Biomed. Phys. Eng. Express 4 (2018) 025022 https://doi.org/10.1088/2057-1976/aa9d76

PAPER

Verification of evaluation accuracy of absorbed dose in the dose-evaluation system for iridium-192 brachytherapy for treatment ofkeloids

MOhta1 , NNakao2, S Kuribayashi3 andNHayashizaki4

1 Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1Ookayama,Meguro-ku, Tokyo, 152-8550 Japan2 Institute of Technology, ShimizuCorporation, 3-4-17 Etchujima, Koto-ku, Tokyo, 135-8530 Japan3 Division of RadiationOncology, NipponMedical SchoolHospital, 1-1-5 Sendagi, Bunkyo-ku, Tokyo, 113-8603 Japan4 Institute of Innovative Research, 2-12-1Ookayama,Meguro-ku, Tokyo, 152-8550 Japan

E-mail: ohta.m.ae@m.titech.ac.jp

Keywords: brachytherapy, iridium, dosimetry,Monte Carlomethods, gamma rays, glass dosimeters

Abstract192Ir high-dose-rate superficial brachytherapy (HDR-SBT)has been delivered as a postoperativeradiotherapy to prevent recurrence of keloids inNipponMedical SchoolHospital. However, extraradiation exposure for organs at risk is a concern because high-energy gamma rays penetratemoredeeply than electrons at the energies typically used for therapy. Therefore, a systemwhich can evaluateabsorbed dose in the affected area and radiation exposure to tissue and organs based on the actualgeometry inHDR-SBTwas developed using the PHITSMonteCarlo code and aMIRD-5 phantom inour previous work. In this study, the absorbed dosewasmeasured in a simple geometry using awater-equivalent phantom and radio-photoluminescence glass dosimeters to verify the evaluation accuracyof absorbed dose calculated by the developed system.We find that, accounting for the transit dosecomponent and the energy dependence analytically, the calculations by PHITS underestimate theabsorbed dose by 10.2% to 19.0% (average 15.5%±1.9%) compared to themeasurements. Thesensitivity-correction procedurewas considered to be themain reason for the difference between themeasured and calculated absorbed dose of the dosimeters.

1. Background

Keloids are extreme overgrowth of scar tissue thatoccurs when fibroblasts produce excessive collagenfiber due to continuous inflammation in the processbywhich injured skin is healed.

Although keloids are a benign tumor of the der-mis, they extend beyond the borders of the originalwound with pain, pruritus, and cosmetic disfigure-ment as they grow formonths or years. Therefore, sur-gical excision to remove keloids is delivered as thetreatment. However, keloids are relatively resistant totreatment and the recurrence rate after surgical exci-sion alone is more than 80% (Sclafani et al 1996, Parket al 2011). Surgical excision with postoperative radio-therapy to prevent recurrence is consequently selectedas an effective treatment for keloids (Guix et al 2001,Hoang et al 2017).

Electron beam radiotherapy (EBT), using a high-energy electron beam generated from a medical elec-tron linear accelerator, has been widely performed as apostoperative radiotherapy after surgical excision.However, the absorbed dose easily decreases at theedge of the radiation field in EBT since the penumbraregion increases due to electron scattering at the sur-faces of the affected area. In addition, EBT is often per-formed by multiple radiation fields because it isdifficult to deliver a uniform dose to affected areaswith complicated shapes or extensive affected areassuch as front and back surfaces of a body; therefore, itcan result in excess or deficiency of absorbed dose atthe junction of thesefields (Hefni et al 2013).

Although EBT had been used for many years aspostoperative radiotherapy for keloids in the NipponMedical School Hospital, high-dose-rate superficialbrachytherapy (HDR-SBT) has been employed since2008 using a remote afterloading device with an

RECEIVED

25August 2017

REVISED

28October 2017

ACCEPTED FOR PUBLICATION

27November 2017

PUBLISHED

30 January 2018

© 2018 IOPPublishing Ltd

Iridium-192 (192Ir) source in order to solve the afore-mentioned problems. HDR-SBT excels in dose con-centration because the encapsulated 192Ir radioisotopeis fixed close to the affected area through a source cableand an applicator, and it has the additional advantageof being able to deliver the prescribed dose more uni-formly during the irradiation while keeping a constantdistance from the skin surface using the cable andapplicator.

