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Corresponding author: Biniam Tesfamicael; Department of Radiation Oncology, McLaren Regional Medical Center, Flint, Michigan, USA.
Cite this article as: Tesfamicael B, Gueye P, Avery S, Lyons D, Mahesh M. A portable secondary dose monitoring system using
scintillating fibers for proton therapy of prostate cancer: A Geant4 Monte Carlo simulation study. Int J Cancer Ther Oncol. 2016;
4(1):4115. DOI: 10.14319/ijcto.41.15

© Tesfamicael et al. ISSN 2330-4049

A portable secondary dose monitoring system using scintillating
fibers for proton therapy of prostate cancer: A Geant4 Monte

Carlo simulation study

Biniam Tesfamicael1, Paul Gueye2, Stephen Avery3, Donald Lyons3, Mahadevappa Mahesh4

1Department of Radiation Oncology, McLaren Regional Medical Center, Flint, Michigan, USA
2Department of Physics, Hampton University, Hampton, Virginia, USA

3Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania, USA
4Department of Radiology and Radiological Sciences, John Hopkins University, Baltimore, Maryland, USAReceived July 05, 2015; Revised January 29, 2016; Accepted February 07, 2016; Published Online February 12, 2016

Original Article

Abstract
Purpose: The main purpose of this study was to monitor the secondary dosedistribution originating from a water phantom during proton therapy of prostatecancer using scintillating fibers. Methods: The Geant4 Monte Carlo toolkit version9.6.p02 was used to simulate a proton therapy of prostate cancer. Two cases werestudied. In the first case, 8 × 8 = 64 equally spaced fibers inside three 4 × 4 × 2.54cm3 Delrin® blocks were used to monitor the emission of secondary particles inthe transverse (left and right) and distal regions relative to the beam direction. Inthe second case, a scintillating block with a thickness of 2.54 cm and equal verticaland longitudinal dimensions as the water phantom was used. Geometrical cutswere implemented to extract the energy deposited in each fiber and inside thescintillating block. Results: The transverse dose distributions from the detectedsecondary particles in both cases are symmetric and agree to within <3.6%. Theenergy deposited gradually increases as one moves from the peripheral row offibers towards the center of the block (aligned with the center of the prostate) by afactor of approximately 5. The energy deposited was also observed to decrease asone goes from the frontal to distal region of the block. The ratio of the energydeposited in the prostate to the energy deposited in the middle two rows of fibersshowed a linear relationship with a slope of (-3.55±2.26) × 10-5 MeV per treatmentGy delivered. The distal detectors recorded a negligible amount of energydeposited due to higher attenuation of the secondary particles by the water in thatdirection. Conclusion: With a good calibration and with the ability to define a goodcorrelation between the radiation flux recorded by the external fibers and the dosedelivered to the prostate, such fibers can be used for real time dose verification tothe target. The system was also observed to respond to the series of Bragg Peaksused to generate the Spread Out Bragg Peak inside the water phantom. Such BraggPeaks were detected by the fibers. The energy deposited inside the lateral blockswere also observed to decrease as one goes away from the beam nozzle due toincreased attenuation.
Keywords: Proton Therapy; Prostate Cancer; Scintillating Fibers; Geant4;Hadrontherapy; Secondary Dose

1. IntroductionRadiation therapy has been one of the most commonlyused treatment options for cancer patients. The externalbeam radiation therapy is performed using high energy photons as well as charged particles, mainly electronsand protons. Due to the sharp characteristic Bragg peakit possesses and the technological ability to spread the

