Comparison of radiation shielding requirements for HDR brachytherapy using Yb 169and Ir 192 sourcesG. Lymperopoulou, P. Papagiannis, L. Sakelliou, E. Georgiou, C. J. Hourdakis, and D. Baltas Citation: Medical Physics 33, 2541 (2006); doi: 10.1118/1.2208940 View online: http://dx.doi.org/10.1118/1.2208940 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/33/7?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Comparison of organ doses for patients undergoing balloon brachytherapy of the breast with HDR I 192 r orelectronic sources using Monte Carlo simulations in a heterogeneous human phantoma) Med. Phys. 37, 662 (2010); 10.1118/1.3292292 The use of directional interstitial sources to improve dosimetry in breast brachytherapy Med. Phys. 35, 240 (2008); 10.1118/1.2815623 A dosimetric comparison of Yb 169 and Ir 192 for HDR brachytherapy of the breast, accounting for the effect offinite patient dimensions and tissue inhomogeneities Med. Phys. 33, 4583 (2006); 10.1118/1.2392408 A dosimetric comparison of Yb 169 versus Ir 192 for HDR prostate brachytherapy Med. Phys. 32, 3832 (2005); 10.1118/1.2126821 A Monte Carlo dosimetry study of vaginal Ir 192 brachytherapy applications with a shielded cylindrical applicatorset Med. Phys. 31, 3080 (2004); 10.1118/1.1810233

Comparison of radiation shielding requirements for HDR brachytherapyusing 169Yb and 192Ir sources

G. Lymperopouloua� and P. Papagiannisb�

Nuclear and Particle Physics Section, Physics Department, University of Athens, Panepistimioupolis, Ilisia,157 71, Athens, Greece

L. SakelliouDosimetry Laboratory, Institute of Accelerating Systems and Applications (IASA),P.O. Box 17214 GR-10024, Athens, Greece

E. GeorgiouMedical Physics Department, Medical School, University of Athens, 75 Mikras Asias,11527, Athens, Greece

C. J. HourdakisGreek Atomic Energy Commission, Licensing & Inspection Department, Agia Paraskeui 15310,Athens, Greece

D. BaltasDepartment of Medical Physics & Engineering, Strahlenklinik, Klinikum Offenbach,63069 Offenbach, Germanyand Nuclear and Particle Physics Section, Physics Department, University of Athens, Panepistimioupolis,Ilisia, 157 71 Athens, Greece

�Received 8 January 2006; revised 8 May 2006; accepted for publication 8 May 2006;published 22 June 2006�169Yb has received a renewed focus lately as an alternative to 192Ir sources for high dose rate �HDR�brachytherapy. Following the results of a recent work by our group which proved 169Yb to be agood candidate for HDR prostate brachytherapy, this work seeks to quantify the radiation shieldingrequirements for 169Yb HDR brachytherapy applications in comparison to the corresponding re-quirements for the current 192Ir HDR brachytherapy standard. Monte Carlo simulation �MC� is usedto obtain 169Yb and 192Ir broad beam transmission data through lead and concrete. Results are fittedto an analytical equation which can be used to readily calculate the barrier thickness required toachieve a given dose rate reduction. Shielding requirements for a HDR brachytherapy treatmentroom facility are presented as a function of distance, occupancy, dose limit, and facility workload,using analytical calculations for both 169Yb and 192Ir HDR sources. The barrier thickness requiredfor 169Yb is lower than that for 192Ir by a factor of 4–5 for lead and 1.5–2 for concrete. Regarding169Yb HDR brachytherapy applications, the lead shielding requirements do not exceed 15 mm, evenin highly conservative case scenarios. This allows for the construction of a lead door in most cases,thus avoiding the construction of a space consuming, specially designed maze. The effects of sourcestructure, attenuation by the patient, and scatter conditions within an actual treatment room on theabove-noted findings are also discussed using corresponding MC simulation results. © 2006American Association of Physicists in Medicine. �DOI: 10.1118/1.2208940�

