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ARTICLE
Year : 2011  |  Volume : 34  |  Issue : 3  |  Page : 166-171  

Bremsstrahlung dose of strontium-89 in therapy application


1 Department of Physics, Government College for women, Kolar, India
2 Shravana, PC Extension, Kolar, India
3 Department of Physics, Bangalore University, Bangalore, Karnataka, India

Date of Web Publication27-Sep-2012

Correspondence Address:
H C Manjunatha
Department of Physics, Government College for women, Kolar
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.101700

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  Abstract 

There has been an increased interest in strontium-89 (Sr-89) therapy, which emits relatively high-energy (1.495 MeV) beta rays. The production in vivo bremsstrahlung radiation sufficient for external imaging and this radiation hazard warrants evaluation. The bremsstrahlung (secondary radiation) of Sr-89 has been traditionally ignored in internal dosimetry calculation. We have estimated the bremsstrahlung dose of Sr-89 source in the muscle and bone to body the various body organs (such as adrenals, brain, breasts, gallbladder wall, LLI wall, small intestine, stomach, ULI wall, heart wall, kidneys, liver, lungs, muscle, ovaries, pancreas, red marrow, bone surfaces, skin, spleen, testes, thymus, thyroid, urine bladder wall, uterus, fetus, placenta, and total body) from distributed sources of Sr-89 in the muscle and bone. In the present study, muscle and bone is considered as source organs. The bremsstrahlung dose of Sr-89 source in a muscle is less than that of cortical bone. In both muscle and bone medium, bremsstrahlung dose decreases with distance. These estimated values show that the bremsstrahlung radiation absorbed dose contribution from an organ to itself is very small, but contribution to other organs is not always negligible especially when large amounts of Sr-89 may be involved as in therapy applications.

Keywords: Strontium-89, bremsstrahlung dose, radiation safety


How to cite this article:
Manjunatha H C, Sankarshan B M, Chandrika B M, Rudraswamy B. Bremsstrahlung dose of strontium-89 in therapy application. Radiat Prot Environ 2011;34:166-71

How to cite this URL:
Manjunatha H C, Sankarshan B M, Chandrika B M, Rudraswamy B. Bremsstrahlung dose of strontium-89 in therapy application. Radiat Prot Environ [serial online] 2011 [cited 2019 Sep 21];34:166-71. Available from: http://www.rpe.org.in/text.asp?2011/34/3/166/101700


  1. Introduction Top


Uchiyama et al., (1997) [1] reported that strontium-89 (Sr-89) is being widely used as a palliative treatment for patients with bone pain caused by bone metastases. The mechanism of pain felt by patients with bony metastases is unclear. However, there appears to be a mechanical as well as biological component. Biological pain is due to the release of cytokines and chemical mediators, regional tissue and nerve irritation, and the effect of tumor mass within the bone. Mechanical pain is attributed to diminished bone density. There is a great need for palliation of pain. This will enhance the quality of life and facilitate health care and the rehabilitation process (Ebrahim et al, 2002). [2] The radio nuclide such as Sr-89 is also been successfully and effectively utilized to provide palliative therapy to patients with multifocal skeletal metastatic lesions in cases of breast and prostatic cancers. The half-life (T 1/2 ) of a radionuclide will determine the initial dose rate and, therefore, the total amount of radioactivity to be administered. A higher initial dose rate may result in more effective cell killing, but the therapeutic ratio of malignant cell destruction to normal cell recovery may be less. Too long T 1/2 creates obvious problems in environmental safety in case of spill or early death of the patient. A very short T 1/2 is problematic so far as shipping and shelf life are concerned. It also requires a larger total of administered activity, which increases the radiation dose to personnel and family members and may require some hospitalization, thus increasing cost. On the other hand, repetitive doses may be given at shorter intervals making it possible to titrate dose to response (Srivastava, 2004). [3]

Strontium-89 is a pure beta-emitting radionuclide with a half-life of 50.6 days. The maximum beta energy is high and penetration (average) of Sr-89 in soft tissue is 2.4 mm. The long physical half-life means that low administered activity is given resulting in a rather low initial dose rate. Nevertheless, it has proven effective (McEwan, 1994). [4] It has a high affinity for all sites of active osteogenesis is preferentially incorporated into multiple sites and has long-term retention in metastatic sites. The biological half-life is about four to five days and there is evidence that 20% of Sr-89 remains in the body (bone matrix) after 90 days. It limits the possibility of repeat doses until much after the initial dosing. Hence Sr-89 is used in the therapy for multiple osseous metastases of prostate and breast cancer (Ebrahim et al., 2002 and Robinson et al.,1989). [2],[3],[4],[5]