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Bragg Peak to cover the entire tumor size in the beamdirection, proton therapy has shown a therapeuticadvantage over the conventional photon therapy insparing much of the surrounding healthy tissue in thetreatment of deep seated malignancies like the prostatecancer.1-8 In proton therapy, the beam can be deliveredin either an active mode or passive mode.5-7 In theformer, an aperture and a compensator that aregenerated from the treatment planning system arefabricated and utilized to conform the beam to the targetshape both laterally and in the beam direction,respectively.The major concern regarding proton therapy, however,has been the production of secondary particles,especially neutrons, through inelastic interactions of theenergetic protons with the beam delivery systemcomponents and the patient body.9,10 The former wasfound to be the major source of neutrons.9, 10 The dosedelivered to patients by such secondary particles hasbeen of great concern and a main topic of study by anumber of researchers.4,11-15 Results showed that thedose from neutrons is very low and the secondarycancer incidence from such a dose is very scarce.16 Theproduction of neutrons was found to be high when usinga passive beam delivery mode. This is due to presence ofthe scatterers and apertures, which are the maincontributors in neutron production in the beam line.Binns et al.12 measured the neutron dose equivalent atthe patient position to be in the range of 33 mSv to 80mSv per treatment Gy when a proton beam of 200 MeVmean energy was delivered using a passive beamdelivery mode. Around the treatment nozzle, anequivalent dose of 0.91 mSv to 15 mSv per treatment Gywas measured at the Harvard Cyclotron Laboratory byYan et al.14 from 160 MeV proton beam with a passivebeam delivery mode.The equivalent dose from secondary neutrons decreaseswith a decrease in the treatment field, an increase in thedistance from the nozzle as well as a decrease in theenergy of the primary proton beam. Agosteo et al.11reported a Monte Carlo simulation estimation of amaximum neutron dose of 10-4Gy per treatment Gy inthe healthy tissue behind the eye from a passivelyscattered ocular treatment beam line using a 65 MeVproton beam at the Center Antoine-Lacassagne inFrance. With an active beam delivery mode, the absenceof scatterers and apertures greatly reduces the neutronproduction.17 Uwe et al.15 measured the dose deliveredto the healthy tissue by the secondary neutrons for largeand medium targets to be approximately 4 mSv and 2mSv per treatment Gy, respectively when a 177 MeVproton beam was used to irradiate the tumor in a spotscanning beam delivery mode.In this work, a Geant4 Monte Carlo simulation toolkit18,19was used to study the distribution of the secondaryparticles around a 36 cm × 22 cm × 24 cm water

phantom. The energy deposited around the waterphantom by these secondary particles was analyzed. Thedose monitoring system constructed in this study ismainly based on the energy deposited external to thewater phantom. This work follows a recent paper thatfocused in the dose distribution near and in the prostategland.20 In the previous paper, a dose monitoring systemwas constructed to study the dose delivered to the rectalwall in the proton therapy of prostate cancer. Thesimulation included thin scintillating fibers attached toan endorectal balloon to record the dose delivered to theballoon surface. This represents the dose to the innerrectal wall, which is in physical contact with the balloon.The results obtained indicated that a good correlationcan be built between the dose delivered to the prostateand the dose to the scintillating fibers, thus an in-vivodose monitoring to the rectal wall as well as the targetprostate can be achieved.
2. Methods and MaterialsThe Geant4 Monte Carlo toolkit version 4.9.6 p02 wasused to simulate a proton therapy of prostate cancerwith an endorectal balloon as an internal immobilizer.The simulation was based on the hadrontherapyexample application available with the Geant4package21-25, modified for the present study. The detailsof the simulation developed are described in theprevious paper.20 For the secondary dose monitoringsystem, three detectors were constructed around thewater phantom. From the beam's eye view, twodetectors were placed in the lateral directions and onein the distal. Two sets of detectors were designed forthis study. In the first case, a 4 cm × 4 cm Delrin® blockwas used to house a total of 8× 8 = 64 fibers ofdimension 1 mm × 1 mm, evenly spaced by a center-to-center distance of 4.556 mm from each other both in thevertical and lateral directions. The length of the fiberswas defined by the thickness of the Delrin® block, whichis 2.54 cm. In the second case, a 36 cm × 22 cm × 2.54 cmblock made entirely of scintillating material was used forradiation detection. Each block was aligned to the centerof the prostate. In both cases, the detectors were placedat a distance of 4 cm away from the surface of the waterphantom. The placement of the scintillating fibers insidethe Delrin® block and the scintillating blocks are shownin Figures 1 and 2.The modulator wheel was rotated from 00 to 3590 insteps of 10 to spread out the Bragg Peak in the beamdirection. The beam was laterally conformed to thetarget shape using patient specific collimator located atthe end of the beam line. With the Delrin® block, thesimulation was run for four different number of eventsper degree of the modulator wheel rotation: 500, 1000,2000, 3000. The results obtained were analyzed to studythe linear response of the scintillating detectors to thedose delivered. For the case of the scintillating block,however, the simulation was run solely for 3000 events