I. INTRODUCTION169Yb is an intermediate photon energy emitter �main emis-sions are in the range of 50–300 keV and the emission prob-ability weighted mean energy is 93 keV� with a half life of32.02 days. It has been extensively studied in the past decademainly as an alternative to 125I and 103Pd low dose rate seeds.Lately it has received renewed focus as a new high dose rate�HDR� brachytherapy source mainly due to the advantages itprovides in terms of radiation protection and shielding re-quirements in comparison to the 192Ir HDR sources.1,2

In our recent work aiming to evaluate 169Yb for use inprostate HDR brachytherapy,1 Monte Carlo �MC� simulationwas used to fully characterize the dosimetric properties of ahypothetical 169Yb source bearing the same design as thenew microSelectron 192Ir source, except for its radioactivecore, which was assumed to consist of pure 169Yb metal. A

comprehensive analysis of different prostate implants utiliz-ing the multiobjective optimization engine of the SWIFT™treatment planning system showed that 169Yb HDR sourcescould be at least as effective as 192Ir HDR sources in prostatebrachytherapy.

Given that the production of an 169Yb source of compa-rable air kerma strength with that of the currently utilized192Ir HDR sources is technically feasible1 and that the MCdosimetry of a new 169Yb HDR source has already beenreported in the literature,2 the aim of this work is to obtainthe basic data required for the design of 169Yb and 192Ir HDRbrachytherapy treatment room facilities using the same MCsimulation code. Results are presented in the form of broadbeam transmission data through lead and concrete as well asin the form of an analytical expression for the calculation oflead and concrete barrier thicknesses needed for any combi-

2541 2541Med. Phys. 33 „7…, July 2006 0094-2405/2006/33„7…/2541/7/$23.00 © 2006 Am. Assoc. Phys. Med.

nation of distance, dose limit, occupancy, and facility work-load. Particular examples are also given to allow for thecomparison of radiation shielding requirements for 169Yb and192Ir HDR brachytherapy treatment room facilities.

The potential effect of source structure, attenuation withinthe patient, and scatter conditions within an actual treatmentroom on radiation shielding calculations using data presentedherein are also evaluated using MC simulation.

II. MATERIALS AND METHODS

A. Monte Carlo simulations

The MCNPX code version 2.4.0 �Ref. 3� was used for MCsimulations in this work. The initially emitted photon spec-trum for 169Yb and 192Ir were taken from the NuDatdatabase,4 and nonpenetrating photons of energy lower than10 keV were not taken into account. The photon simulationcut off energy was set to 5 keV.

For broad beam transmission calculations the geometrysimulated consisted of an 169Yb or 192Ir bare point sourcelocated 100 cm away from the center of a 5 m�5 m lead��=11.35 g cm−3� or concrete ��=2.3 g cm−3� wall barrierwith a varying thickness. The assumed percentage elementalcomposition by weight for concrete was as follows: 2.2% H,0.3% C, 57.5% O, 1.5% Na, 0.1% Mg, 2.0% Al, 30.5% Si,1.0% K, 4.3% Ca, and 0.6% Fe.5 The air kerma rate after thebarrier was scored using spherical air voxels of 10 cm radiuscentered 30 cm away from the barrier. The number of photonhistories initiated was adjusted to achieve statistical uncer-tainties below 1% for transmission results of B�10−2. Maxi-mum statistical uncertainties �corresponding to the simula-tions of the largest barrier thicknesses� were on the order of5%.

In order to asses the influence of attenuation and scatter-ing for a real source structure within a patient on shieldingcalculations, the new microSelectron HDR source designwas employed. Separate MC simulations were performed forthe actual new 192Ir microSelectron source as well as for ahypothetical source of the same design bearing a radioactivecore of pure 169Yb metal.1 Each source was situated 100 cmaway from a lead barrier of thickness equal to the broadbeam half value layer �HVL� calculated for the correspond-ing bare point source. In view of the dose rate anisotropyaround real brachytherapy source designs, the sources weremodeled with their transverse bisector perpendicular to the

barrier to ensure that the maximum dose rate was incident onthe barrier. The transmission through the lead barrier as wellas the photon spectrum before and after the lead barrier werescored in four separate MC simulations: two with eachsource in free space and two with each source centered in a15 cm radius, spherical water phantom.

