effect of uniform magnetic field on dose distribution in...

International Journal of Radiation Research, April 2014 Volume 12, No 2

Effect of uniform magnetic field on dose distribution in the breast radiotherapy

INTRODUCTION

It is well known that radiation is acarcinogen; therefore, it is very importance toreduce exposure to normal tissues whilemaximizing thedose to the targetedarea (s). Itis this principle of minimizing unwanted dosethat has prompted many studies in the reduc-tionofexposuretotheinternalandcontralateralbreast.However,deliveringtoohighadosewillcausecomplicationstonormaltissues including

radiation-induced pneumonitis, cardiac toxicity,secondary cancers and skin reactions. Theprobabilityofoccurringof sucheffectsdependsprimarilyonthedelivereddoseandthevolumeofthereceiveddose.Boiceetal.(1992)(1)foundacorrelationbetweentheamountofdosetothecontralateral breast and the likelihood of asecondarymalignancyformation.

Multiple groups have proposed designs forreducing the dose to the internal andcontralateral organs; for example, choosing aradiation technique like breast intensity

A.D. Esmaeeli1*, M. Pouladian1, A.S. Monfared2, S.R. Mahdavi3, D. Moslemi4

1DepartmentofMedicalRadiationEngineering,ScienceandResearchBranch,IslamicAzadUniversity,Tehran,Iran

2DepartmentofMedicalPhysics,BabolUniversityofMedicalSciences,Babol,Iran3DepartmentofMedicalPhysics,TehranUniversityofMedicalSciences,Tehran,Iran4DepartmentofRadiationOncology,BabolUniversityofMedicalSciences,Babol,Iran

ABSTRACT Background: To reduce the dose to normal ssues surrounding the treated breast, a uniform magne c field was used within a humanoid phantom in breast radiotherapy. Materials and Methods: Monte Carlo simula ons were performed with GEANT4, irradia ng humanoid phantoms in a magne c field. To reconstruct phantoms, computed tomography (CT) data slices of four pa ents were used for the Monte Carlo simula ons. All of them had le breast cancer either or not mastectomy. In the simula ons, the planning and methods of chest wall irradia on were similar to the actual clinical planning. Results: U lizing magne c field will help to produce uniform dose distribu on to the breast with a sharp dose‐volume histogram (DVH) curve for the planning target volume (PTV), however, for the ipsilateral lung and chest wall skin the mean dose was reduced by a mean of 16% and 12% at 1.5 T, and 9% and 7% at 3 T, respec vely. The magne c field was shown to restrict the lateral spread of secondary electrons to the contralateral organs, resul ng in significient dose reduc ons to the contralateral breast (CB) and contralateral chest wall skin (CCWS) by a mean (range) of 28% (21‐37%) and 58% (44‐75%) at 1.5 T, and 48% (32‐81) and 66% (54‐73%) at 3 T, respec vely. Conclusion: The simula ons established that the magne c field can reduce the dose to the internal and contralateral ssues and increase it to the PTV with sharper edge DVH curve. Keywords: ERE, magnetic field, breast, GEANT4.

*Correspondingauthor:Dr.AliEsmaeeliDafchahi,Fax:+981314247056E-mail:

[email protected]

Received: Jan. 2013 Accepted: April 2013 Int. J. Radiat. Res., April 2014; 12(2): 151-160

► Original article

Esmaeeli et al. / Effect of uniform magnetic field on dose distribution in breast radiotherapy

modulated radiothetapy (IMRT), 3-dimensionalconformalradiotherapy(3D-CRT),partialbreastradiotherapy and high-dose rate (HDR)brachytherapyorusingaplatform-basedbreastshield that minimizes exposure of internalorgans(2-7).

