Monte Carlo dosimetry of a new [sup 192]Ir high dose rate brachytherapy source

Monte Carlo dosimetry of a new 192Ir high dose rate brachytherapy sourceA. Angelopoulos,a) P. Baras, and L. SakelliouNuclear and Particle Physics Section, Physics Department, University of Athens, Panepistimioupolis, Ilisia,157 71 Athens, Greece

P. Karaiskos and P. SandilosDepartment of Radiology, Medical School, University of Athens, Areteion Hospital, 76 Vas. Sofias Avenue,115 28 Athens, Greece and Medical Physics Department, Hygeia Hospital, Kiffisias Avenue,24 Erythroy Stavrou, Marousi, 151 23 Athens, Greece

~Received 13 March 2000; accepted for publication 14 August 2000!

This work provides full dosimetric data for a new high-strength192Ir source currently launched byVarian Oncology Systems for use in their high dose rate remote afterloading systems. The activecore length of the new source is reduced to 5 mm compared to a value of 10 mm for the existingVariSource source design, with all other geometric source and encapsulation details being similar.Dose-rate constant, radial dose functions, geometry factors, and anisotropy functions, utilized in theAAPM Task Group 43 dose calculation formalism, were calculated using Monte Carlo simulation.Results are compared with corresponding data published for the existing VariSource andmicroSelectron high dose rate sources. The dose-rate constant for the new Varian source was foundto be equal to 1.10160.006 cGy h21 U21, compared to values of 1.04360.005 and 1.11660.006 cGy h21 U21 calculated for the existing VariSource and microSelectron sources, respec-tively. The radial dose functions between the three sources are similar with the exception of theirvalues at radial distances very close to the source (r'2 mm) where differences of;3% areobserved. The new Varian source demonstrates a smaller anisotropy relative to the existing Vari-Source source design for polar angles close to the source longitudinal axis, due to its smaller activecore length. ©2000 American Association of Physicists in Medicine.@S0094-2405~00!00511-3#

Key words: Monte Carlo, dosimetry, VariSource, microSelectron,192Ir, HDR, brachytherapy

ig

as

rc

llim

reosncsrlollw

oanth

to

is

34-d-

end

bley

moxi-aeo-head

sk

al

I. INTRODUCTION

Most of the commercially available high dose rate~HDR!remote afterloading systems utilize a single cylindrical, hstrength192Ir source. Varian Oncology Systems~Palo Alto,CA! currently launches a new192Ir source design~new Vari-Source wire model VS2000, referred to in the followingthe new Varian source! for their HDR remote afterloadingsystems. Compared to the existing VariSource HDR sou~referred to in the following as VariSource!, the new sourceis shorter in active core length~5 mm instead of 10 mm!,aiming to provide increased flexibility and cycle life, with aother geometric source and encapsulation details being slar.

The clinical use of a HDR brachytherapy source requiaccurate determination of all relevant dosimetric data. Drate distributions around such a source depend significaon the source and encapsulation geometric characteristi

In this work we used our well-established Monte Casimulation code1–4 to derive accurate calculations of the fudosimetric characteristics of the new Varian source, folloing the formalism introduced by AAPM TG 43.5 Dose rateconstant, radial dose functions, geometry factors, and danisotropy functions were calculated for the new sourcecompared with corresponding data we have published forVariSource4 and microSelectron3 ~Nucletron Corporation,The Netherlands! HDR sources, which have been foundbe in good agreement with similar studies.6–9

2521 Med. Phys. 27 „11…, November 2000 0094-2405 Õ2000Õ2

h

e

i-

se

tly.

-

sede

II. MATERIALS AND METHODS

A. Radioactive source

A schematic diagram of the new Varian HDR sourcepresented in Fig. 1~a!. According to the data obtained fromthe manufacturer, the active source consists of two, 0.mm-diam, 2.5-mm-long cylinders with semispherical enings. These two seeds are made of pure iridium metal~den-sity 22.42 g/cm3! within which the radioactive material isuniformly distributed. The source is encapsulated at theof an '260-cm-long titanium/nickel wire cylinder of 0.59mm in outer diameter, of which 150 cm are actually usalength, with a composition of 44.4% Ti and 55.6% Ni bweight ~density 6.5 g/cm3!. The encapsulation extends 1 mbeyond the distal end of the active core and can be apprmated by a 0.59-mm-diam, 0.705-mm-long cylinder withsemispherical end of 0.295 mm radius. The above-given gmetric details are similar to those of the VariSource, with texception of the total active core length being 5 mm insteof 10 mm.