Although HDR-SBT after surgical excision hasdelivered satisfactory clinical results, extra radiationexposure for organs at risk near the affected area is aconcern because high-energy gamma ray penetratesdeeper than the electron beam of 4–6MeV used inEBT (Kuribayashi et al 2011). However, irradiationand construction of shielding geometry are currentlyperformed based on the doctor’s experience and sim-ple dose calculations, without quantitative evaluationof the radiation exposure.

Therefore, in our previous work, a system wasdeveloped to evaluate the absorbed dose in the affectedarea and the radiation exposure to tissue and organswhen the prescribed dose is delivered uniformly to aflat surface based on the actual geometry in HDR-SBT(Ohta et al 2016). In this system, the radiation trans-port simulation is performed using the particle andheavy ion transport code system (PHITS)Monte Carlocode and aMIRD-5 phantom as the stylized computa-tional phantom, which can be compiled by the MonteCarlo code.

In this study, to verify the evaluation accuracy ofthe absorbed dose calculated by PHITS in the devel-oped system, the absorbed dose was measured in asimple geometry using a water-equivalent solid phan-tomand radio-photoluminescence glass dosimeter.

2.Material

2.1. 192Ir radiation sourceAmicroSelectron HDR-v3 remote afterloading device(Elekta AB, Stockholm, Sweden) was used for theirradiation in this study. The mHDR-v2r 192Ir source(0.6 mm in diameter and 3.5 mm in length) waswelded to the end of a cable and stored in theafterloading device while not in use. As observed withother HDR 192Ir sources, the mHDR-v2r sourceexhibits anisotropy due to the source material deposi-tion and encapsulation (0.1 mm stainless steel) (Gran-ero et al 2011). The intensity of the 192Ir source storedin the microSelectron HDR-v3 remote afterloadingdevice was measured with a well-type ionizationchamber (HDR 1000 Plus Well Chamber, StandardImaging Inc., Wisconsin, USA) and an electrometer(MAX 4000 Plus Electrometer, Standard Imaging Inc.,Wisconsin, USA). The chamber was initially calibratedby the University of Wisconsin Accredited DosimetryCalibration Laboratory, and the calibration has beendone with the electrometer routinely every two to

three years by the Japan Radioisotope Association. Acalibration factor traceable to national standards wassupplied, and it has an expanded uncertainty of±2.2% (k=2) at the confidence level of 95%.

2.2. Applicator for superficial irradiationSpecialized applicators are used for superficial bra-chytherapy, placed directly on the affected area. TheFreiburg Flap Applicator (Elekta AB, Stockholm,Sweden) has been used for HDR-SBT of keloids inNippon Medical School Hospital. Figure 1 shows aphotograph of a Freiburg Flap Applicator. The Frei-burg Flap Applicator is a flexible mesh-style applicatorcomposed of many silicon rubber beads with adiameter of 1 cm and mass density of 1.12 g cm−3

arrayed in a plane. The center of each bead has a holewith a diameter of 2 mm which enables catheterinsertion to transfer the 192Ir source. The 192Ir sourcetravels through each catheter, and it stops in 1 cm stepsat the center of the beads, the so-called ‘dwellpositions’ for respective optimized times. The max-imum size of this applicator is 24 cm×24 cm, and anapplicator of six rowswas used in this study since it canbe cut to any size depending on the desired irradia-tion area.

2.3. Radio-photoluminescence glass dosimeterA radio-photoluminescence (RPL) glass dosimeter is asolid-state passive dosimeter. It can measure absorbeddose based on the following principle: The silveractivated phosphate glass irradiated by ionizing radia-tion emits luminescence when it is excited by ultravio-let light. This phenomenon is called ‘radio-photoluminescence’. The intensity of the lumines-cence is proportional to the absorbed dose. Thematerial of the dosimeter is a phosphate glass dopedsilver as the color center, which is composed of51.16 wt% oxygen, 31.55 wt% phosphorus, 11.0 wt %sodium, 6.12 wt% aluminum, and 0.17 wt% silver.The mass density is 2.61 g cm−3, and the effectiveatomic number is 12.039 (Tsuda 2000).When ionizingradiation enters into the dosimeter, electron-holepairs are created in the valence band. The electrons arepromoted from the valence band into the conductionband by receiving the energy, and are subsequentlycaptured by Ag+ ions, causing the Ag+ ions tobecome neutralized. In addition, holes are initiallycaptured by PO4 tetrahedra which then migrate to Ag+ ions, causing the production of more stable Ag2+ions as time passes. When the dosimeter is irradiatedby a pulsed ultraviolet laser, these Ag0 and Ag2+ ionsare excited as the color centers. Therefore, lumines-cence is emitted as the surplus energy with de-excitation, and the absorbed dose of the dosimeter canbe evaluated by the amount of luminescence. Further-more, RPL glass dosimeters maintain their signalthrough multiple readouts, providing a possibleadvantage over thermoluminescent dosimeters which