Many studies have proposed designs forcombininga linearaccelerator(Linac)ora60Cotele-therapy unit with a magnetic resonanceimaging (MRI) system for taking the real-timeimage guidance along with the treatment (8-19).Furthermore,otherstudieshaveconsideredthepossibilityofusingamagnetic ield inavarietyof geometries in order to investigateperturbationsondosedistributions to improvevolumetricconformance inradiotherapy (9,20-23).The consequences of a magnetic ield ondosimetry have been studied by all of thesegroupsusinganalytical,MonteCarlo (MC)and/or experimental techniques. Largely, the focushas been on changes to the spatial distributionof dose resulting from the magnetic ieldin luence on charged particle transport. Theyhave established that high magnetic ieldstrength can have signi icant perturbations ondose distributions, such as changes to thepercentage depth dose, tissue interface effects(electronreturneffect)andlateralshiftsindosedistributions in the photon beam radiotherapy(17,24-26).Theseeffectsaremostnoticeableinthetissue-lung interface where the ield passesthroughtheinterface(13,17,25).

The aim of thisworkwas to investigate theconsequences to radiation dose distributionsthat occur in differentmagnetic ield strengthsin order to understand any advantages it mayprovide in comparison with the zero magneticieldinbreastradiotherapy.Possibleadvantagescouldbeuniformeddose to the treatedvolumeand decreased dose to the internal andcontralateralbreastorgans.

MATERIALSANDMETHODS

MonteCarlosimulationsThe MC toolkit GEANT4 (v9.3) has been

used to calculate the dose distribution in areconstructed patient-derived phantom in the

presence of a magnetic ield. GEANT4 is amultipurpose MC code, developed andmaintainedbytheGEANT4collaboration(27).All regular physical processes for medical

applications from the LowEPhysics Processespackagewereactuated.The low-energycut-offsfor different materials used in this work wereconsidered according to default values whichwere de ined in GEANT4 (31). The human bodytissues like lung, liver, chest wall, bones weremodeled according to the InternationalCommission on Radiation Units andmeasurements(ICRU)report46(32).For the calculation of the trajectories of

chargedparticlesinamagnetic ield,themethod‘ClassicalRK4’ was used (for more informationsee GEANT4.9.3 user's guide for applicationdevelopers). This code was shown to provideaccurate results for gamma ray dosedistributionsinthepresenceofamagnetic ield(29). The simulations were performed 20 timeswith independently created phase-spaceiles. From the resulting dose distributions, anaveraged dose pro ile and a standard deviationpro ile were obtained. The chosen number ofparticlesensuredastatisticaluncertaintyof1%,1 SD (for all voxelswith thedoseofmore than1%ofmaximumdose).

Target anatomy and treatment planningsetups

In this work, we have chosen four patientswho had breast cancer with mastectomy andnon-mastectomy cases. Breast treatment plansetups were tabulated in table 1, and the PTVandorgansatrisk(OARs)togetherwithwedgesand treatment beam directions were shown inigure1(a).Theexternal surfaceofpatientsandlung contours were de ined by automated den-sity gradient tracking; then, they were editedandveri iedbyphysicians.ThePTVwasde inedby adding 5 mm to the clinical target volume(CTV).ThecontourofthePTVwasoutlinedwithdepthof1.6mmtotheskinsurfaceanteriorlytoevaluateDVHs.Toassess thevalueof skindoseaccurately, another volume including 1.6 mmsurface thickness of the CTVwas contoured astheskinstructure.

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Table 1. Treatment plan setups and pa ent specifica ons.

a Source to Surface Distance b Body Mass Index c Lung Orthogonal Distance: is defined as the maximum breast and lung distance (see figure 1 (a)).

Figure 1. (a) Target anatomy and structure, OARs, medial and lateral beams, and lung orthogonal distance in pa ent # 1. (b) One slice of the reconstructed phantom (pa ent # 1) along with Linac head simulated in GEANT4.