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per degree of modulator wheel rotation to evaluate theeffect of Delrin® material as a host to the dosemonitoring fibers as in the former case.
3. ResultsThe fibers were uniformly spaced within the blocks,with the fiber ID running from 1 to 64 as shown inFigure 1. The beam direction is from left to right,irradiating the whole prostate. In the root file26generated from the simulation run, the energy depositedin all the fibers inside the three blocks were returnedunder one tree by implementing a cut in the volumename. To analyze the energy deposited in each fiber, theknowledge of the geometrical location of the individualfibers inside the blocks is required. The plots of theinteraction points in the fibers in the X-Y (along thebeam direction) and Z-Y (along the length of the fibers)planes in the lateral blocks are shown in Figure 2. The

geometry simulating the prostate is centered at Y = Z = 0and X = 21.5 cm. The right block (Z > 0) is the reflectionof the left block (Z < 0) with the X-axis as a mirror at Z =0. Hence, it is sufficient to obtain the informationregarding the geometric location of the fibers from oneblock only. The information for the other lateral blockfibers will be a mirror reflection of the first block fibersalong the X-axis, i.e. setting Z > 0 will give thecoordinates of the fibers in the right block and Z < 0 willbe that of the fibers in the left block from the beam's eyeview.From the two plots above, the coordinates that definethe volume of the individual fibers was generated. Theinformation was later used to calculate the totalintegrated energy deposited in each fiber located insidethe left and right blocks.

Figure 1: Left panel: the top view of the simulation design in and around the water phantom. Right panel: the placement ofthe 64 scintillating fibers inside the lateral Delrin® blocks. The blocks are aligned to the center of the prostate and wereplaced 4 cm away from the surface of the water phantom both in the two lateral and distal regions.

Figure 2: The interaction points inside the left block fibers along the fiber length (Y-Z plot) and in (Y-X) the beam direction(beam direction is along the X axis).



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Figure 3: The interaction points of the radiation within the distal block fibers. The fiber ID goes from 1 to 64 from the top -leftto the bottom-right corner.

Figure 4: The total integrated energy deposited inside the right and left fiber within the Delrin® block for the 2000 and 3000events per degree of modulator rotation. The same results were observed for the 500 and 1000 events per modulator wheelrotation – (not shown for clarity).

Figure 5: The energy deposited in the left and right fiber blocks with and without the virtual cut. Both simulation runs werefor 3000 events per degree of modulator rotation.



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Figure 6: The energy deposited in the prostate and the total energy in the 16 fibers located in rows 4 and 5 above (+1.8 mm)and below (-1.8 mm) the center of the prostate, respectively. The plot also shows the ratio of the two energies deposited(E_Prostate/E_ fibers), which is scaled up by 8,000 for clarity.For the fibers in the distal region, the beam was movedforward with its momentum in the positive X direction.Such setup results in more interaction points inside thedistal block fibers, hence giving a clear image of thegeometry of the fibers. From the plot of the interactionpoints, the geometrical information regarding the 64fibers was generated and later used to calculate the totalintegrated energy deposited inside each fiber. The plotof the interaction points in the fibers inside the distalblock are shown in Figure 3.Once the geometrical coordinates for all the fibers wereextracted, the simulation was run for the differentnumber of events per modulator wheel rotation forproton beam energy of 200 MeV as listed in section II.The plot of the integrated energy deposited in the fibersfor the lateral blocks is shown in Figure 4.Figure 4 shows a clear symmetry in the energydeposited between the left and right block fibers. Theplot also shows symmetry in the energy deposited in thefibers across the rows. The fibers in the first row, whichare physically located at the top of the block (see Figure1), record relatively the same amount of energydeposited as those in the eighth row, which are locatedat the bottom of the block. The same relationship existsbetween the second and seventh, third and sixth as wellas fourth and fifth row fibers. Such pairs of row fibersare located at the same distance from the center of theprostate.

The energy deposited in the fibers increase from row 1(and 8) to row 3 (and 6), then slightly decreases in row 4(and 5). Fibers in rows 1 and 8 are at comparativelylonger distances from the center of the prostate. Hence,the secondaries will travel a longer distance to arrive atthose fibers, experiencing a relatively higherattenuation. For the fibers in rows 3 and 6, the distancedecreases, resulting in lower attenuation and higherdeposited energy as compared to the energy recordedby the fibers in rows 1 and 8. The energy depositedcontinues to increase as one move from the edge, rows 1and 8, towards the middle, rows 4 and 5. A slightdecrease in the energy deposited in rows 4 and 5 couldbe attributed to the spherical geometry of the prostate,which makes it thicker at the center. This affects theenergy deposited inside the fibers in rows 4 and 5 thatare located slightly above and below the center of theprostate, respectively.In the second case, the Delrin® block was replaced by afull solid scintillating slab of dimensions 36 cm × 22 cm× 2.5 cm. This simulation was run for 3000 events permodulator rotation and virtual cuts were applied toobtain the energy deposited in the geometrical locationof the fibers. The geometrical information of the fibersgenerated from Figure 2 was used for the virtual cut andthe integrated energy deposited was calculated and isdepicted in Figure 5.The plots overlap very well indicating that the Delrin®material used to house the individual fibers has no effect