B. Dose calculation formalism

The dose calculation formalism proposed by AAPM TaGroup 43 has been followed.5 The dose rate,D(r ,u), atpoint (r ,u) in the medium, wherer is the distance in centi-meters from the active source center andu is the polar anglerelative to the positive direction of the source’s longitudinaxis, is expressed as

25217„11…Õ2521Õ7Õ$17.00 © 2000 Am. Assoc. Phys. Med.

o

rtde

oiatur

u

a

,

theb-

-rialss,

re.ho-

erlyof

asedossihe

ntoatert-

is

e-

d inde

thisthe

g inatces

erw

el

nd.th,heeri-als

-al

to

om

2522 Angelopoulos et al. : Monte Carlo dosimetry 2522

D~r ,u!5SkLG~r ,u!

G~1 cm,p/2!g~r !F~r ,u!, ~1!

whereSk is the air kerma strength of the source in unitsU~1U51 mGy m2 h2151 cGy cm2 h21!.

L is the dose rate constant defined as

L5D~1 cm,p/2!/Sk . ~2!

G(r ,u) is the geometry factor, defined as

G~r !5Ev

r~r 8!dV8

ur 82r u2, ~3!

where r(r 8) is the radioactive density at the pointr(r 8)5r(x8,y8,z8) within the source,V denotes integration ovethe active source volume,dV8 being the volume elemenlocated atr 8, and r denotes the point of interest locateoutside the source. For brachytherapy sources the geomfactor can be approximated5 by

G~r ,u!5H 1/r 2 for point source approximation

b

Lr sinufor line approximation

~4!

with L being the active length of the source, andb the anglesubtended by the active source with respect to the p(r ,u). The use of the point source approximation to calculthe geometry factors of an elongated brachytherapy sodesign is valid only for radial distancesr .2L while forsmaller radial distances the line source approximation shobe utilized.10

g(r ) is the dimensionless radial dose function, defined

g~r !5D~r ,p/2!G~1 cm,p/2!

D~1 cm,p/2!,G~r ,p/2!~5!

andF(r ,u) is the dimensionless dose anisotropy function

F~r ,u!5D~r ,u!G~r ,p/2!

D~r ,p/2!G~r ,u!. ~6!

FIG. 1. ~a! Schematic diagram of the new Varian HDR192Ir brachytherapysource and~b! the coordinate system used in the simulation.

Medical Physics, Vol. 27, No. 11, November 2000

f

try

ntece

ld

s

C. Monte Carlo simulation code

We used our Monte Carlo~MC! particle-transport simu-lation code to derive all necessary dosimetric data fornew Varian HDR source. The MC code, which is well estalished and experimentally verified,1–4 incorporates the detailed active core and encapsulation geometry and mateof the investigated192Ir source design. The primary photonwith a spectrum taken from Glasgow and Dillman,11 are pro-duced isotropically and uniformly through the source coThe transport and interactions of primary and scattered ptons are sampled stochastically according to the propnormalized probabilities for the dominating processesphotoabsorption, coherent and incoherent scattering, bon a set of self-consistent total, partial, and differential crsections12–16 and the procedure outlined by Chan and Do17

for the sampling of coherent and incoherent scattering. Telectron binding energy of the scattering atom is taken iaccount in the incoherent scattering process. The liquid wform factors of Morin18 were not found to have any detecable influence in the calculations, since for192Ir the coherentto incoherent interactions ratio in a bounded water mediumsmall ~0.03 for a 30-cm-diam water phantom!. A photon his-tory is terminated if the photon’s energy falls below a slected cutoff value~in this work 10 keV! or if its spatialcoordinates lie outside the boundaries of the phantom usethe simulation. Further detail and analysis on the MC cocan be found elsewhere.1

Transport of secondary electrons was not simulated instudy and the collision kerma was used to approximateabsorbed dose.8,9,19Most recent work demonstrates20 that notonly secondary electrons but also beta particles originatinthe 192Ir source core contribute significantly to the doseshort distances from the source. However, for distangreater than 1 mm from the source~for which the results inthis study refer!, water-kerma well approximates dose.