Additionally, in order to ascertain that radiation shieldingcalculations are not affected by scattering conditions withinreal room geometries, MC simulations were performed withan 169Yb point source centered in a room of 5 m by 7 m witha height of 4 m and the floor and the ceiling consisting ofordinary concrete slabs of 15 cm thickness. Two separateruns were performed; one with the room walls consisting ofone TVL of lead and one with the room walls consisting ofone TVL of concrete, as these were calculated by broadbeam transmission data. These wall thicknesses were se-lected to ensure full scatter conditions within the room, whilebeing reasonably small.

B. Analytical calculations

Transmission results, B, versus lead or concrete thickness,x, obtained by MC simulations for 169Yb and 192Ir bare pointsources in broad beam geometries were fitted using the equa-tion originally proposed by Archer et al.6

B = ��1 +�

�� · e��x −

�

��−1/�

, �1�

where �, �, and � are free parameters to be determined bythe fit.

This equation facilitates the direct calculation of the bar-rier thickness, x, needed to reduce the transmission to a re-quired value B according to

x =1

��· ln

B−� + ��/��1 + ��/��

. �2�

The factor R�=B−1� by which a shielding barrier shouldreduce the dose to a point of interest lying at distance d �m�from the radiation source, located in an area of occupancy T,in order to reach a shielding design goal P ��Gy week−1�, forthe workload W ��Gy week−1� of a given HDR brachy-therapy facility, can be calculated using the following ana-lytical formalism:7

TABLE I. Annual dose limits, E, and corresponding shielding design goals, P, recommended by NCRP 147report �Ref. 7� and EUROATOM directive 96/29 �Ref. 15�.

Area

NCRP 147 EUROATOM directive 96/29

Dose limitE, mSv year−1

Shieldingdesign goal

P, �Gy week−1Dose limit

E, mSv year−1

Shieldingdesign goal

P, �Gy week−1

Controlled 5 100 20 200Supervised ¯ ¯ 6 60

Non controlled 1 20 1 10

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Medical Physics, Vol. 33, No. 7, July 2006

R =WT

Pd2 =NKpT

Pd2 , �3�

where N is the average number of patients treated per weekand Kp equals the product of the air kerma rate at 1 m dis-tance from the source by the fraction of the treatment timeper patient for which the source is outside its housing. Kp isequivalent to the quantity TRAK �Total Reference Air Kermarate� of the ICRU 58 report.8

The air kerma strength, SK, of an 192Ir HDR source upondelivery is on the order of 4�104 �Gy m2 h−1 �370 GBqapparent activity�. For this SK in prostate HDR brachy-therapy, which constitutes a multi-catheter/multi-sourcedwell position application, the sum of source dwell timesneeded to deliver a total dose of 950 cGy to the PTV surfaceis on the order of 5 min for typical prostate volumes�20–50 cm3�.1 In this work, a conservative assumption of10 min per patient is used to account for higher fractiondoses �e.g., boost therapy� or large PTV volumes.9–14 Hence,the Kp value employed for 192Ir calculations, Kp�192Ir�, in Eq.�3� is 6700 �Gy m2 per patient.

In our previous work,1 it was found that an 169Yb activityof 1166 GBq is required to achieve the same air kermastrength as that of a 370 GBq 192Ir HDR source with the

same design. Since this order of activity is technicallyfeasible1 and given that the same treatment time is requiredfor 169Yb and 192Ir HDR sources of equal SK, it is assumed inthis work that Kp�169Yb� equals Kp�192Ir�.

