results may increase the few database present in the literature contributing to a better understanding of the relationship between the breast tissues and their electron densities for using in radiology (diagnostic) and in radiotherapy (treatment) of breast cancer. Variations in electron density values for the same tissue type were verified which can be attributed either to external parameters, associated to the sampling of different patients, or to internal parameters associated to particular histological characteristic of each tissue. This work also discussed the relationship between histology of the tissues and the obtained electron densities, showing to be possible to use the technique both as a
tool for diagnosis of histologic biopsies as well as an imaging technique which will permit add spatial information of the tissue in the breast (boundaries, infiltration, shape, position, extension) to the contrast information obtained by the scattering radiation (associated with electron density) enabling the use of the technique as a complementary tool to the breast cancer diagnosis.
Acknowledgments We would like to acknowledge the support by the Brazilian agencies Fundac- a~ o de Amparo a Pesquisa do Estado de Sa~ o Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) and by the Brazilian Synchrotron Light Laboratory (LNLS). In addition, we also would like to thank the Department of Pathology of Hospital das Clı´nicas da Faculdade de Medicina de Ribeira~ o Preto, Universidade de Sa~ o Paulo, Brazil, for allow collection of the human breast samples. References Al-Bahri, J.S., Spyrou, N.M., 1998. Electron density of normal and pathological breast tissues using a compton scattering technique. Appl. Radiat. Isotopes 49, 1677–1684. Antoniassi, M., Conceic- a~ o, A.L.C., Poletti, M.E., 2010. Characterization of breast tissues using Compton scattering. Nucl. Instrum. Methods Phys. Res. A 619, 375–378. Cesareo, R., Hanson, A.L., Gigante, G.E., Pedraza, L.J., Mathaboally, S.Q.G., 1992. Interaction of keV photons with matter and new applications. Phys. Rep. 213, 117–178. Cusatis, C., Franco, M.K., Kakuno, E., Giles, C., Morelha~ o, S., Mello, V., Mazzaro, I., 1998. A versatile X-ray diffraction station at LNLS (Brazil). J. Synchrotron Radiat. 5, 491–493. Hubbell, J.H., Veigele, W.J., Briggs, E.A., Brown, R.T., Cromer, D.T., Howerton, R.J., 1975. Atomic form factors, incoherent scattering functions, and photon scattering cross sections. J. Phys. Chem. Ref. Data 4, 471–538. ICRU, 1989. Tissue Substitutes in Radiation Dosimetry and Measurement. International Commission on Radiation Units & Measurements, Bethesda. Johns, P.C., Yaffe, M.J., 1987. X-ray characterisation of normal and neoplastic breast tissues. Phys. Med. Biol. 32, 675–695. Kane, P.P., 1992. Inelastic scattering of X-rays and gamma rays by inner shell electrons. Phys. Rep. 218, 67–139. Poletti, M.E., Gonc- alves, O.D., Mazzaro, I., 2002a. Coherent and incoherent scattering of 17.44 and 6.93 keV X-ray photons scattered from biological and biological-equivalent samples: Characterization of tissues. X-Ray Spectrom. 31, 57–61. Poletti, M.E., Gonc-alves, O.D., Mazzaro, I., 2002b. X-ray scattering from human breast tissues and breast-equivalent materials. Phys. Med. Biol. 47, 47–63. Poletti, M.E., Gonc-alves, O.D., Schechter, H., Mazzaro, I., 2002c. Precise evaluation of elastic differential scattering cross-sections and their uncertainties in X-ray scattering experiments. Nucl. Instrum. Methods Phys. Res. B 187, 437–446. Ryan, E.A., Farquharson, M.J., Flinton, D.M., 2005. The use of Compton scattering to differentiate between classifications of normal and diseased breast tissue. Phys. Med. Biol. 50, 3337–3348. Sharaf, J.M., 2001. Practical aspects of Compton scatter densitometry. Appl. Radiat. Isotopes 54, 801–809. Tomal, A., Mazarro, I., Kakuno, E.M., Poletti, M.E., 2010. Experimental determination of linear attenuation coefficient of normal, benign and malignant breast tissues. Radiat. Meas. 45, 1055–1059. Woodard, H.Q., White, D.R., 1986. The composition of body tissues. Br. J. Radiol. 59, 1209–1218.