In this study, the192Ir source was positioned at the centof a 30-cm-diam spherical liquid water phantom to allocomparison with similar studies.8,9,19 The coordinate systemused in the simulation is shown in Fig. 1~b!; the origin Ocoincides with the center of the radioactive core~between thetwo seeds! and the positivey axis points to the source drivwire side. The polar angleu50° is defined at the proxima~drive wire! side of the source, while the polar angleu5180° corresponds to the side of the source’s distal eThe Ti/Ni wire was not simulated in its entire usable lengbut only within the 30-cm-diam spherical phantom. Tphantom sphere was divided into discrete concentric sphcal shells of 1 mm thickness, each split into angular intervof 1° both with respect to polar angleu~0, p! and azimuthalangle w ~0, 2p!. However, as all dosimetric quantities involved in this study are isotropic with respect to azimuthangle w, this three-dimensional~3D! segmentation can besimplified to a two-dimensional one, only with respectcoordinatesr ~ranging from 1 to 150 mm, in 1 mm intervals!andu ~ranging from 0° to 180°, in 1° intervals!. In this waythe phantom was divided in a large number (1503180527 000) of scoring cells. The above-mentioned phant

ingyaap

ss-s

ll

Dutea

rs0

pa

ceis-iste

ue

ulaula

i--.

ta

esred

nceer,, aof

ce,p-

l

he

re-ata

insteere

ntlyilar

ns.

01

e ,re


configuration allows the versatility of scoring either withthe volume of the cell~when the code is used to score enerdeposition! or through the surface element of the individucell, as is the case in this study where water kerma wcalculated by weighting the photon energy fluence, at a scific r, u surface element, with the corresponding maenergy absorption coefficient.16 The direct scoring of waterkerma used here, is void of binning artifacts as thoobserved in the energy binning procedure21 and results in a3D histogram containing all 27 000 individual, (r ,u), pointwater-kerma values. Thus, 150 polar dose profiles as we180 radial dose profiles, for any radiusr and/or polar angleuin the range discussed earlier, may be directly obtained.to the fine calculation grid used, a large number of simulahistories is required in order to alleviate statistical fluctutions within scoring elements. In this work 53108 primaryphotons were individually tracked, yielding statistical errofor the derived data of less than 1% for polar angles 1,u,170°, and 3% for polar anglesu,10° andu.170°.These errors are not actually dependent on distancer sincethe scoring surface element increases liker 2 and for192Ir inwater, scattering compensates for absorption.

The air-kerma strength of the sourceSk , needed for thecalculations outlined in Sec. II B, was evaluated in two serate MC simulations.

~a! Simulation in free space: In an infinite vacuum spaair-kerma ratesK(d) were calculated for transverse-axis dtances,d, ranging from 2 to 30 cm in 1 cm intervals. In thdistance range, the new Varian source is well approximaby a point source and the strength,Sk , was specified as

Sk5K~d!d2. ~7!

The maximum deviation observed between individual valof Sk was 0.4% of their mean value~their standard deviationwas found to be equal to 0.15% of the mean! confirming theaccuracy of the scoring procedure with regard to the simtion of geometry. The uncertainty assigned to these calctions is their statistical error estimated as 0.5% of the me

~b! Simulation in dry air: In a dry air sphere of 5 m dameter, air-kerma ratesK(d) were calculated for transverseaxis distances,d, ranging from 2 to 100 cm in 1 cm intervalsThe source strengthSk was obtained by fitting these daaccording to the linear equation:8,19

K~d!d25Sk1ad ~8!

TABLE I. Dose rate constantsL for the new Varian, the VariSource, and thmicroSelectron192Ir HDR brachytherapy sources.

Source design L ~cGy h21 U21!

New Variana 1.10160.006VariSourceb 1.04360.005microSelectronc 1.11660.006

aThis study.bReference 5.cReference 4.


lse--

e

as

ed-

°

-

,

d

s

-a-n.

where the slopea describes the deviation of air-kerma ratK(d) from inverse-square law due to the buildup of scattephotons in air.