TPS.Plan iles,includingCTdata,wereexportedfromtheTPSandimportedintoGEANT4forMCsimulation.Inthesimulations,aglobalmagnetic ieldwas

implemented in the x direction ( igure 2).Unidirectional photons with a realistic 6 MVlinearacceleratorenergywereusedasasourcethatwaspreviouslycomparedwitharealLinac(SiemensPrimus) in thewaterphantom(30).Allvoxelswereplacedasparameterizedvolumesonabackgroundmaterialofair.Different components of the simulated linear

accelerator(SiemensPrimus)fora6-MVphotonbeamwereshownin igure1(b).Foreachbeam,a phase spacewas generated below thewedgeandwithout thepresenceofanymagnetic ield.The assumption implicit in thismodelwas thatthe B0 ieldwas negligible to this point due to

CTdataslicesofeachpatientwereusedintheMCcodeasinputdata.Dimensionsofeachvoxelin the reconstructed image of patients inGEANT4were0.8×0.8×3mm3.Onesliceofthereconstructed phantom (patient # 1) and thepath of the medial beam together with Linachead setups simulated in GEANT4 werepresented in igure1(b). In thesimulations, theplanning andmethods of chest wall irradiationweresimilar to theactualclinicalplanning.Thetreatment planning system (TPS) used in thisstudy is the TiGRT TPS, External TreatmentPlanning version 1.0.140.5061.L1.244584(LinaTech,LLC,Sunnyvale,CA,94086).ThisTPSsoftware uses super-position convolutionalgorithm to calculate the dose distribution.Commissioning and veri ication underexperimental condition are done with TiGRT

(a) (b)

Treatment plan parameters

Pa ent # 1 Pa ent # 2 Pa ent # 3 Pa ent # 4 Beam 1(Medial)

Beam 2

Beam 1(Medial)

Beam 2

Beam 1(Medial)

Beam 2

Beam 1(Medial)

Beam 2

SSDa (cm) 90.5 90.8 94.6 94.5 92.6 93.3 93.3 93.3 Collimator Width (cm) 9.6 9.6 6.4 6.4 7.6 7.6 9.8 9.8 Collimator Length (cm) 22.0 22.0 22.4 22.4 21.0 21.0 20.6 20.6 Gantry Angle (degree) 305 131 305 128.5 297 121 295 121

Wedge (degree) 30 30 15 15 15 15 15 15 Mastectomy/age/BMIb No/42/32 Yes/59/27 Yes /45/24 No/44/35

LODc (cm) 1.4 0.7 2.8 1.8

Int. J. Radiat. Res., Vol. 12 No. 2, April 2014


IL,contralateral lung(CL), ipsilateralchestwallskin (ICWS), CCWS and CB in humanoidphantomswereconsideredfor investigatingtheeffect of the magnetic ield on the dosedistribution to the internal and contralateralorgans.

RESULTSANDDISCUSSION

ComparisonoftheMCandTPS

ComparisonsoftheDVHsfortheMCandTPSinthecaseof6MVbreastpatientswereshownin igure3.ThePTVdosecalculationevaluationsshowed a good comparison with the MCsimulations and TPS. Table 2 demonstratesmean dose values calculated by the TPS andGEANT4.Comparisonsof themeandose for theTPSandGEANT4showeddeviationsintheorderof5%maximum.DosedistributiontotheipsilateralorgansTo illustrate differences in the relative dose

distribution, results of the simulations werepresented for different magnetic ields withrespectto thezeromagnetic ield in themiddlesliceofthecentralplaneforpatientsin igure4.Figure 5 depicts the corresponding differencemaps of igure 4 from the zero magnetic ieldcase. Dose distributions to the breast and lungwerenearlyuniformat0T;but, at1.5and3T,sharp regions were produced at breast-lungboundaries ( igure 4). This is explained by thefactthatwhenphotonbeamsirradiatewithoutamagnetic ield, charged particles travel into achest wall and this process produces a largenumberofelectronsinapredominantlyforwarddirection. Dose deposition arises from theseelectronsalongtheirpath.Inthepresenceofthemagnetic ield,electron trajectoriesarebent,

Figure 2. Schema c representa on of the magne c field geometry for the medial and lateral beams, showing rela ve

posi oning of the Linac, magnets and the magne c field direc on.