2

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

can only be read out once and then are annealed athigh temperature (400 °C) (Araki et al 2003).

In this study, a GD-302M RPL glass dosimeter(Asahi Techno Glass Co., Shizuoka, Japan) and FGD-1000 automatic readout system (Asahi Techno GlassCo., Shizuoka, Japan) were used to measure absorbeddose. Figure 2 shows a photograph of the GD-302MRPL glass dosimeter. The dosimeter element is acylindrical glass rod with a diameter of 1.5 mm andlength of 12 mm (figure 2(a)) inserted into the dosi-meter holder with a diameter of 2.8 mm and length of13 mm made of ABS resin (figure 2(b)). Irradiationwas performed while the dosimeter was inserted intothe holder.

3. Experimental setup

The prescribed dose in HDR-SBT for keloids is 6 Gyper fraction (total 18 Gy in 3 fractions) at the referencedepth of 2 mm under skin in Nippon Medical SchoolHospital.

A tough WE-type water phantom (Kyoto KagakuCo., Ltd, Kyoto, Japan), which is a water-equivalentsolid phantom, was used for the measurement; it iscomposed of 66.33 wt% carbon, 20.65 wt% oxygen,8.21 wt%hydrogen, 2.21 wt%nitrogen, 2.20 wt% cal-cium, and 0.40 wt% chlorine. The mass density is1.017 g cm−3, and the effective atomic number is 7.42.

Figure 3 shows a schematic illustration of theexperimental setup. As shown in figure 3(a), the size ofthe tough water phantom is 30 cm×30 cm in a hor-izontal plane. In the vertical axis, it has two layerscorresponding to the human trunk (20 cm) and skin(0.2 cm), respectively. As shown in figure 3(b), 42dosimeters were placed in a depression on the uppersurface of the trunk layer of the tough water phantom.The dosimeters were arrayed in 3 rows and 14 col-umns. In addition, the geometric center of the detec-tors of 3 rows were x=−1.3, 0, and 1.3, as shown infigures 3(a) and (b). The 0.2 cm thick skin layer of thetough water phantom was covered over the 20 cmthick trunk layer phantom, and a six-row FreiburgFlap Applicator was placed on top. As shown infigure 3(c), an area of 5 cm×5 cm from the centerwhere the dosimeters were placed was defined as theradiation field to deliver the prescribed dose uni-formly, and only five rows of the applicator were usedfor irradiation. The center of the radiation field was (0,55) in the (x, z) coordinate frame (units of cm).

Furthermore, the positive x-axis points to the leftas viewed from above, the positive y-axis points todepth from the surface of the tough water phantom,and the positive z-axis points to the advancing direc-tion of the 192Ir source in the coordinate system of theexperimental setup. In this study, the absorbed dose ofeach dosimeter was evaluated when the evaluation

Figure 1. (a)The patient’s actual treatment inHDR-SBT. (b)Photograph of a six-row Freiburg FlapApplicator.

Figure 2. (a)Photograph of aGD-302MRPL glass dosimeter element. (b)Dosimeter holder inwhich theGD-302MRPL glassdosimeter element is inserted.

3

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

area of 5 cm×5 cm (depth 0.2 cm to 0.48 cm fromthe surface of the tough water, which corresponds tothe depth region of the RPL dosimeters) uniformlyreceived a prescribed dose of 6 Gy.

4.Measurement

Firstly, the RPL glass dosimeters were annealed at400 °C for 60 min, and each predose was read repeat-edly a total of 10 times using FGD-1000 beforehand.Secondly, test irradiation was performed to verify theaccuracy of the source stop position assuming that the(0, 55) of the (x, z) coordinate was set as the optimaldwell position, and the actual source stop position wasconfirmed by real-time x-ray fluoroscopic images.