In the above-mentioned calculations, x-ray fluorescewas not modeled in the simulations. Williamson, howevwas the first to observe, on the basis of MC calculationssignificant impact of x-ray fluorescence on the dosimetry125I seeds.22 Recently, Borg and Rogers reported on a;0.2%contribution ofK-shell x-ray fluorescence21 to the air-kermastrength of the microSelectron-HDR192Ir source. In order tohave an estimation of this effect on the new Varian sourwe simulatedK-shell fluorescence, following photoabsortion only within the192Ir active core, adopting a weighted~interms of relative intensities! average energy for alK-fluorescence photonsKa,b566.296 keV. This contribu-tion was found to be less than 0.35%, thus falling within tstatistical error of 0.5%.

III. RESULTS AND DISCUSSION

Dosimetric quantities of the new Varian source are psented in this section, in comparison with corresponding dfor the VariSource4 and the microSelectron.3 Results for thelast two sources have been cross-checked agaexperimental6,7 and MC8,9 data of other authors and havbeen found to be in good agreement. It must be noted hthat for the microSelectron two source designs are currein use, model Nos. 080950 and 105.002, which have simgeometric and dosimetric characteristics.19 In this work mi-croSelectron model No. 080950 is used in all compariso

A. Dose rate constant

The dose rate constantL for the new Varian HDR brachy-therapy source was found to be equal to (1.1

TABLE II. Radial dose functions,g(r ), for the new Varian, the VariSourceand the microSelectron192Ir HDR brachytherapy sources. Calculations adone with the sources centered at a spherical water phantom ofd530 cm indiameter.

r~cm!

g(r )

New Varian VariSource microSelectron

0.1 0.975 0.950 0.9900.2 0.985 0.969 0.9930.3 0.990 0.981 0.9940.5 0.995 0.992 0.9960.7 0.998 0.995 0.9981.0 1.000 1.000 1.0001.5 1.002 1.004 1.0032.0 1.005 1.006 1.0042.5 1.006 1.007 1.0043.0 1.006 1.004 1.0054.0 1.002 1.002 1.0005.0 0.993 0.993 0.9916.0 0.981 0.980 0.9798.0 0.941 0.941 0.940

10.0 0.881 0.882 0.88012.0 0.803 0.802 0.80014.0 0.693 0.694 0.69215.0 0.609 0.609 0.608

han

th

u

l toetry

ce-

d

al

n

al-

s (seof

se

la


60.006) cGy h21 U21 and is presented in Table I, along witthe corresponding values calculated for the VariSourcemicroSelectron HDR sources.3,4 The value ofL for the newVarian source is 5.5% higher than that calculated forVariSource and 1.4% lower than the value ofL derived forthe microSelectron. As expected from our previo

FIG. 2. Radial dose functionsg(r ), as a function of radial distancer. OurMC calculations for the new Varian HDR source, as well as MC calcutions for an192Ir point source are presented for comparison.


d

e

s

works,4,23 these percentage differences are almost equathe percentage differences observed between the geomfactors,G(1 cm,p/2), of the three sources, at radial distanr 51 cm and polar angleu5p/2, where the dose rate constant is defined.

The air-kerma strength,Sk , used in the above-mentionecalculations was determined by Eq.~7! as the average of theSk values in free space for transverse-axis distances,d, rang-ing from 2 to 30 cm in 1 cm intervals, and was found equto (10.2760.05) 1028 U/Bq of source activity. Calculationsin air, according to Eq.~8!, yielded results that agree withierrors@Sk5(10.2860.05) 1028 U/Bq of source activity#.

B. Radial dose functions

Our calculations of the radial dose functions,g(r ), of thenew Varian source at radial distances,r 50.1– 15 cm are pre-sented in Table II, along with the corresponding values cculated for the VariSource4 and the microSelectron,3 for thesame phantom dimensions~30 cm diameter!. It must benoted here that, at radial distances near phantom edger515 cm!, radial dose functions have lower values than thocalculated for an unbounded phantom due to the lackbackscatter.1–3 In Fig. 2 a comparison between radial dofunctions of the new Varian source and a192Ir point source,

-

rce
TABLE III. Anisotropy functions,F(r ,u), calculated for the new Varian HDR source~data not shown correspond to points within the encapsulated sou!.
Polar angle~deg!