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on the reading of the fibers. In the four different runs,the energy deposited in the prostate as well as the totalenergy deposited in the sixteen fibers in rows 4 and 5were recorded and analyzed. The plot of the energydeposited as a function of the number of events permodulator wheel rotation (i.e., dose) for both theprostate and these fibers is shown in Figure 6. Linear fitsapplied for each plot showed a slop of 17.2±0.35 for theprostate and 6.5±0.13 for the fibers in the two middlerows.

The plot for the ratio of the energy deposited inside theprostate to that recorded by the fibers is relativelyconstant, with a slope of (-3.55±2.26) × 10-5 MeV pertreatment Gy to the prostate. The results obtainedensures a direct scaling between the two energiesdeposited. Thus, defining a precise correlation betweenthe energy deposited in the prostate and the energydeposited in the 16 fibers located outside of the waterphantom will enable one to get a good prediction of thedose to the prostate from the measurement of theradiation flux in the fibers.

Figure 7: The ratio of the energy deposited for the 3000 events per degree of modulator wheel rotation run. The plot showsthe ratio of the energy deposited in the left fibers to those in the right fibers for both the block and virtual cut cases.

Figure 8: Contour plot of the interaction points in the distal scintillating block. Beam direction in the X Axis. Left panel is inbeam's eye view and Right panel transverse to beam direction.



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Figure 9: The energy deposited in thin slabs of the right scintillating block at different locations along the beam direction.

Figure 10: The energy deposited as a function of depth in the lateral region.
4. DiscussionFrom the results obtained, one could notice a definitesymmetry in the distribution of the secondary particlesin the lateral (right and left) regions in both cases asdepicted in Figures 4 and 5. This indicates a uniformdistribution of the radiation exiting the water phantom.The plot of the ratio of the energy deposited in the leftand right regions for both cases is shown in Figure 7.

Figure 7 shows the ratio of the energy deposited in theleft to that of the right fibers for both cases (inside theDelrin® block and for the full scintillating slab). Thedata analysis showed that the flux in the two lateralregions match to within 3.6% in both cases. The linear fitperformed over a selected region also showed a verysmall slope, with a y-intercept close to 1, with the twodata sets fluctuating around 10% (1σ). The selected



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region for the linear fit was the fibers located betweenrows 2 and 7, where there is sufficient statistics.Moreover, the ratio of the data obtained from the fibersinside the Delrin® block to that of the virtual cuts insidethe scintillating slab was analyzed for both regions. Thedata match to within 3.5%, which indicates that thematerial housing the dose monitoring fibers has lesseffect on the energy deposited inside those fibers.The secondary dose monitoring fibers and thescintillating block in the distal region, however,recorded very minimum energy deposited. This is due tothe higher attenuation the secondary particlesexperience in arriving at those fibers for the selectedbeam energy. The primary purpose of this detector wasto obtain information on the beams centroid in order toprovide a correction on the beam alignment.To generate the contour plot in Figure 8, a proton beamof energy 250MeV was used with the beam sourcepositioned at around the center of the water phantom toovercome the higher attenuation. Observed from thebeam's eye view, the left panel shows the detection ofmore secondaries around the center. The right panelshows the side view of the scintillating block detector,i.e. transverse to the beam direction. More secondarieswere detected at the frontal edge around the center ofthe detector.Further analysis of the data showed that the energydeposited in the fibers located in each row fluctuates,following a relatively similar pattern in all the rows. Asshown in Figures 4 and 5, the "up-down" patternappears in both cases, i.e. when using a Delrin® block tohouse the monitoring fibers as well as when the wholescintillating block was used with a virtual cut. The fibersthat are housed inside the Delrin® block in each rowwere spaced at 4.6 mm from each other. To understandthis fluctuation, the scintillating block located on theright side of the water phantom, viewed from the beam'seye view, was virtually dissected in the vertical direction(slices run along beam direction) in thicknesses of 0.5mm. In the Y and Z directions (height and thickness,respectively), the whole size of the block wasconsidered. Such slices (0.5 mm × 22 cm × 2.45 cm)were used to calculate the integrated energy depositedand the results obtained are shown in Figure 9.The panel in the top left shows the scattered plot of thesecondaries within the scintillating block. Panel (a)represents the energy distribution in the proximalregion. The spread in the energy variation is equivalentto one standard deviation (SD). Hence, this variation isattributed to statistical fluctuation. In (b), however, thefluctuation follows a certain pattern that repeats over agiven region, i.e. around the location of the spread outBragg peak (SOBP). The repetitive spikes correspond tothe large number of Pristine Bragg peaks involved to