Thirdly, the dosimeters, Freiburg Flap Applicator, andtough water phantom were arranged as described infigure 3. Fourthly, based on the original optimizationalgorithm developed in our previous work (Ohtaet al 2016), the dwell time to deliver 6 Gy uniformly tothe evaluation area was calculated at each dwellposition. Finally, each dwell time was input to theremote afterloading device manually, and the irradia-tion planwas delivered.

The dosimeters were taken from the tough waterphantom by a tweezers after the irradiation, and theywere preheated at 70 °C for 30 min to stabilize thecolor centers. In addition, the dosimeters were readrepeatedly a total of 10 times using FGD-1000 with

Figure 3. Schematic illustration of the experimental setup. (a)Cross-sectional view in z-axis. (b)Horizontal view in the planewherethe RPL glass dosimeters were placed. (c)Horizontal view in the planewhere the Freiburg FlapApplicator was placed.

4

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

readout time 4.25 s per dosimeter after the dosimeterswere fully cooled.

Figure 4 shows a real-time x-ray fluoroscopicimage taken to verify the accuracy of the source stopposition. The image was analyzed by Image J, which isa Java-based image-processing program developed atNational Institutes of Health. During the analysis pro-cess, the 192Ir source stopped at the position shiftedfrom the setting dwell position by 0.01 mm to thenegative z-axis direction. However, this was negligiblebecause 1 mm is the minimum acceptable unit in theremote afterloading device (Nucletron 2009), and itwas considered that the 192Ir source stopped at the pre-cise position.

5. Calculations

5.1.MonteCarlo simulationMonte Carlo codes for particle transport simulation(i.e., MCNP, GEANT4, EGS5, and PHITS, etc) havebeen widely used to evaluate absorbed dose distribu-tions of 192Ir sources (Islam et al 2012, Candela-Juanet al 2013, Sarabiasl et al 2016). The PHITS (Satoet al 2013) version 2.88 used in this study is a general-purpose Monte Carlo code developed mainly in Japanthrough a collaboration between the Japan AtomicEnergy Agency (JAEA), the Research Orginzation forInformation Science and Technology (RIST) and theHigh Energy Accelerator Research Organisation(KEK). PHITS has been used in the fields of acceleratortechnology, radiotherapy, space radiation, and othersbecause it can analyze the dosimetric behavior ofvarious particles (i.e., electron, photon, neutron andproton, etc) over a wide energy range in three-dimensionalmodeling systems. From version 2.76, theelectron and photon transport algorithm has beenbased on Electron Gamma Shower Version 5 (EGS5)(Hirayama et al 2005), which is an electromagneticcascade Monte Carlo code incorporated into the

PHITS code in the so-called EGS5 mode. Theabsorbed dose was calculated using the EGS5 mode inthis study.

In the PHITS simulation, geometries were madefor the Freiburg Flap Applicator, dosimeters, andtough water phantom based on the shapes and ele-ment compositions disclosed from themanufacturers.For the Freiburg Flap Applicator, the element compo-sition was defined as silicon rubber (C5H6Si) becausethe detailed element composition was not disclosed.For the 192Ir source geometry, the design of themHDR-v2r source as described by Granero et al(2011)was used in the PHITS simulation. For beta andgamma rays released from the 192Ir source, variousenergies and their emission probabilities based on theupdated nuclear decay data (ICRP Publication 107)were taken into account in the simulation(ICRP 2008).

5.2.Optimization of dwell timeIn our previous work (Ohta et al 2016), each dwell timeto deliver a uniform dose to the evaluation area wasoptimized based on absorbed dose distribution of the192Ir point source at a depth including the dosimeterssimulated by the PHITS code.

The radioactive 192Ir source, which has a half-lifeof 73.83 days, is renewed every four months to keepthe irradiation time within the limits required by clin-ical practice at NipponMedical School Hospital. For afour-month-old 192Ir source, the actual treatmentirradiation time is under 15 min to deliver 6 Gy uni-formly to the evaluation area. If the total time of theirradiation is 15 min, the decrease of radioactivity ofthe 192Ir source is 0.01%. Therefore, radioactivitydecrease due to the decay during irradiation wasneglected in this study.