Radial distance~cm!

0.25 0.5 1 3 5 7 10 12 15

179.5 0.564 0.530 0.550 0.616 0.663 0.720 0.736 0.728178.5 0.574 0.538 0.581 0.642 0.685 0.727 0.748 0.756177.5 0.588 0.557 0.601 0.657 0.697 0.746 0.760 0.773176.5 0.620 0.591 0.634 0.687 0.722 0.762 0.777 0.787175.5 0.646 0.624 0.663 0.706 0.736 0.778 0.790 0.796174.5 0.675 0.653 0.690 0.730 0.762 0.794 0.805 0.813172.5 0.849 0.736 0.721 0.745 0.773 0.802 0.824 0.831 0.836170.5 0.880 0.787 0.766 0.779 0.808 0.827 0.847 0.853 0.860167.5 0.910 0.837 0.816 0.821 0.841 0.856 0.876 0.882 0.883165.5 0.925 0.859 0.843 0.845 0.859 0.872 0.884 0.889 0.894160.5 0.948 0.905 0.890 0.889 0.901 0.907 0.915 0.916 0.918150.5 0.968 0.949 0.940 0.938 0.943 0.945 0.948 0.952 0.951140.5 0.983 0.970 0.966 0.965 0.968 0.968 0.971 0.970 0.972130.5 0.989 0.985 0.982 0.983 0.982 0.983 0.985 0.984 0.984110.5 0.999 0.998 0.996 0.997 0.995 0.996 0.996 0.995 0.99790.5 0.999 1.000 1.001 0.999 1.002 1.002 1.000 1.000 0.99970.5 0.998 0.995 0.995 0.995 0.998 0.997 0.995 0.996 0.99850.5 0.990 0.986 0.982 0.984 0.985 0.983 0.982 0.984 0.98440.5 0.982 0.972 0.968 0.968 0.969 0.969 0.971 0.971 0.97130.5 0.970 0.952 0.945 0.941 0.947 0.949 0.952 0.952 0.95320.5 0.950 0.910 0.894 0.896 0.902 0.910 0.918 0.922 0.92215.5 0.929 0.871 0.854 0.856 0.868 0.880 0.895 0.898 0.89712.5 0.911 0.836 0.816 0.827 0.843 0.857 0.874 0.875 0.87910.5 0.891 0.807 0.784 0.793 0.814 0.832 0.848 0.860 0.8597.5 0.844 0.741 0.711 0.725 0.762 0.789 0.808 0.825 0.8296.5 0.702 0.677 0.706 0.741 0.769 0.802 0.810 0.8195.5 0.670 0.641 0.671 0.716 0.747 0.778 0.789 0.7984.5 0.627 0.594 0.632 0.685 0.718 0.756 0.772 0.7823.5 0.549 0.593 0.646 0.687 0.731 0.753 0.7712.5 0.544 0.611 0.652 0.710 0.730 0.7361.5 0.460 0.544 0.596 0.661 0.684 0.711

t

s.


FIG. 3. Anisotropy functionsF(r ,u),as a function of polar angleu, relativeto the longitudinal axis of the source aradial distancesr 50.25, 0.5, 1, 3, 5,and 7 cm. MC calculations for the newVarian HDR source, as well as MCcalculations for the VariSource~Ref.4! and microSelectron HDR source~Ref. 3! are presented for comparison

to

va

disarth

rerc

ueDR

eth

portd

an-on-

ac-e

atue

ger

rcetion

e

g.

dis-of

e

-nted,

calculated in a separate MC run using the same phandimensions, is presented. Results confirm2,4 that source andencapsulation geometry does not significantly affect theues of the radial dose functions. Differences of;3% areobserved between radial dose function values only attances very close to the source (r'2 mm) and they are lesthan 1% for all other radial distances. Furthermore, compson of point source results of this study, derived usingspectral data of Glasgow and Dillman,11 and previouscalculations2 using the spectral data from Amersham~16gamma rays with a probability of 216.85% per decay! re-vealed that radial dose functions,g(r ), are not sensitive tospectrum details.