the magnetic shielding implicit in the Linac-magnetic resonance design. The contaminantelectrons were included in the presentphase-space data. Each phase space was thenused as a source in the GEANT4 platform. Itshouldbenotedthat, thecontaminantelectronswere thus subjected to themagnetic ield fromthispointoninthismodel.In order to verify the Linac head along with

the wedge and treatment plan setups in theGEANT4MC code, dose distributions calculatedby the MC simulations were evaluated usingdose distributions calculated by the TPS bycomparingDVHsofthePTV,ipsilaterallung(IL)andheartvolumes.Two opposed parallel beams described as

being tangential to the chest wall wereimplemented. Each beam was simulatedseparately and the results were combined andnormalized to 100% at isocenter for the zeroield case. Absolute dose can be obtained bymultiplying all voxel doses by the dose-toisocenter/100% for the zero ield plan. Thenominal prescribed dose was 50 Gy in 25fractions using 6-MV photons. The calculateddosewas normalized to a relevant point in thePTV to provide dose homogeneity. The meandoseofeachorganwasnormalizedtomeandoseofPTVat0T.The mean dose, DVHs and dose-area

histograms (DAHs) obtained for the PTV, heart,

Table 2. Mean organ dose calculated by the TPS and MC.

Mean Dose (Gy)

Organ Pa ent # 4 Pa ent #3 Pa ent #2 Pa ent # 1

TPS MC TPS MC TPS MC TPS MC

50.0 50.0 50.0 50.0 50.0 50.5 50.0 49.4 PTV

3.5 3.4 8.2 7.8 3.8 3.7 7.5 7.4 IL

1.3 1.4 5.7 6.0 2.7 2.8 2.7 2.8 Heart

154 Int. J. Radiat. Res., Vol. 12 No. 2, April 2014


chest wall. This phenomenon is called theelectron return effect (ERE) (25),which causesdosereductioninlow-densitymediaclosetothedenseones.

describing a helical path. In the chest wall, themeanfreepath length isshort incomparisontothe spiral radius. However, as soon as theelectronsleavethechestwallandenterintothelow-density media, like lung, their length ofmean free path gets longer compared to theirhelicalradius.Therefore,thehelicalpathcanbefollowed by small interaction and the electronwill re-enter the chest wall. This will happenregardless of the exit angle of the electrons.Thereforeeffectivelyallelectronsreturnintothe

Figure 4. Energy deposi on in the middle slice of the central plane of pa ents for different magne c fields; (a, b and c) pa ent #1, (d, e and f) pa ent #2, (g, h and i) pa ent

#3 and (j, k and l) pa ent #4.

Figure 3. DVHs of the PTV, IL and heart for comparison of the TPS and MC calculated dose distribu ons in the 6MV breast case for (a) pa ent # 1, (b) pa ent # 2, (c) pa ent # 3 and (d)

pa ent # 4.

a

b

c

d

a b

c d

e f

g h

i j

k l



156

It is important to note that low densitytissues,likelung,wereshowntoleadtoregionsof dose decrease in the presence of aconventional magnetic ield due to electronsreturningtomoredensetissues(16,17,25,28,29).

Differentmapsweredrawntoshowchangeson the dose distribution due to the presence ofthemagnetic ield.At1.5and3T,hotspotcouldbeobservedinthebreast,whichwere indicatedby dashed black arrows in the igure, and hotand cold spots in the lung that were indicatedbysolidblackandwhitearrows,respectively.

In regions near the breast-lung boundaries,the dose was reduced at 1.5 and 3 T withrespect to the without magnetic ield case,because high-energy charged particles thatwerescatteredfromthebreastintothelunghada large enough gyration radius for curving backintothebreast.

Low-energy charged particles originated farfrom the breast-lung boundaries in the lunghave small gyration radius; therefore, theyrelease their energy in the lung. Furthermore,therearenumerousscatteredphotonsonwhichthe magnetic ield did not have any in luence.Accordingly, theeffectofmagnetic ieldreducesinthedeeperregionsinthelungandtherateofdose reduction decreases to these areas of thelungwithrespecttoregionsclosetothebreast-lungboundariesinthelung( igure5).