A total of 81 tally grids over an area of9 cm×9 cm were defined to evaluate the absorbeddose in an area larger than the actual source area of5 cm×5 cm. Since the two-dimensional spatial dis-tributions of absorbed dose due to different sourcepositions are considered to be relatively similar, thesimulation result performed with the 192Ir pointsource at the center of the radiation field(5 cm×5 cm) can be copied to the other source posi-tions with different contribution factors which areproportional to the dwell time in this case. Using thisassumption, an algorithm was developed to optimizethe dwell time at each dwell position to deliver a uni-form dose (6 Gy) to the evaluation area of5 cm×5 cm (depth 0.2 cm to 0.48 cm from the sur-face of the tough water, which corresponds to thedepth region of the RPL dosimeters) in the centralsquare inside the area of 9 cm×9 cm.

First of all, the absorbed doses at each of the 25tally grids were sequentially summed up for all 25dwell positions with 1 s as an initial dwell time. Thealgorithm then finds the tally grid with the lowest

Figure 4.Real-time x-rayfluoroscopic image analyzed byImage J.

5

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

absorbed dose in the summed data, and the distribu-tion for a 0.1 s contribution of the correspondingsource position is added to the summed distribution.The minimum acceptable unit in the remote after-loading device is 0.1 s. This process repeats until thedifference in absorbed dose between the highest andthe lowest positions is less than 0.01% of the lowestvalue where the optimized dwell times at each positionconverge to the same values. Finally, the lowest absor-bed dose over the 25 tally grids is normalized to 6 Gyand a set of optimized dwell times for the 25 sourcepositionwas obtained.

5.3. Absorbed dose evaluation for the RPL glassdosimetersTo evaluate the absorbed dose of the dosimeters in thewhole irradiation process, the absorbed dose rate ofeach dosimeter was calculated for the 25 differentdwell positions of the 192Ir source by PHITS in anothertally, multiplied by the corresponding optimized dwelltimes described previously, and summed up. Inaddition, the absorbed dose of each dosimeter wasevaluated by the actual detector size respectively, basedon the actual experimental setup in the PHITSsimulation.

The number of source gamma-rays was set to 107

in the simulation, and the statistical errors of the cal-culated absorbed dose of the dosimeters were 0.22%or under.

6. Results and discussion

6.1. Comparison betweenmeasurement andcalculationA total of 42 dosimeters were used in this study; thecoefficient of sensitivity variation was 0.87%, and thecoefficients of readout variations for each dosimeterwere less than 0.14%.

Figure 5 shows a comparison between the absor-bed dose of the dosimeters as measured and calculatedby PHITS. The absorbed dose calculated by PHITSwas underestimated by 13.3% to 21.8% (average18.4%±1.8%). The discrepancies betweenmeasuredand calculated absorbed dose of the dosimeters wasconsidered to result from the factors caused by themicroSelectron HDR-v3 remote afterloading device,the RPL glass dosimeter system, and the PHITSsimulation.

6.2. Correction of transit dose componentThe intensity agreed by 0.72% with the value of themanufacturing specification. In addition, it was con-firmed that the 192Ir source stopped at the preciseposition described in theMeasurement section.

The remote afterloading device uses a single 192Irsource, and irradiation is performed sequentially rowby row. In addition, the 192Ir source firstly stops at theproximal dwell position near the remote afterloadingtreatment unit, and the source moves to each dwellposition sequentially in the distal direction in 1 cmsteps. When irradiation at the distal position is fin-ished, the source moves back immediately to theremote afterloading treatment unit. This processrepeats in the next row.

Figure 6 shows the moving process of the 192Irsource. The ‘transit dose’ is the absorbed dose receivedduring the time the source steps between successivedwell positions and moves back from the catheter tip.Since it was not considered in PHITS simulation, anadditional analysis was performed to evaluate the tran-sit dose component. To get good statistics in a realistictime, we set the number of source gamma-rays in thesimulation to 5×106, and the statistical errors of cal-culated absorbed dose of the dosimeters were 0.08%or under.

The 192Ir source travels between successive dwellpositions with average speed of 35 cm s−1 (Nucle-tron 2009), and the transit time in successive dwellpositions was calculated for each in the section

Figure 5.Comparison between themeasured and calculatedabsorbed dose of the RPL glass dosimeters. X coordinateindicates the center of the coordinate of the dosimeters shownin figures 3(a) and (b).