National Council on Radiation Protection and Measurement (NCRP) model (1970) [6] explains the procedure of dose assessment of therapeutic nuclides. This model incorporates the contributions of only penetrating radiations (i.e., photons such as x- and g-rays). However, a therapeutic radionuclide should emit principally nonpenetrating radiations such as beta particles. Zanzonico et al. (1999) [7] extended this for point source beta emitters. The bremsstrahlung component (secondary radiations) of beta emitters has been traditionally ignored in internal dosimetry calculations. The radiation therapy needs experimental studies on the dose of bremsstrahlung component of beta emitters in tissues. But these experiments are very difficult to undertake and analyze, since many biochemical processes are taking place at the same time, competing with radiation effects. The resulting hazard of bremsstrahlung radiation released during beta therapy, may therefore be some of concern, at least theoretically, and should be systematically evaluated. In the present study, we have estimated the radiation dose from the bremsstrahlung component of Sr-89 beta emitter distributed in the muscle and cortical bone to various body organs. The calculations employed the accurate energy absorption buildup factors and the bremsstrahlung photon spectrum. This estimated spectrum is accurate because it is based on more accurate Zmod , Seltzer-Berger data where an electron-electron interaction is also included.


  2. Present Work Top


The computations of bremsstrahlung radiation dose have been divided into three parts, which are as follows:

2.1 Estimation of bremsstrahlung cross section

Markowicz et al., (1984) [8] proposed an expression for modified atomic number (Zmod ) of compound target defined for bremsstrahlung process to take into account the self-absorption of bremsstrahlung and electron back scattering,



Here, W i, A i, and Z i are atomic weight, weight fraction, and atomic number of ith element, respectively. The six elements whose atomic numbers adjacent to that of muscle (Zmod = 5.8742) chosen are Li, Be, B, C, N, and O and their Z values are 3, 4, 5, 6, 7, and 8, respectively. Zmod is evaluated using Eq. (1) and their composition. Muscle is assumed to have a composition as H 63 C 6 O 28 N. The bremsstrahlung cross section for muscle is evaluated using Lagrange's interpolation technique, Seltzer-Berger (1986) [9] theoretical bremsstrahlung cross-section data given for elements using the following expression:



where lower case z is the atomic number of the element of known bremsstrahlung cross section σz adjacent to the modified atomic number (Zmod ) of the compound whose bremsstrahlung cross section σZ mod is desired, and upper case Z are atomic numbers of other elements of known bremsstrahlung cross-section adjacent to Zmod . The estimated σZ mod (milli barn/MeV) is used for evaluation of spectrum. The evaluated σZ mod as a function of electron energy is shown in [Figure 1] and [Figure 2].
Figure 1: Variation of Bremsstrahlung cross section with photon energy in muscle

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Figure 2: Variation of Bremsstrahlung cross section with photon energy in cortical bone

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2.2 Evaluation of bremsstrahlung spectrum

The number n(T,k) of bremsstrahlung photons of energy k when all of the incident electron energy T completely absorbed in thick target is given by Bethe and Heitler (1934) [10] is



where σ(E, k) is bremsstrahlung cross section at photon energy k and electron energy E, N is the number of atoms per unit volume of target, and E is the energy of an electron available for an interaction with nucleus of the thick target after it undergoes a loss of energy per unit length (−dE/dx). For a beta emitter with end point energy T max , spectral distribution of bremsstrahlung photons (S (k)) is given by



where P(T) is the beta spectrum. Evaluated results of σ(E, k) of Eq. (2) and tabulated values of (−dE/dx) of Seltzer-Berger (1986) [9] data are used to get S(k) for the target compounds.

2.3 Evaluation of bremsstrahlung dose

We used the following expression (Manjunatha et al., 2011a) [11] for the calculation of specific absorbed fraction of energy at distance x from the point source monoenergetic photon emitter



Here μ en is linear absorption coefficient of photons of given energy, μ is linear attenuation coefficient of photons of given energy, Ben is energy absorption buildup factor; r is density of the medium. The energy absorption buildup factors in bone have been computed using geometric progression fitting method (Manjunatha et al., 2011b). [12] The values of μ en and μ of photons have been taken from Hubbel (1982). [13] The specific absorbed fraction for a given beta source was estimated by integrating over the entire bremsstrahlung spectrum



where T max is the maximum energy of beta. Estimation of the value of F allows calculation of the absorbed dose at fixed distances from the point source in the infinite, homogeneous medium