The shielding design goals, P, in Eq. �3� are levels of airkerma rate at a point behind a shielding barrier which ensurethat the annual values for effective dose, E �also referred toas annual dose limit�, for occupationally exposed individualsor members of the public are not exceeded. The annual doselimits recommended by the EURATOM directive 96/29�Ref. 15� and the NCRP 147 �Ref. 7� report are presented inTable I along with the corresponding shielding design goals,P, expressed in �Gy week−1 for the aforementioned work-

TABLE II. Fitting parameters �, �, and � determined by Eq. �1� for broadbeam transmission curves in lead and concrete, for 169Yb and 192Ir bare pointsources.

� �cm−1� � �cm−1� �

Lead 169Yb 0.4113 3.337 1.085192Ir 0.1234 0.164 3 0.6257

Concrete 169Yb 0.2005 0.037 81 1.809192Ir 0.1642 −0.088 82 1.267

FIG. 1. MC calculated, broad beam transmission, B, for bare point 169Yb and 192Ir sources through �a� lead and �b� concrete, plotted vs lead and concretebarrier thickness, x. Solid lines correspond to fitting results using Eq. �1�.

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load of a brachytherapy treatment room facility. In Table I, itshould be noted that the P values recommended by theEURATOM directive 96/2915 incorporate a constraint of halfthe established dose limits, E.

III. RESULTS AND DISCUSSION

A. Transmission data for lead and concrete

MC calculated, broad beam transmission results throughlead and concrete for 192Ir and 169Yb bare point sources arepresented in Figs. 1�a� and 1�b�, respectively. The data pre-

sented in these figures were fitted using Eq. �1� and results ofthe parameters �, �, and � are given in Table II.

Using results in Table II with Eq. �2� allows for the cal-culation of the successive HVLs in lead and concrete for the169Yb and 192Ir bare point sources. Corresponding calcula-tions are plotted in Figs. 2�a� and 2�b� and depict the depen-dence of the photon beam quality on the thickness of shield-ing material traversed. For lead �Fig. 2�a��, photoelectricabsorption is the main interaction process for photons of en-ergy lower than 600 keV. Thus, a severe hardening of boththe 169Yb and 192Ir photon spectra occurs with the increase of

FIG. 2. Successive HVLs for 169Yb and 192Ir bare point sources in �a� lead and �b� concrete. Successive TVLs for 169Yb and 192Ir bare point sources in �c� leadand �d� concrete. Dotted lines denote the corresponding equilibrium half- and tenth-value layers �HVLe and TVLe�.

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lead thickness traversed by the photon beam, which is mani-fested as a gradual increase of the successive HVL thickness.This effect is more pronounced for the 169Yb source, sincethis source has relatively more low energy photons than 192Ir.For concrete �Fig. 2�b��, the main interaction process for thephoton energies emitted by 169Yb and 192Ir sources is inco-herent scattering. This leads to the gradual build up of ascattered, lower energy, photon component of the broadbeam spectra traveling through the concrete barrier. As aresult of this average photon energy degradation, a gradualreduction of the successive HVL thickness for the 192Ir barepoint source can be observed.

Table III presents the first HVL and TVL in lead andconcrete for both the 169Yb and 192Ir bare point sources alongwith the corresponding hard or equilibrium HVL and TVL�HVLe and TVLe�. The latter values correspond to that pen-etrating region where the radiation directional and spectraldistributions are practically independent of thickness, so thata single value of the HVL or TVL is valid,19 and can becalculated by

HVLe =ln 2

�, TVLe =

ln 10

�. �4�

In lead, “equilibrium” occurs at approximately 10 mm thick-ness for the 169Yb spectrum and 60 mm for the 192Ir spec-trum �see also Figs. 2�a� and 2�c��. Corresponding thick-nesses in concrete are approximately 10 cm for 169Yb and25 cm for 192Ir �see also Figs. 2�b� and 2�d��. In Table III,first as well as equilibrium HVL and TVL results available inthe literature are also shown and they agree very well withthe results of this work.