C. Anisotropy functions

In Table III full data for the anisotropy functionsF(r ,u)are presented for radial distancesr 50.25– 15 cm and polaanglesu50° – 180° relative to the longitudinal axis of thsource. In Fig. 3 anisotropy data for the new Varian souare presented graphically for radial distancesr 50.25, 0.5,1.0, 3.0, 5.0 and 7.0 cm, along with corresponding valpublished for the VariSource and the microSelectron Hsources, for comparison.3,4

The comparison between the anisotropy data of the nVarian source design and the VariSource reveals thatnew source demonstrates decreased anisotropy atangles close to the longitudinal axis, as a result of the sholength of its active core~5 mm for the new source compare


m

l-

s-

i-e

e

s

welarer

to 10 mm for the VariSource!. The same comparison withthe microSelectron shows that both sources have similarisotropy characteristics, with the new Varian source demstrating slightly smaller anisotropy~up to 5%! for polarangles close to the source midsection, due to its smallertive source diameter~0.34 mm for the new Varian sourccompared to 0.60 mm for the microSelectron!. Larger anisot-ropy, of up to 15%, is observed for the new Varian sourcepolar angles close to the longitudinal source axis, mainly dto the fact that the new Varian source has a slightly lonactive core relative to the microSelectron~5 mm for the newVarian source versus 3.5 mm for the microSelectron sou!and second, due to the thicker distal end of the encapsulaof the new Varian source.

The discontinuity in the distribution of the radioactivmaterial within the new Varian source, as shown atr50 cm ~Fig. 1! is not reflected in the anisotropy data of Fi3 even for the radial distance ofr 50.25 cm. This is ex-plained by the fact that this distance (r 50.25 cm) is an orderof magnitude greater than the extent of the radioactivecontinuity, which is of the order of the seeds’ diameter0.34 mm.

D. Dose rate distributions

In Fig. 4, dose rate distributions in water multiplied by thsquare of radial distancer ~cGy cm2 h21 per unit air-kermastrength!, to remove the strongr 22 dependence for comparison purposes, around the new Varian source are prese

-,e


FIG. 4. Dose rate distributionsD(r ,u)multiplied byr 2, as a function of polarangleu relative to the longitudinal axisof the source, at radial distancesr50.25, 0.5, 1, and 3 cm. MC calculations for the new Varian, VariSourceand microSelectron HDR sources arpresented for comparison.

ledi

ososointiotle

ivutht

F

ise

und

lon-ter-

ive

re-ore

alandrceula-s.t val-ces

for selected radial distances ofr 50.25, 0.5, 1.0, and 3.0 cmand polar angles in the rangeu50° – 180°. Also in Fig. 4 thecorresponding results for the VariSource and the microSetron are shown for comparison. It can be seen that for radistances comparable to the lengths of the sourcesr,2L), there are considerable differences in the polar drate profiles, which become significant at polar angles clto the longitudinal axis. This is mainly due to the fact that fthese short distances the geometry factors of the examsources, which play a predominant role in the determinaof dose rate distributions in brachytherapy, are significandifferent as a result of the differences in the sources geomric characteristics, especially with respect to their actlength. For greater radial distances, where the inverse sqlaw accurately determines geometry factors for all ofexamined sources, differences in dose rate values followdifferences observed in anisotropy functions.

Results are also presented in Cartesian coordinates in5, where dose rate distributions of all three sources~with andwithout the r 22 dependence! are given atx50.5 cm awayfrom the source as a function of distance,y, along the sourceaxis, confirming that differences are significant at short dtances and decrease as one moves away from the sourc


c-al(ee

rednyt-

eareehe

ig.

-.

This can also be seen in Fig. 6 where isodose lines arothe investigated sources are drawn on thexy plane containingthe longitudinal axis of the source@see Fig. 1~b!#. The dif-ferences observed close to the sources and/or along theirgitudinal axes are due to their different geometric characistics.