Figure 6. Schematic illustration of the transit dose compo-nents.

6

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

between Z=53 and Z=57. In addition, the 192Irsource is moved back from the distal tip with averagespeed of 25 cm s−1 (Nucletron 2009), and the transittime was calculated for each in the section between thedistal dwell position (Z=52.5) and the end of theradiation field (Z=57). The absorbed dose rate ofeach dosimeter was calculated by PHITS assumingthat the source travels in 1 mm steps, and the totaltransit dose was evaluated by summing up the absor-bed dose for each source position in 1 mmsteps.

Furthermore, in the microSelectron HDR-v3remote afterloading device, when irradiation in theprevious dwell position is finished, the dwell timecounter starts immediately, and therefore the dwelltime for the next position includes 0.1 s of the transittime from the previous dwell position for adjustment(Nucletron 2009).

Therefore, the actual dwell time is 0.1 s shorterthan the planned dwell time except in the first dwellposition for each row. The actual dwell time at eachdwell position was estimated, and absorbed dose ofeach dosimeter was evaluated in the sameway.

Figure 7 shows the absorbed dose of each dosi-meter calculated by PHITS, considering for transitdose component and actual dwell time. The correctedabsorbed dose was increased by 0.48% to 0.87% (aver-age 0.71%±0.11%) compared with that before thecorrection, and the transit dose component was can-celed by the adjustment of dwell time so that the effectwas negligible.

6.3. Energy dependence of RPL glass dosimeterIn regard to the glass dosimeter reader system, thestandard glass dosimeter for calibration of the readersystem was read to verify the validity of reading value,and the reading was less than the calibration value by0.79%, which was in good agreement with the calibra-tion value.

On the other hand, the dosimeter is calibrated bymono-energetic 662 keV photons emitted from Cs-

137, which is widely used for dosimetry in radio-therapy using high-energy ionizing radiation. How-ever, the GD-302M RPL glass dosimeter has a highersensitivity for lower-energy photons because theamount of luminescence increases due to the photo-electric effect.

Therefore, a sensitivity correction of each dosi-meter was performed based on the relative sensitivity,which is based on the energy dependence of the GD-302M glass dosimeter reported by Huang and Hsu(2011). In addition, GSYS2.4 digitizing software devel-oped at the Japan Nuclear Reaction Data Centre(Suzuki 2010, Semkova 2013, Suzuki 2013) was usedto obtain numerical data and the obtained data waslinearly interpolated, whichwas used for the analysis.

Firstly, each dosimeter was defined an evaluationregion based on the actual experimental setup, and theenergy spectrum of photons incident on each dosi-meter in the whole irradiation process was simulatedby PHITS. Because the energy dependence describesthe sensitivity between the absorbed dose of the RPLglass dosimeter and the absorbed dose of the mediumas a contribution of photon, in addition to each pho-ton energy value, it was necessary to take into accountthe energy deposition of photons in each dosimeter.Therefore, the kerma factors of photons in the RPLglass dosimeter were evaluated by PHITS as a functionof photon energy. Note that ‘kerma’ represents kineticenergy per unit mass, and is equal to the absorbed dosewhen the equilibrium between incoming and out-going secondary particles is established. In addition,the relative sensitivity of theGD-302M glass dosimeterfor each energy value was weighed by the kerma factorand the photon energy distribution, and they wereintegrated for all energies. Therefore, each sensitivitycorrection factor f was calculated using the followingequation:

fE K E R E E

E K E E

d

d, 1

E

E0

max

0

max

ò

ò=

F

F

( ) ( ) ( )

( ) ( )( )

where E is the photon energy, Φ(E) is the energyspectrum of the photon fluence, K(E) is the kermafactor, and R(E) is the relative sensitivity of thedosimeters.

The relative sensitivity reported by Huang wasobtained with a photon energy lower than 662 keV.However, the rates of photons which have an energyhigher than 662 keV incident on each dosimeter were0.139% to 0.146% (average 0.142%), which is negli-gible. Therefore, the sensitivity correction factor wascalculated based on the assumption that the relativesensitivity for photons higher than 662 keV was 1. Theuncertainty of the f factor for each 42 dosimeter is0.018% to 0.021%, with the confidence level of onestandard deviation.