Thesamescenario takesplaceat thebreast-air boundaries at the entrance of the medialtangent ield that causes charge particles curveback into the breast from the air and breast-airboundaries (chest wall skin). Due to the EREeffect, the chestwall skin dosewill be reduced(withdottedblackarrowsshownin igure5).

For the treatment planning with twoopposing beams for the breast, DVHs for thePTV, heart, IL and DAHs for ICWS for differentmagnetic ieldswereplottedin igure6.

Asageneral trend, thehigh-doseareaof theDVHwas seen to be shifted to the right for 1.5and3Twithsharpdropofthecurve,whichwasdue to the ERE, increasing of the dose tovolumes of the breast near the breast-air andbreast-lunginsidethePTV.

In the IL, igure 6 shows only small changesintheDVHsat1.5and3T.V20(where20was

Figure 5. Rela ve dose differences in the middle slice of the central plane at 1.5 and 3.0 T from the zero

magne c field case for (a and b) pa ent # 1, (c and d) pa ent # 2, (e and f) pa ent # 3 and (g and h) pa ent #

4. Solid white arrows indicate dose reduc on in the lung. Dashed black arrows indicate hot spot in the

breast. The net effect of the Lorentz force ac ng on the electrons shi s the dose outside of the pa ent’s body

to the right. Dashed white arrows show dose reduc on in the air above the contralateral breast. Solid black arrows indicate the hot spot produced in the lung.

Do ed black arrows show dose reduc on above the ipsilateral breast.

a b

c d

e f

g h



expressed as the percentage of theprescriptiondose)remainsreasonablyconstant.From igure5,thereareobviouschangesonthedose distribution within the lung. As the EREoccurs in the breast-lung boundaries, there ishighest dose reduction in these areas, incontrast, inregionsnotclosetothebreast-lungboundaries, the effect of the magnetic ield isreducedandmakessomesub-regionshotter.

TheICWSshowedgoodchangesintheDAHacross the range of the investigated magneticieldstrengths.InDVHsoftheheart,thereisnotanysigni icantdifferenceforallpatients,but inpatient# 3 due to the increase of the dose forthe PTV, after re-normalization, noticeablemeandosereductioncanbeseen( igure7).

Utilizingmagnetic ieldwillhelptoproduceuniform dose distribution to the breast with asharpDVHscurveforthePTV,however,fortheipsilateral lung and chest wall skin the meandosewasreducedbyameanof16%and12%at 1.5 T, and 9% and 7% at 3 T, respectively,depending on different patients and varioustreatmentplansetups.

Two groups studied the effect of themagnetic ield on dose distribution in ahumanoidphantominthepresenceofthestrongmagnetic ieldbyuseofIMRTtechniquewiththehelpoftheMonteCarlocode.

The irstgroup (29) investigatedthemagneticieldeffectsondosedistributionintheprostate,larynx and oropharynx tissues. They concludedthatthepresenceofa1.5Tmagnetic ielddoesnot compromise the ability to achieve theprescribeddosedistributionswithIMRT.

Other group (16,17) investigated lungdosimetryinalinac-MRIradiotherapyunitwitha longitudinal magnetic ield. The longitudinalcon igurationexhibitedasigni icantdecreaseintissue interfaceeffectsanddemonstratedan in-creaseinthedosetothePTVasafunctionofin-creasedmagnetic ield. The same resultswerefound in our cases with an increase in dose tothePTV.