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Study of electron densities of normal and neoplastic human breast tissues by Compton scattering using synchrotron radiation M. Antoniassi, A.L.C. Conceic- a~ o, M.E. Poletti n ~ Preto, Sao ~ Paulo, Brazil Departamento de Fı´sica—Faculdade de Filosofia Ciˆencias e Letras de Ribeira~ o Preto—Universidade de Sa~ o Paulo, Ribeirao
a r t i c l e i n f o
abstract
Available online 10 January 2012
Electron densities of 33 samples of normal (adipose and fibroglangular) and neoplastic (benign and malignant) human breast tissues were determined through Compton scattering data using a monochromatic synchrotron radiation source and an energy dispersive detector. The area of Compton peaks was used to determine the electron densities of the samples. Adipose tissue exhibits the lowest values of electron density whereas malignant tissue the highest. The relationship with their histology was discussed. Comparison with previous results showed differences smaller than 4%. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Compton scattering Electron density Breast cancer Mammography
1. Introduction The Compton scattered radiation has been widely reported as an important tool for characterization of materials since it allows assessing their electron densities (Cesareo et al., 1992; Sharaf, 2001). Specifically for breast tissue, the Compton scattering technique has been applied by some authors. Al-Bahri and Spyrou (1998) used a monoenergetic source (Am241) and a high purity germanium detector in a scattering angle of 901 to obtain the electron densities of normal and pathological breast samples of nine breast cancer patients. In this study a water sample was used to calibrate the system and the electron densities were determined neglecting all possible corrections (background subtraction, multiple scattering and attenuation effects). Ryan et al. (2005) measured the electron densities of 44 breast samples of four different classifications: adipose, malignant, fibroadenoma and fibrocystic change, using a polienergetic beam from a tungsten target industrial X-rays unit and a scattering angle of 301. The system was calibrated using solutions of known electron density and their method considered background and attenuation corrections. More recently, Antoniassi et al. (2010) determined the electron densities of 109 samples of breast tissues, composed of normal (adipose and fibrous), benign (fibroadenoma) and malignant (carcinoma) neoplasias using the Ka line from a molybdenum industrial X-ray tube and a scattering angle of 901. The system was calibrated using standard reference materials, liquids and solids, with similar properties to breast tissues. In the present work, the electron densities of normal and neoplastic breast tissues were determined through measurements of Compton scattering photons from a monoenergetic synchrotron radiation source. The advantages of the use of this type of source are
high intensity, natural collimation, linear polarization and energy tunability, features that allow time reduction and minimal spectral correction, reducing the experimental uncertainties. The distributions of electron densities were presented and correlated with histological characteristics of each tissue type. The results were compared to previously published data for normal adipose (both theoretical and experimental) and neoplastic breast tissues (only experimental).
2. Materials and method 2.1. Samples The breast tissue samples were obtained from surgical removal of the entire breast (mastectomy) or plastic surgery for breast reduction (mastoplasty) at the Hospital das Clı´nicas da Faculdade de Medicina de Ribeira~ o Preto, Universidade de Sa~ o Paulo, Ribeira~ o Preto, Brazil, whose collection and handling of these fresh samples were performed in compliance with the requirements established by the local ethical committee. Upon removal, the samples were stored at room temperature in formalin (4% formaldehyde in water) in order to preserve the structures within the tissues such as cells walls and nuclei which are needed for accurate histopathological analysis. A total of 33 samples were histopathologically classified, being 15 normal adipose breast tissues and 18 neoplastic tissues comprising 6 fibroadenomas (benign tissues) and 12 invasive ductal carcinomas (malignant tissues). 2.2. Experimental arrangement
n
Corresponding author. Tel.: þ55 16 3602 4442; fax: þ 55 16 3602 4887. E-mail address: [email protected] (M.E. Poletti).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2012.01.001
The experiments were performed at the D12A-XRD-1 beamline in the Laborato´rio Nacional de Luz Sı´ncrotron in Campinas,
1352
M. Antoniassi et al. / Applied Radiation and Isotopes 70 (2012) 1351–1354
Fig. 1. Experimental setup.