Figure 8 shows the calculated absorbed dose of thedosimeters compared with the measured data,

Figure 7.Absorbed dose of the RPL glass dosimeterscalculated by PHITS, accounting for transit dose and adjust-ment of dwell time.X coordinate indicates the center of thecoordinate of the dosimeters shown infigures 3(a), (b).

7

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

including a correction of the sensitivity due to the pho-ton energy dependence, calculated by PHITS with thecorrection of transit dose component. Accounting forthese corrections, the absorbed dose of the dosimeterscalculated by PHITS was underestimated by 10.2% to19.0% (average 15.5%±1.9%).

6.4. Summary of factors of difference betweenmeasured and calculated valuesFirstly, the compositions andmass density ofmaterialsexcluding the Freiburg FlapApplicatorwere accuratelydefined in the PHITS simulation. Iwamoto et al (2017)reported that the benchmark test result to verify theaccuracy of photon and electron transport using theEGS5 mode agreed with the experimental data wellexcept for neutrons produced by photonuclear reac-tions with energy range from keV to GeV. In addition,the transit dose component was canceled by theadjustment of dwell time in the microSelectron HDR-v3 remote afterloading device, so that the effect wasnegligible.

On the other hand, the sensitivity corrections ofthe dosimeters were performed based on the energydependence of the GD-302M RPL glass dosimeterreported by Huang and Hsu (2011). However, sincethe photon sensitivity curve varies greatly in the energyrange from 20 to 120 keV, it is considered that someuncertainty remains, depending on the photon energyand interpolationmethod.

Furthermore, the sensitivity correction factor ofeach dosimeter was calculated based on the relativesensitivity for photon energy because almost all betaparticles emitted from the 192Ir source stop within thebeads of the applicator due to the stopping range.However, itmay be assumed that not only photons butalso electrons enter the dosimeters since electrons areemitted by the interaction between 192Ir gamma rayand materials such as the applicator and the 2 mmthick tough water assumed as human skin. Since it is

difficult to evaluate the contribution of the sensitivitycaused by the electron contamination in this study, thesensitivity-correction procedure was considered to beone of the main reasons for the difference between themeasured and calculated absorbed dose of thedosimeters.

7. Conclusion

In this study, absorbed dose was measured in a simplegeometry using a water-equivalent solid phantom andRPL glass dosimeter to verify the accuracy of absorbeddose by the combination of the PHITS calculation andthe developed evaluation system. Accounting for thetransit dose component and energy dependence of thedosimeters analytically, the absorbed dose of thedosimeters calculated by PHITS was underestimatedby 10.2% to 19.0% (average 15.5%±1.9%). Thesensitivity-correction procedure was considered to bethe main reason for the difference between measuredand calculated absorbed dose of the dosimeters. Toperform verificationmore accurately in future work, itis necessary to (1) evaluate the energy dependence ofthe RPL glass dosimeter measured for more photonenergy points and (2) develop a measurement systemcapable of sensitivity calibration in the radiation fieldequivalent to the RPL glass dosimeter based on theactual geometry inHDR-SBT.

Acknowledgments

The authors would like to thank the radiologicaltechnologists of the Department of Radiation Oncol-ogy, Nippon Medical School Hospital, for technicalassistance with the experiments. The authors alsothank Dr T Sato, from the Japan Atomic EnergyAgency, for helpful discussions about the PHITSsimulation.

ORCID iDs

MOhta https://orcid.org/0000-0002-6535-9220

References

Araki F, Ikegami T, Ishidoya T andKuboHD2003MeasurementsofGamma-Knife helmet output factors using aradiophotoluminescent glass rod dosimeter and a diodedetectorMed. Phys. 30 1976–81

Candela-JuanC, Perez-Calatayud J, Ballester F andRivardM J 2013Calculated organ doses usingMonte Carlo simulations in areferencemale phantomundergoingHDRbrachytherapyapplied to localized prostate carcinomaMed. Phys. 40 033901

GraneroD, Vijande J, Ballester F andRivardM J 2011Dosimetryrevisited for theHDR192Ir brachytherapy sourcemodelmHDR-v2Med. Phys. 38 487–94

Guix B,Henriquez I, Andrés A, Finestres F, Tello J I andMartínez A2001Treatment of keloids by high-dose-rate brachytherapy:a seven-year study Int. J. RadiationOncology Biol. Phys. 50167–72

Figure 8.Absorbed dose of the RPL glass dosimeterscompared between themeasured data, with a correction ofsensitivity energy dependence, calculated by PHITSwithcorrection of transit dose.X coordinate indicates the center ofthe coordinate of the dosimeters shown in figures 3(a), (b).