where D(x) is the absorbed dose at distance x per unit initial activity (Gy/MBq); t is the residence time of activity; and Δ is the mean energy emitted per unit cumulated activity and it is numerically equal to (2.13 n i E i), where n i is the frequency of occurrence of emissions with energy E i; the quantities n and E are provided by the calculated bremsstrahlung spectrum using Eq. (4). We have estimated D(x) between x = 0.01 and x = 10 mm, through complete decay of a Sr-89 beta source. After obtaining the absorbed dose at a number of chosen distances from the source, we have also plotted the calculated estimates of absorbed dose per unit initial activity as a function of distance from the point source. We then developed S-values (Snyder et al., 1975) [14] for Sr-89 bemsstrahlung emissions for activity uniformly distributed throughout the muscle and cortical bone of the standard reference male phantom (Stabin Michael et al., 2002) [15] by folding the bremsstrahlung spectrum over the specific absorbed fractions (Stabin Michael et al., 2002) as a function of energy for these source regions.


  3. Results and Discussion Top


The evaluated bremsstrahlung spectra employed in the calculations are shown in [Figure 3]. The variation of specific absorbed fractions of energy (F) with photon energy in cortical bone and muscle is as shown in [Figure 4] and [Figure 5]. The specific absorbed fraction of energy (F) increases up to 0.1 MeV and then decreases. The variation of F with energy is due to dominance of photoelectric absorption in the lower end and dominance of pair production in the higher photon energy region. During the calculation of F values thickness of penetration depth is considered up to 40 mean free paths. The estimated beta-induced bremsstrahlung dose per unit activity (in Gy/MBq) of a Sr-89 point source in a muscle and cortical bone medium, through complete decay is as shown in [Figure 6]. It is also given in [Table 1]. We find that the bremsstrahlung dose at about 0.01 mm from Sr-89 source in a muscle would be 0.9 Gy/MBq and that of bone is 18.7 Gy/MBq.
Table 1: Bremsstrahlung absorbed dose (in Gy/MBq) near a point source of Strontium-89

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Figure 3: The evaluated Bremsstrahlung spectra [S(k)] of 89Sr expressed as number of photons per moc2 per beta

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Figure 4: The variation of specific fraction of absorbed energy (Φ ) with incident photon energy in bone at penetration depth=40mfp

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Figure 5: The variation of specific fraction of absorbed energy (Φ ) with incident photon energy in muscle at penetration depth=40mfp

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Figure 6: Bremsstrahlunag beta dose per unit activity (in Gy/MBq) of a 89Sr point source in a muscle and cartical bone medium, through complete decay

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The bremsstrahlung dose of Sr-89 source in a muscle is less than that of cortical bone because it depends on modified atomic number (Zmod ) of the target medium (Zmod of muscle is less than that of cortical bone). In both muscle and bone medium, bremsstrahlung dose decreases with distance. Bremsstrahlung dose depends on the specific absorbed fraction of energy (F) of the target medium. The specific absorbed fraction of energy (F) also increases up to the Epe and then decreases. Here, Epe is the energy value at which the photoelectric interaction coefficients match with Compton interaction coefficients for a given material. For bone and muscle, Epe is almost equal to 0.1 MeV. The variation of F with energy is due to dominance of photoelectric absorption in the lower end and dominance of pair production in the higher photon energy region. In the lower energy end, photoelectric absorption is dominant photon interaction process; hence, F values minimum. As the energy of incident photon increases, Compton scattering overtakes the photoelectric absorption. It results multiple Compton scattering events, which increases the value of F up to the Epe and becomes maximum at Epe . Thereafter (above Epe ), pair production starts dominating (absorption process) which reduces the value of F to minimum. Sr-89 emits relatively high-energy (1.495 MeV) beta particles, so that it produces high-energy photons (<1.495 MeV), hence dose of Sr-89 induced bremsstrahlung dose decreases with distance.

The estimated S-values for bremsstrahlung dose (mGy/MBq-hr) to various target organs from a uniform source of Sr-89 in the muscle and cortical bone are given in [Table 2]. We have computed the total equivalent dose due to sum of all other decays of Sr-89 using radiation tool box (2004) [16] and compared this with the equivalent dose due to bremsstrahlung radiation [Table 3]. These estimated values show that the bremsstrahlung dose contribution from an organ to itself is very small, but contribution to other organs is not always negligible especially when large amounts of Sr-89 may be involved as in therapy applications. For example, when using Sr-89-strontium chloride for the palliation of bone pain, the standard administrated is 5 × 10 3 MBq-hr. Then it would result in a bremsstrahlung dose to the gallbladder of ~0.878 mGy (0.087 rad), to the adrenals of 0.642 mGy (0.064 rad), to the pancreas of 10.17 mGy (1.017 rad), and to the kidneys of 2.5 mGy (0.25 rad), assuming no other significant sources of exposure. Bremsstrahlung dose depends on the radionuclide kinetics in a given situation; however, all contributions to total dose should be considered in therapy applications.
Table 2: S-values for Bremsstrahlung dose (mGy/MBq-hr) to various target organs from a uniform source of strontium-89 in the muscle and cortical bone