Brazil (Cusatis et al., 1998). The experimental setup is exhibited in Fig. 1. A double-crystal Si(111) monochromator was used to provide ˚ with an energy an X-ray beam energy of 14 keV (l ¼0.8856 A, bandwidth of Dl/l 10 4) and a beam size of (5.0 mm 1.0 mm) on the sample. During measurements, the samples were positioned inside an acrylic sample holder (15 mm 10 mm 4 mm) and covered by thin kapton foils (0.125 mm thickness) at both sides. The sample holder was positioned into the Huber diffractometer assembled on a rotative table and a scattering angle of 1501 (corresponding to a momentum transfer x ¼1.09 A˚ 1) was fixed during the experiment. In order to minimize the angular spread of the scattering beam, a collimation system was placed in front of the detector, being composed of a cylindrical collimator housing of 30 mm-length and 14 mm-external radius made of stainless steel and two Tungsten (W) collimator disks 2 mm-thick with a 3 mm hole placed at the beginning and the end of the collimator housing. The scattering photons (coherent and incoherent) were detected in reflection mode using a detector of Si-Pin XR-100CR by AmpTek (145 eV of energetic resolution in 5.9 keV) coupled with a multichannel analyzer and controlled by a main computer. The sample to detector distance was fixed at 210 mm and the measurement counting time (around 300 s for adipose tissues and 550 s for neoplastic tissues) was chosen in order to provide a statistical uncertainty in peak area of less than 1% for the tissues samples. An ionization chamber was used for monitoring the incident beam in order to permit the normalization of the measurements due to the diminution of the electron current in the storage ring. 2.3. Data collection For each sample, three datasets were collected from different regions of the sample to minimize the influence of a possible sample inhomogeneity. After measured the scattered intensity by each sample, additional measurements were performed in order to quantify the contributions from every other spurious scattering sources (air, kapton foil and sample holder) from the original data. The corrected data, Ninc, was obtained from Ninc ¼ Ntot Si T i Ni , where Ntot is the total photon counting measured, Ni represents each spurious scattering counting, and Ti is the appropriate transmission factor to be applied to each Ni, as described by Poletti et al. (2002c). 2.4. Basis of the experimental model The number of photons single incoherently (Compton) scattered (Ninc ) from the material at a scattering angle y, can be written in the
form (Kane, 1992): ds ds Ninc ðyÞ ¼ N 0 nat DOdet eVA ¼ N 0 nat Sðx,ZÞ DOdet eVA, dO inc dO KN ð1Þ where N0 is the number of incident photons per unit area; nat is the number of atoms by volume; ðds=dOÞ inc corresponds to the mean differential inelastic cross-section for the range of scattering angle which can be written as ðds=dOÞ inc ¼ Sðx,ZÞðds=dOÞ KN where S(x,Z) is the incoherent scattering function (Hubbell et al., 1975) which 1 depends on the momentum transfer, x, defined as x ¼ l sinðy=2Þ (where l is the wavelength of the incident photon) and the atomic number Z; ðds=dOÞ KN is the well-known Klein-Nishina differential scattering cross section averaged for the range of scattering angle considered; the term DOdet is the mean solid angle subtended by the detector; e is the detector efficiency at the scattering energy; V is the scatterer volume and A is the attenuation factor given in a R general form as: A ¼ ð1=VÞ V eðm1 l1 þ m2 l2 Þ dV, where eðm1 l1 þ m2 l2 Þ is the attenuation factor through the distances l1 from surface to the elemental scattering volume (dV) in the sample and l2 from this element to the surface of the sample in the direction of the detector, being m1 and m2 the linear attenuation coefficients for incident and scattered energy respectively. Writing nat ¼ re =Z, we can rewrite Eq. (1) as: S ds Ninc ðyÞ ¼ N 0 re DOdet eVA ð2Þ Z dO KN It is convenient to eliminate N0, DOdet and e and simultaneously express (S/Z) and A in terms of suitable ratios. For this purpose, it is needed to use a reference material for this simplification. In this experience, water was chosen for presenting similar scattering properties to breast tissues (Al-Bahri and Spyrou, 1998; Poletti et al., 2002b) and because its electron density is known. Thus, from the ratio between the number of incoherent scattered photons by tissue sample (Ntissue inc ) and by water (Nw inc ), using the Eq. (2) and further manipulation, yields the ratio of the electron density of the tissue, rtissue e , to the electron density of water, rw e , as:
rtissue N tissue ðS=ZÞw Aw e ¼ inc , w tissue tissue re Nw A inc ðS=ZÞ
ð3Þ
where the (S/Z) values for compounds and mixtures (water and tissues) were obtained using the independent atomic model (IAM) approximation (Poletti et al., 2002b): P ðo =M i ÞSi ðxÞ ð4Þ ðS=ZÞ ¼ P i i i ðoi =M i ÞZ i ðxÞ
M. Antoniassi et al. / Applied Radiation and Isotopes 70 (2012) 1351–1354
1353
It is worth noting that the accuracy of this method is limited by the accuracy of the quantities ½S=Zw =½S=Ztissue and Aw =Atissue . The ½S=Zw =½S=Ztissue ratio, at x ¼1.09 A˚ 1, was calculated using mean chemical composition of adipose (Woodard and White, 1986) and neoplastic (Poletti et al., 2002a) breast tissues. For adipose tissue the ½S=Zw =½S=Zadipose ratio was 0.980,while the ½S=Zw =½S=Zneoplastic ratio, for neoplastic tissue, was 0.994. For both tissue type the uncertainty of ½S=Zw =½S=Ztissue values around the adopted value (mean value) was evaluated from the variation range of the chemical composition present in the literature (Woodard and White, 1986; Poletti et al., 2002a) and it was lesser than 0.5%. The attenuation factors Aw and Atissue were calculated using an in-house-developed Monte Carlo code considering just geometrical conditions (reflection geometry), without introducing any other physical constraint when sampling the photon paths during the scattering process. The linear attenuation coefficients for the incident and scattering energies used as sources in the code were obtained experimentally for each breast tissue and for water during the experiment. The experimental uncertainties associated with the values of electron densities have been estimated by error propagation in Eq. (3), assuming all variables uncorrelated. Then, the statistical uncertainties related to Ninc (Eq. (3)) was from 1% to 1.6%, while the statistical uncertainties arising from simulation code (including the experimental uncertainties in the linear attenuation coefficients) was between 1% and 1.5% and the uncertainty arising from calibration process was estimated in 2.1%, yielding a final uncertainty ranging from 4.1% to 5.0%.
3. Results and discussion
Fig. 2. (a) Average processed spectra in arbitrary unity (a.u.) for each tissue type and (b) corrected following Eq. (3).
Fig. 2a shows the average processed spectra of normal (adipose) and neoplastic (benign and malignant) human breast tissues and water. For each spectrum, two predominant features can be observed, as previously mentioned: coherent scattering photopeak (14 keV) and incoherent scattering photo-peak (13.3 keV). Considering that these spectra have information about attenuation, associated with the linear attenuation coefficient, and electron density of the tissues, it is clearly observed that neoplastic breast tissues (benign and malignant) and water spectra are similar, while normal adipose tissue spectrum shows higher intensity mainly due to the smaller attenuation of radiation inside of this tissue (Johns and Yaffe, 1987; Tomal et al., 2010). The differences of linear attenuation coefficient (m) and consequently of attenuation factor (A) of adipose (m adipose ¼ 1:23 cm1 , A adipose ¼ 0:45) and neoplastic (m neoplastic ¼ 2:09 cm1 , A neoplastic ¼ 0:26) tissues are significant at the experimental energy of 14 keV. In this way the attenuation w
adipose
w
neoplastic
¼ 0:60 and A =A ¼ 1:03Þ correction Aw =Atissue (A =A for each sample, must be taken in account to determine the electron densities of the tissues. Fig. 2b shows these spectra relative to water (including all correction factors present in Eq. (3)), in which the intensity of the incoherent photo-peak is due basically to the electron density of these tissues. Fig. 3 shows a box and whisker plot of the distribution of electron density obtained in this work. The thick line inside the boxes shows the median of each tissue type. The inter-quartile range is shown by the box and encloses the 25th to the 75th percentile. The whiskers contain all values within 1.5 box lengths. From this figure it is possible to observe that electron density distributions are broad, exhibiting overlapping values between the different types of breast tissues. The variations of electron densities obtained for each tissue type are expected, since these samples belonged to different
Fig. 3. Box plot of the electron density results for each tissue type.