Data presented in Table III and Fig. 2 suggest that if leadshielding calculations are performed using the required num-ber of TVLs instead of the broad beam transmission data inthe form of Eq. �2�, significant differences could occur. Forexample, using three TVLe of lead to achieve a reduction ofR=103 �B=10−3� for 169Yb would lead to an overestimationof lead shielding requirements by 30% while use of threefirst TVLs would lead to a corresponding underestimation by60%. For concrete, however, where no severe alteration in

the spectrum characteristics is observed for 169Yb, the firstTVL does not differ significantly from the TVLe. Hence,calculations based on the number of TVLe required for agiven R constitutes a safe approach that overestimates shield-ing requirements only slightly.

B. Shielding requirements

Transmission and HVL results presented in the previoussection readily delineate the advantage of 169Yb relative to192Ir HDR sources in terms of radiation shielding, as ex-pected by the average energy of their emitted photon spectra�the emission probability weighted, average energy of 169Yband 192Ir are 93 and 355 keV, respectively�. In order to quan-tify the difference in shielding requirements of a HDR treat-ment room facility analytical calculations were performedusing Eq. �3�.

Lead shielding requirements for an 169Yb and an 192Ir barepoint HDR source are plotted in Fig. 3�a� for any combina-tion of patients treated per week, N, distance to the source, d,occupancy factor, T, and shielding goal, P �see Sec. II B�.The assumption of Kp=6700 �Gy week−1 in these calcula-tions does not undermine their general applicability, sinceany Kp� assumption can be used by weighting the x axis val-ues in Fig. 3�a� by the ratio Kp� /Kp. In Fig. 3�a�, comparisonof the two curves for 169Yb and 192Ir shows that for the sameworkload assumption, lead shielding requirements are about4–5 times lower for the 169Yb source. Corresponding resultsfor concrete are plotted in Fig. 3�b�, and 169Yb can be seen torequire about 1.5–2 times less shielding compared to 192Ir.

Figure 4 presents some indicative examples of 169Ybshielding requirements for a supervised area �e.g., controlstation, P=60 �Gy week−1� and a noncontrolled area �P=10 �Gy week−1�. Lead shielding requirements �Fig. 4�a��and concrete shielding requirements �Fig. 4�b�� are plottedversus the facility workload, assuming that Kp�169Yb�=6700 �Gy m2 per patient, for selected distances of d=4, 5,and 7 m. These findings suggest that besides the significantreduction in shielding thickness requirements, use of 169YbHDR sources could eliminate the need for a maze design,which is commonly necessitated by the higher energy of192Ir, in order to obviate an unpractical heavy lead shieldingof the room door.

C. Source structure, patient, and room scatter effects

Broad beam transmission data and shielding calculationspresented in the previous sections could be sensitive to at-tenuation and scattering within real source structures and thepatient geometry. MC simulations for an 169Yb point sourceand an 169Yb source of similar design to that of the newmicroSelectron yielded air kerma rate results of 0.043 and0.030 mGy h−1 MBq−1, respectively.1 Corresponding resultsin a water sphere of 15 cm radius simulating a patient geom-etry showed that the most important effect to be consideredis kerma attenuation within the patient. This attenuation wasfound to be slightly increased for the real source design rela-

TABLE III. 192Ir and 169Yb HVL and TVL values in lead and concrete ob-tained by MC calculated transmission data. Corresponding results availablein the literature are also presented for comparison.

First HVL HVLe First TVL TVLe

Lead thickness �mm�192Ir 2.8 5.7 11.0 18.7

3 �Ref. 16� 6 �Ref. 17� 12 �Ref. 16� 20 �Ref. 17�169Yb 0.25 1.6 1.8 5.3

0.23 �Ref. 18� ¯ 1.8 �Ref. 18� ¯

�1 �Ref. 16� ¯ 2 �Ref. 16� ¯

Concrete thickness �cm�192Ir 6.5 4.2 17.6 14.1

¯ 4.3 �Ref. 17� ¯ 14.7 �Ref. 17�169Yb 3.2 3.4 10.6 11.4

2.7 �Ref. 18� ¯ 10.4 �Ref. 18� ¯

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Medical Physics, Vol. 33, No. 7, July 2006

tive to the point source �56% vs 52%� and in close agreementwith the corresponding results of Granero et al.18 calculatedfor a bare point 169Yb source.