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Table 3: Comparision of the total equivalent dose (Sv/Bq) due to sum of all other decays of 89Sr using radiation tool box (2004) with the dose due to Bremsstrahlung radiation

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  4. Conclusion Top


Calculated S-values of bremsstrahlung radiation fromSr-89 have been compared with S-values of beta and all other radiations of same source. These estimated values show that the bremsstrahlung radiation absorbed dose contribution from an organ to itself is very small. But contribution to other organs is not always negligible especially when large amounts of Sr-89 may be involved as in therapy applications.

 
  References Top

1.Uchiyama, Mayuki, Narita Hiroto RT., Makino, Motoji, Sekine, Hiroshi, Mori, Yataka, Fukumitsu, Nobuyoshi, Kawakami, Kenji., Strontium-89 therapy and imaging with bremsstrahlung in bone metastases. Clin. Nucl. Med. 1997;22:605-9.   Back to cited text no. 1
    
2.Ebrahim Ashayeri, Adedamola Omogbehin, Rajagopalan Sridhar, Ravi A. Shankar., strontium-89 in the treatment of pain due to dise osseous metastases: a university hospital experience, Journal of the national medical association 2002; 94:706-11.  Back to cited text no. 2
    
3.Srivastava SC., Treatment of bone and joint pain with electron emitting Radiopharmaceuticals, Indian journal of nuclear medicine 2004;19:89-97  Back to cited text no. 3
    
4.McEwan AJB., Radiopharmaceuticals for palliative treatment of painful bone metastases, New Perspec. Cancer. Diagn.Management 1994;1:24-31.  Back to cited text no. 4
    
5.Robinson RG, Spicer JA, Blake GM, Martin NL, Preston DF, Wegst AV, McEwan AJ, Ackery DM, Strontium-89 treatment results and kinetics in patients with painful metastatic prostate and breast cancer in bone, RadioGraphics 1989;9:271-81.   Back to cited text no. 5
    
6.National Council on Radiation Protection and Measurements. Precautions in the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides. NCRP Report No. 37. Bethesda, MD: National Council on RadiationProtection and Measurement; 1970.  Back to cited text no. 6
    
7.Zanzonico P B, Binkert, Barbara L, Goldsmith, Stanley J, Bremsstrahlung exposure from pure beta emitters, J. Nucl. Med. 1999;40:1024-8  Back to cited text no. 7
    
8.Markowicz AA., VanGriken RE., Composition dependence of Bremsstrahlung background in electron-probe X-ray microanalysis. Anal. Chem. 1984;56:2049-51.  Back to cited text no. 8
    
9.Seltzer SM., Berger MJ., Bremsstrahlung energy spectra from electrons with kinetic energy 1keV to 10GeV incident on screened nuclei and orbital electrons of neutral atoms With Z=1-100. At. Data Nucl. Data Tables 1986;35:345-418  Back to cited text no. 9
    
10.Bethe, H., Heitler, W., On the stopping of fast particles and on the creation of positive electrons. Proc R Soc Lond A. 1934;146:83-112.   Back to cited text no. 10
    
11.Manjunatha HC, Rudraswamy B, A study of thickness and penetration depth dependence of specific absorbed fraction of energy in bone,Annals of Nuclear Energy. 2011;38:2271-82  Back to cited text no. 11
    
12.Manjunatha HC, Rudraswamy B , Computation of exposure build-up factors in teeth, Rad. Phy. Chem. 2011;80:14-21.  Back to cited text no. 12
    
13.Hubbell JH, Photon mass attenuation and energy-absorption, Int. J. Appl. Rad. and Isot. 1982;33:1269-1310.  Back to cited text no. 13
    
14.Snyder W, Ford M, Warner G, Watson S., MIRD Pamphlet No. 11, New York: Society of Nuclear Medicine; 1975.  Back to cited text no. 14
    
15.Stabin Michael G, da Luz, Lydia C.Q.P., Decay Data for internal and external dose assessment, Health Physics 2002;83:471-5.  Back to cited text no. 15
    
16.Radiation tool box ORNL/TM-2004/27R1  Back to cited text no. 16
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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Radiation Effects and Defects in Solids. 2018; : 1
[Pubmed] | [DOI]



 

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Abstract
1. Introduction
2. Present Work
3. Results and D...
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