generate the SOBP (18 in number). The SOBP wasgenerated by a modulator wheel of 18 slices, each with athickness of 0.3 cm stacked together to form a staircase.The secondary dose monitoring fibers as well as thescintillating block located outside of the water phantomwere able to "sense" those Pristine Bragg Peaks,resulting in the formation of the spikes. In (c), which islocated in the distal region of the right block, no suchspikes were observed due to attenuation and smearingin water. Such spike patterns were also observed insidethe prostate. The spikes generated, however, were lesspronounced due to a higher contribution of low energysecondaries that are suppressed from reaching theexternal detectors due to attenuation.From Figures 9(a, b and c), one notices that the energydeposited gradually decreases as one moves towards thedistal section of the scintillating block. To understandthe distribution of the energy deposited, further analysisof the data was conducted. In this case, the scintillatingblock was virtually dissected into thin slices of thickness3.5 cm each along the beam direction. The energydeposited in each slice was calculated and plotted asshown in Figure 10.From the plot, it can be noticed that the energydeposited is higher in the proximal region and decreasestowards the distal region. The peak at the depth ofaround 12 cm shows a turning point, which is inagreement to the plots in Figure 9 (a) and (b). Beyondthat point, the energy deposited starts decreasing and isclose to zero at depths of around 30 cm and beyond. Thisis due to the attenuation of the mainly low energysecondaries that are generated at depths beyond thelocation of the SOBP. The result obtained is in closeagreement with those obtained by Wroe et al.27, whoused a different setup for their experiment. The reasonattributed for higher energy deposited in the proximalregion is due to the higher production of high energysecondaries, like neutrons and electrons inside the beamline components which are closer to the proximal edgeof the scintillating block. For the distal region, however,the main contribution comes from the secondaryparticles generated inside the water phantom. Thesecondaries from the water phantom are produced withcomparatively lower energies.
5. ConclusionA Geant4 Monte Carlo simulation was designed tomonitor the total external radiation flux and a possiblecorrelation of the measured flux with the dose deliveredto the prostate. The previous work20 was mainlyfocusing on using thin scintillating fibers attached to awater filled endorectal balloon to monitor the dose tothe rectal wall as well as the prostate. In this work,however, the goal was to develop a dose monitoringsystem based on the radiation flux recorded fromoutside of the water phantom.



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The results obtained were in good agreement to resultsfrom literature review. A linear response of the fibers tothe dose delivered to the prostate was observed, aproperty of the fibers studied by a number ofresearchers. Secondary dose monitoring withscintillating fibers could be used to give a relatively goodprediction (with a precision higher than 97%) of thedose to the prostate. The multiple Bragg peaks usedduring clinical treatment to generate an SOBP could beobserved and monitored to estimate the non-uniformdistribution of the dose within the SOBP region. Furtherstudies on the biological effect of such dose noneuniformity distribution is planned.The grant allocated for this study was mainly forprostate cancer case. The simulation, however, can bemodified, with little work, to study other cases. In thefuture, there is a plan by the authors to apply the MonteCarlo simulation developed to investigate the treatmentof other anatomical sites. The authors are also lookingfor a fund for equipment purchase and beam time toconduct experimental measurements based on thesimulation design. The results from the experimentalmeasurements will be compared with the results fromthe simulation.
Conflict of interestThe authors declare that they have no conflicts ofinterest. The authors alone are responsible for thecontent and writing of the paper.
AcknowledgementThis work was partly supported by the Department ofEnergy - National Security Administration under awardnumber DE-NA0000979.
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