patients, then it can be attributed either to external parameters (associated to the sampling of different patients) such as diet, medication, environment, age, hormonal status, genetics, or to internal parameters associated to particular histological characteristic of each tissue type. The results also show that adipose breast tissue present the smallest electron density values when compared with neoplastic breast tissues. It is well-known that normal adipose tissue is composed principally by cells specialized in storing lipids (triacylglycerols and cholesterol ester), called adipocytes, rich in carbon, presenting a low density, while neoplastic tissues (benign
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Table 1 Summary of electron density values for breast tissue present in literature and in this work. Tissue
Normal Adipose Benign Malignant
Experimental
Theoretical
Al-Bahri and Spyrou 1998
Ryan et al. 2005
Antoniassi et al. 2010
This work
ICRU report 44
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
re (SD) 1023 (e/cm3)
3.46 NM 3.56
3.24 (0.14) 3.31 (0.14) 3.53 (0.15)
3.24 (0.16) 3.47 (0.12) 3.60 (0.17)
3.21 (0.14) 3.39 (0.17) 3.48 (0.15)
3.18 NA NA
NM¼ not measured. NA¼ not available.
and malignant) are characterized, basically, by an excessive proliferation of cells constricted in the stroma, formed by conjunctive tissue (rich in oxygen) of higher density. In the particular case of malignant tissue, the cells of carcinoma form solid cords which infiltrate the breast tissue and confine the tissue growth. An increase of metabolic activity related to glucose consumption (Ryan et al., 2005) and an extensive fibroblast proliferation and production of collagen fibers, of elevated electron density, (desmoplastic reaction) increases its density, which explains the macroscopic hard consistency of the tumor and its elevated electron density. In other hand, fibroadenomas are less fibrous and the increase of its electron density is only related to cell proliferation. Table 1 compares the electron density values obtained in this work (average electron density and standard deviation SD) for each tissue type with experimental values previously reported by Al-Bahri and Spyrou (1998), Ryan et al. (2005) and Antoniassi et al. (2010) and theoretical data evaluated from the ICRU report 44 (1989), where the electron densities were obtained using elemental composition data of the breast tissues. Finally, our experimental average values are in good agreement with results obtained by Ryan et al. (2005) and Antoniassi et al. (2010) for all investigated tissue types and by Al-Bahri and Spyrou (1998) for the malignant tissues. A reasonable agreement (differences smaller than 1%) was also obtained comparing our normal adipose breast tissues values with theoretical predictions evaluated from the ICRU report 44 (1989).
4. Conclusion In this work the electron densities of normal and neoplastic human breast tissues were determined through measurements of Compton scattering using monoenergetic synchrotron radiation. The methodology was discussed being extensible to other tissues types and other monoenergetic beams, such as radioactive g-ray source beam or even a monochromatized X-ray tube beam, which would make the technique more accessible to a possible clinical application. The values of electron densities obtained in this work are in good agreement with previously published data. It is expected that the obtained results may increase the few database present in the literature contributing to a better understanding of the relationship between the breast tissues and their electron densities for using in radiology (diagnostic) and in radiotherapy (treatment) of breast cancer. Variations in electron density values for the same tissue type were verified which can be attributed either to external parameters, associated to the sampling of different patients, or to internal parameters associated to particular histological characteristic of each tissue. This work also discussed the relationship between histology of the tissues and the obtained electron densities, showing to be possible to use the technique both as a
tool for diagnosis of histologic biopsies as well as an imaging technique which will permit add spatial information of the tissue in the breast (boundaries, infiltration, shape, position, extension) to the contrast information obtained by the scattering radiation (associated with electron density) enabling the use of the technique as a complementary tool to the breast cancer diagnosis.