8

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al

HefniMA, El-Attar A L, AlyMMO, El-SaidM I andHashemMA2013 Experimental study of usingwaxwedge filter in electronbeamsProc. of the Eleventh Radiation Physics and ProtectionConf. (Cairo, November 2012) pp 209–16

HirayamaH,Namito Y, BielajewAF,Wilderman S J andNelsonWR2005The EGS5Code System SLAC-R-730 andKEKReport 2005-8High EnergyAccelerator ResearchOrganization, Stanford Linear Accelerator Center

HoangD, Reznik R,OrgelM, LiQ,Mirhadi A andKulberDA 2017Surgical excision and adjuvant brachytherapy vs externalbeam radiation for the effective treatment of keloids: 10-yearinstitutional retrospective analysisAesthet. Surg. J 37 212–25

HuangDYC andHsu SM2011Radio-photoluminescence glassdosimeter (RPLGD)Adv. Cancer Therapy edHGali-Muhtasib (InTech) (https://doi.org/10.5772/23710)

ICRP 2008Nuclear decay data for dosimetric calculationsAnn.ICRP 38 7–96 ICRPPublication 107

IslamMA,AkramuzzamanMMandZakariaGA 2012Dosimetriccomparison between themicroSelectronHDR192Ir v2source and the BEBIG 60Co source forHDRbrachytherapyusing the EGSnrcMonte Carlo transport code J.Med. Phys.37 219–25

Iwamoto Y, Sato T,Hashimoto S,OgawaT, Furuta T, Abe S, Kai T,MatsudaN,HosoyamadaR andNiita K 2017 Benchmarkstudy of the recent version of the PHITS code J. Nucl. Sci.Technol. 54 617–35

Kuribayashi S,Miyashita T,OzawaY, IwanoM,OgawaR, Akaishi S,Dohi T,HyakusokuHandKumita S 2011 Post-keloidectomyirradiation using high-dose-rate superficial brachytherapyJ. Radiat. Res. 52 365–8

Nucletron 2009Oncentra® External Beam v4.3Oncentra ® Brachyv4.3 170. 730 Physics andAlgorithmsManual (Stockholm:Elekta AB) 7 66–70

OhtaM,NakaoN,Kuribayashi S,Miyashita T, ShigematsuN andHayashizakiN 2016 Evaluation of radiation exposure in Ir-192 brachytherapy for treatment of keloids Proc. of the 5th Int.Symp. on Innovative Nuclear Energy Systems (Tokyo)November (accepted)

Park TH, Seo SW,Kim JK andChangCH2011Management ofchest keloids J Cardiothorac Surg. 6 49

Sarabiasl A, AyoobianN, Jabbari I, PoorbaygiH and JavanshirMR2016Monte Carlo dosimetry of the IRAsource high dose rate192Ir brachytherapy sourceAustralas. Phys. Eng. Sci.Med. 39413–22

SatoT et al 2013 Particle and heavy ion transport code systemPHITSVersion 2.52J. Nucl. Sci. Technol. 50 913–23

Sclafani A P,Gordon L, ChadhaMandRomoT 3rd 1996Prevention of earlobe keloid recurrencewith postoperativecorticosteroid injections versus radiation therapy: arandomized, prospective study and review of the literatureDermatol. Surg. 22 569–74

SemkovaV 2013 Benchmarking of digitization software (MemoCP-D/761): results and discussionsConsultants’Meeting onBenchmarking of Digitization Software (Vienna, November2012) pp 42–57

Suzuki R 2010GSYS2.4manualNRDFAnnual Report 20 3–25Suzuki R 2013 Introduction, design and implementation of

digitization softwareGSYSConsultants’Meeting onBenchmarking of Digitization Software (Vienna, November2012) pp 19–26

TsudaM2000A few remarks on photoluminescence dosimetrywith high energy x-rays Igaku Butsuri 20 131–9

9

Biomed. Phys. Eng. Express 4 (2018) 025022 MOhta et al