IV. CONCLUSIONS

We have used our Monte Carlo simulation code to derfull dosimetric data for the new Varian192Ir HDR brachy-therapy source. The new source design is different withspect to that of its predecessor in the length of the active c~L50.5 cm for the new Varian source compared toL51 cm for the VariSource!. The dose rate constant, radidose functions, geometry factors, anisotropy functions,dose rate distributions were calculated for the new souand the results were compared to the corresponding calctions for the VariSource and microSelectron HDR source

Percentage differences between the dose rate constanues of the three sources follow the percentage differenobserved between their geometry factors,G(1 cm,p/2) at

a-,e

FIG. 5. Dose rate distributionsD(x,y)as a function of distance,y, along thesource, atx50.5 cm away from thesource ~a! in cGy U21 and ~b! incGy U21 r 2 thus excluding the inversesquare law dependence. MC calcultions for the new Varian, VariSourceand microSelectron HDR sources arpresented for comparison.

s,heg,


FIG. 6. MC calculated isodose curvearound the VariSource, new Varianand microSelectron HDR sources witair-kerma strength of 100 U. The dosrates for the isodose curves startinfrom the outside are 20, 30, 50, 100200, and 500 cGy/h.

m

gis

ibaitti

mary-hniat

ma

is,at

is,

u-

, Ln

, Km-y

ndc-

ou

igh

G.

i-

rce

ofom-

ure

too.

r,c-ta

on

oss

ef-fort,’’

in

g

hys.

hy-

lated

s in

as,ter-nalsica

radial distancer 51 cm and polar angleu5p/2, where thedose rate constant is defined.

Differences in the geometric characteristics of the exained sources~including their encapsulations! do not seem toaffect radial dose function calculations. The decreased lenof the new Varian source design results in decreased anropy characteristics compared to the VariSource.

The significant differences between the dose rate distrtions calculated around the investigated sources, whichobserved at short radial distances and/or along their longdinal axes, are due to the different geometric characterisof these sources.

ACKNOWLEDGMENTS

The authors wish to thank T. Clark and A. Mader, froVarian Oncology Systems, for providing all the necessdata for the simulation of the new Varian HDR brachtherapy source and H. Papanikolaou for her valuable teccal assistance. This work was supported in part by VarOncology Systems and by the Special Research Accounthe University of Athens.

a!Author to whom correspondence should be addressed; [email protected]. Angelopoulos, A. Perris, K. Sakellariou, L. Sakelliou, K. Sarigiannand G. Zarris, ‘‘Accurate Monte Carlo calculations of the combinedtenuation and buildup factors, for energies~20–1500 keV! and distances~0–10 cm! relevant in brachytherapy,’’ Phys. Med. Biol.36, 763–718~1991!.

2L. Sakelliou, K. Sakellariou, K. Sarigiannis, A. Angelopoulos, A. Perrand G. Zarris, ‘‘Dose rate distributions around60Co, 137Cs, 198Au, 192Ir,241Am, 125I ~models 6702 and 6711! brachytherapy sources and the nclide 99Tcm, ’’ Phys. Med. Biol.37, 1859–1872~1992!.

3P. Karaiskos, A. Angelopoulos, L. Sakelliou, P. Sandilos, C. AntypasVlachos, and E. Koutsouveli, ‘‘Monte Carlo and TLD dosimetry of a192Ir high dose rate brachytherapy source,’’ Med. Phys.25, 1975–1984~1998!.

4P. Karaiskos, A. Angelopoulos, P. Baras, L. Sakelliou, P. SandilosDardoufas, and L. Vlachos, ‘‘A Monte Carlo investigation of the dosietric characteristics of the VariSource192Ir high dose rate brachytherapsource,’’ Med. Phys.26, 1498–1502~1999!.

5R. Nath, L. L. Anderson, G. Luxton, K. A. Weaver, J. F. Williamson, aA. S. Meigooni, ‘‘Dosimetry of interstitial brachytherapy sources: Reommendations of the AAPM Radiation Therapy Committee Task Gr43,’’ Med. Phys.22, 209–234~1995!.

6R. Muller-Runkel and S. H. Cho, ‘‘Anisotropy measurements of a hdose rate Ir-192 in air and in polystyrene,’’ Med. Phys.21, 1131–1134~1994!.