Acknowledgments We would like to acknowledge the support by the Brazilian agencies Fundac- a~ o de Amparo a Pesquisa do Estado de Sa~ o Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) and by the Brazilian Synchrotron Light Laboratory (LNLS). In addition, we also would like to thank the Department of Pathology of Hospital das Clı´nicas da Faculdade de Medicina de Ribeira~ o Preto, Universidade de Sa~ o Paulo, Brazil, for allow collection of the human breast samples. References Al-Bahri, J.S., Spyrou, N.M., 1998. Electron density of normal and pathological breast tissues using a compton scattering technique. Appl. Radiat. Isotopes 49, 1677–1684. Antoniassi, M., Conceic- a~ o, A.L.C., Poletti, M.E., 2010. Characterization of breast tissues using Compton scattering. Nucl. Instrum. Methods Phys. Res. A 619, 375–378. Cesareo, R., Hanson, A.L., Gigante, G.E., Pedraza, L.J., Mathaboally, S.Q.G., 1992. Interaction of keV photons with matter and new applications. Phys. Rep. 213, 117–178. Cusatis, C., Franco, M.K., Kakuno, E., Giles, C., Morelha~ o, S., Mello, V., Mazzaro, I., 1998. A versatile X-ray diffraction station at LNLS (Brazil). J. Synchrotron Radiat. 5, 491–493. Hubbell, J.H., Veigele, W.J., Briggs, E.A., Brown, R.T., Cromer, D.T., Howerton, R.J., 1975. Atomic form factors, incoherent scattering functions, and photon scattering cross sections. J. Phys. Chem. Ref. Data 4, 471–538. ICRU, 1989. Tissue Substitutes in Radiation Dosimetry and Measurement. International Commission on Radiation Units & Measurements, Bethesda. Johns, P.C., Yaffe, M.J., 1987. X-ray characterisation of normal and neoplastic breast tissues. Phys. Med. Biol. 32, 675–695. Kane, P.P., 1992. Inelastic scattering of X-rays and gamma rays by inner shell electrons. Phys. Rep. 218, 67–139. Poletti, M.E., Gonc- alves, O.D., Mazzaro, I., 2002a. Coherent and incoherent scattering of 17.44 and 6.93 keV X-ray photons scattered from biological and biological-equivalent samples: Characterization of tissues. X-Ray Spectrom. 31, 57–61. Poletti, M.E., Gonc-alves, O.D., Mazzaro, I., 2002b. X-ray scattering from human breast tissues and breast-equivalent materials. Phys. Med. Biol. 47, 47–63. Poletti, M.E., Gonc-alves, O.D., Schechter, H., Mazzaro, I., 2002c. Precise evaluation of elastic differential scattering cross-sections and their uncertainties in X-ray scattering experiments. Nucl. Instrum. Methods Phys. Res. B 187, 437–446. Ryan, E.A., Farquharson, M.J., Flinton, D.M., 2005. The use of Compton scattering to differentiate between classifications of normal and diseased breast tissue. Phys. Med. Biol. 50, 3337–3348. Sharaf, J.M., 2001. Practical aspects of Compton scatter densitometry. Appl. Radiat. Isotopes 54, 801–809. Tomal, A., Mazarro, I., Kakuno, E.M., Poletti, M.E., 2010. Experimental determination of linear attenuation coefficient of normal, benign and malignant breast tissues. Radiat. Meas. 45, 1055–1059. Woodard, H.Q., White, D.R., 1986. The composition of body tissues. Br. J. Radiol. 59, 1209–1218.