TheCBreceivedasmallmeandosegivingbya mean (range) of 36 cGy (23-51 cGy) at 0 T.Afterapplyingmagnetic ieldof1.5and3T,themeandoseofCBwasreducedbyamean(range)

Figure 6. Comparison of the breast treatment plan DVHs and DAHs between B = 0, 1.5 and 3 T in pa ent #1 (a), pa ent # 2 (b), pa ent # 3 (c) and pa ent #4 (d). Note that, those volumes within the radia on field are considered for the ICWS DAHs.



of 28% (21-37%) and 48% (32-81%),respectively. There was the same dosereduction to the CCWS by a mean (range) of58% (44- 75%)and66% (54-73%)at 1.5 and3 T. This is explained by the fact that the magnetic ield is shown to restrict the lateralspread of secondary electrons to the contralateral organs, resulting in signi icantdosereductionstotheCBand CCWS.DosedistributiontothecontralateralorgansIt may be assumed that, at the tissue-lung

interface and on the chest wall skin to thecontralateral, the ERE increases the dose ofsurrounding tissues; but, the net effect of theLorentz force acting on the electrons shifts thedose far from the CB (as indicated by dashedwhitearrowin igure5).MeandoseoftheCBandinternalorganswas

tabulated in table3 for the zeromagnetic ieldcase. For external beam radiotherapy, thephysicalwedgecompensationtechniqueyieldedthelargestdosetotheneighboringsolidorgans,

Table 3. Mean organ dose of the pa ents for the zero magne c field case.

Organs Mean dose at 0 T (cGy)

Pa ent # 1 Pa ent # 2 Pa ent # 3 Pa ent # 4

CCWS 408 249 166 84

CB 51 33 23 37

CL 18 10 16 11

IL 739 366 786 344

Heart 282 279 605 139

Figure 7. Rela ve mean organ dose varia ons in all pa ents at 1.5 T (a) and 3 T (b) with respect to the zero field case.

like the IL or the heart. Figure 7 compared therelativemeandosewith theselectedorgans fordifferent magnetic ields with respect to nomagnetic ieldcase.TheCBreceivedasmallmeandosegivingby

a mean (range) of 36 cGy (23-51 cGy) at 0 T.Afterapplyingmagnetic ieldof1.5and3T, themeandoseofCBwasreducedbyamean(range)of 28% (21-37%) and 48% (32-81%),respectively.Therewasthesamedosereductionto the CCWS by a mean (range) of 58% (44-75%)and66%(54-73%)at1.5and3T.This isexplained by the fact that the magnetic ield isshowntorestrictthelateralspreadofsecondaryelectronstothecontralateralorgans,resultinginsigni icientdosereductionstotheCBandCCWS.Although radiation-induced pneumonitis is

consideredatrivialissueinmostcasesofbreastradiotherapy, it can be problematic in somepatients with unfavorable anatomy and aninadequate radiation technique (33,34). Asradiotherapy techniques have improved, therisks of radiation-induced pneumonitis andcardiaccomplicationhavedeclined.Separate excess relative risks (ERRs) have

been reported in patients treated withradiotherapy (35) for breast cancers. Boice (1)foundacorrelationbetweentheamountofdoseto the CB and the likelihood of a secondarymalignancy forming. Dasu's competition modelprojected a continuously increasing risk withdose up to approximately 4-5 Gy followed by areduced risk in the CB (36). However, using themagnetic ield in standard breast radiotherapyfor patients with different anatomies andconsidering the mean dose reduction shown inigure 7, one could clearly conclude thereductionofthesecomplications.

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CONCLUSIONS

Magnetic ieldstrengthsof1.5and3Twereapplied in the x direction in the thorax area inwhich the breast and lung underwent thephoton irradiation.The simulationsestablishedthat the magnetic ield can reduce the dose tothe internal and contralateral tissues andincrease it to the PTV with sharper edge DVHcurve. In the all patients, there was noticeabledosereductiontotheICWS,acrosstherangeofmagnetic ieldstrengthsinvestigated.Therefore,this technique could be more feasible forpatients whose skins are not considered atarget.

ACKNOWLEDEGEMENT

We are thankful from physicists ofradiotherapydepartmentsinSh.RajaeeandParsHospital for their support toproperuseofdataand planning outputs and special thanks toMr.Rezaii and Rezazadeh for their kindcooperations.

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