Apart from kerma attenuation, MC simulations showedthat the hardening of the photon spectrum incident on ashielding barrier by the source structure is counterbalancedby the corresponding softening due to multiple Comptonscattering in the patient. Hence, shielding calculations of areal 169Yb HDR source within a patient using transmission

results presented herein for a bare point source constitutes asafe assumption �i.e., the first HVL remains the same whilethe first TVL reduces from 1.8 mm lead to 1.65 mm�.

MC simulations for the 192Ir microSelectron source withina 15 cm radius water sphere showed a corresponding reduc-tion of shielding requirements. This does not however under-mine the validity of the conclusions drawn in the previoussections for the comparison of analytically calculated shield-ing requirements for 169Yb and 192Ir sources.

FIG. 3. The lead thickness in mm �a� and concrete thickness in cm �b� required to reduce the dose rate of 169Yb and 192Ir HDR sources to a shielding designgoal P, plotted as a function of NT/Pd2. P is in �Gy week−1 �see Table I�, N is the number of patients treated per week and d is the distance of the sourceto the occupied area in m.

FIG. 4. The lead thickness in mm �a� and concrete thickness in cm �b� required to reduce the dose rate of an 169Yb HDR source for a supervised area �P=60 �Gy week−1 solid lines� and a noncontrolled area �P=10 �Gy week−1 dotted lines� plotted vs the total number of patients treated per week �N� by theoccupancy factor �T� of the area for selected source—area distances.

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MC simulation results for the 169Yb source in the 5 m�7 m room geometry yielded approximately 25% increasedair kerma rate values inside the lead-walled room. The cor-responding increase inside the concrete-walled room was onthe order of 55%. However, air kerma rate results beyond theroom walls were found to have excellent agreement with theprevious results for a single wall with one TVL �first TVL�thickness with the same source air kerma strength. This isexplained by the combined effect of the degraded energy ofthe scattered photons reaching each wall barrier and the rela-tive importance of the photoelectric effect for the energyspectrum of this source. Lower energy, scattered photons arereadily absorbed in the walls and they only increase the airkerma rates inside the room.

IV. CONCLUSIONS

The broad beam transmission data through lead and con-crete barriers were calculated and compared for 169Yb and192Ir bare point sources, and an analytical expression wasprovided to readily calculate the barrier thickness requiredfor any given dose rate reduction. Lead or concrete wallthickness were analytically computed for any combination ofworkload, distance, occupancy factor, and shielding designgoal for HDR brachytherapy employing 169Yb and 192Irsources. The required lead shielding thickness for 169Yb is 4to 5 times lower than that for 192Ir. Given that the maximumlead thickness requirement for 169Yb HDR brachytherapydoes not exceed 15 mm even in highly conservative sce-narios, the construction of a space consuming, specially de-signed maze could be avoided by building a lead door inmost cases. The concrete shielding thickness requirementsfor 169Yb are 1.5–2 times lower than that for 192Ir. Bare pointsource broad beam transmission data results are not signifi-cantly affected by the combined effect of attenuation andscatter within the actual source design structures and the pa-tient geometry. Therefore, shielding calculation results usingthese data for the design of 169Yb HDR brachytherapy treat-ment room facilities constitute a safe approach.

ACKNOWLEDGMENT

This work was supported by a PYTHAGORAS II re-search grant by the Greek Ministry of National Educationand Religious Affairs within the framework of the EPEAEKII program.

a�Electronic mail: glymper@phys.uoa.grb�Author to whom all correspondence should be addressed. Electronic

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