-

thot-

u-reu-cs

y

i-nof

il:

-

.

.

p

7A. S. Meigooni, M. T. Kleiman, J. L. Johnson, D. Mazloomdoost, andS. Ibbott, ‘‘Dosimetric characteristics of a new high-intensity192Ir sourcefor remote afterloading,’’ Med. Phys.24, 2008–2013~1997!.

8J. F. Williamson and Z. Li, ‘‘Monte Carlo aided dosimetry of the mcroselectron pulsed and high dose rate192Ir sources,’’ Med. Phys.22,809–819~1995!.

9R. Wang and R. S. Sloboda, ‘‘Monte Carlo dosimetry of the VariSouhigh dose rate192Ir source,’’ Med. Phys.25, 415–423~1998!.

10P. Karaiskos, L. Sakelliou, P. Sandilos, and L. Vlachos, ‘‘Limitationsthe point and line source approximations for the determination of geetry factors around brachytherapy sources,’’ Med. Phys.27, 124–128~2000!.

11G. P. Glasgow and L. T. Dillman, ‘‘Specific-ray constant and exposrate constant for192Ir, ’’ Med. Phys.6, 49–52~1979!.

12H. H. Scofield, ‘‘Theoretical photoionization cross sections from 11500 keV,’’ Lawrence Livermore National Laboratory Report NUCRL-51326, Livermore, CA 94550-9234, 1973.

13J. H. Hubbel, Wm. J. Veigele, E. A. Briggs, R. T. Brown, D. T. Cromeand R. J. Howerton, ‘‘Atomic form factors, incoherent scattering funtions, and photon scattering cross sections,’’ J. Phys. Chem. Ref. Da4,471–538~1975!.

14J. H. Hubbel and I. Overbo, ‘‘Relativistic atomic form factors and photcoherent scattering cross sections,’’ J. Phys. Chem. Ref. Data8, 69–105~1979!.

15E. B. Saloman, J. H. Hubbel, and J. H. Scofield, ‘‘X-ray attenuation crsections for energies 100 eV to 100 keV and elementsZ51 to Z592,’’At. Data Nucl. Data Tables38, 1–197~1988!.

16J. H. Hubbel and S. M. Seltzer, ‘‘Tables of x-ray mass attenuation coficients and mass energy-absorption coefficients 1 keV to 20 MeVelementsZ51 to 92 and 48 additional substances of dosimetric interesTechnical Report No. NISTIR 5632, NIST, Gaithersburg, MD, 1995.

17H.-P. Chan and K. Doi, ‘‘Physical characteristics of scattered radiationdiagnostic radiology: Monte Carlo simulation studies,’’ Med. Phys.12,152–165~1985!.

18L. R. M. Morin, ‘‘Molecular form factors and photon coherent scatterincross sections of water,’’ J. Phys. Chem. Ref. Data11, 1091–1098~1982!.

19G. M. Daskalov, E. Lo¨ffler, and J. F. Williamson, ‘‘Monte Carlo-aideddosimetry of a new high dose-rate brachytherapy source,’’ Med. P25, 2200–2208~1998!.

20D. Baltas, P. Karaiskos, A. Angelopoulos, L. Sakelliou, E. Lo¨ffler, and N.Zamboglou, ‘‘Role of beta particles in dosimetry close to IR-192 bractherapy sources,’’ Int. J. Radiat. Oncology, Biol. Phys.~submitted!.

21J. Borg and D. Rogers, ‘‘Spectra and air-kerma strength for encapsu192Ir sources,’’ Med. Phys.26, 2441–2444~1999!.

22J. F. Williamson, ‘‘Comparison of measured and calculated dose ratewater near125I and 192Ir seeds,’’ Med. Phys.18, 776–786~1991!.

23P. Karaiskos, A. Angelopoulos, L. Sakelliou, P. Sandilos, K. Dardoufand L. Vlachos, ‘‘Influence of source geometry on dosimetric characistics of 192Ir brachytherapy sources,’’ presented at the VI InternatioConference on Medical Physics-Patras Medical Physics ’99, PhyMedicaXV , 196 ~1999!.

Documents

Monte Carlo dosimetry of a new [sup 192]Ir high dose rate brachytherapy source