I. INTRODUCTIONBrachytherapy of intraocular tumors with 125I eye plaqueshas become a successful treatment modality and a viable al-ternative to enucleation.1–4 The eye plaques designed by theCollaborative Ocular Melanoma Study (COMS) group arethe most frequently used type for brachytherapy of ocularmelanomas. The eye plaque of COMS design (COMS plaque)consists of a 0.5 mm thick bowl-like backing made of gold al-
loy (trade name Modulay) with a cylindrical collimating lipand a Silastic seed carrier insert with grooves into which the125I seeds are loaded.5–7 The carrier offsets the seeds by 1 mmfrom the concave (front) surface of the plaque assembly.5, 6
Both the gold-alloy backing and Silastic insert have highereffective atomic number Zeff compared with water.6
In 1985, COMS defined dose prescription, calculation, andreporting criteria for 125I eye plaque treatments. In the COMS
011708-2 Acar et al.: Material heterogeneity dosimetric effects for 125I eye plaques 011708-2
protocol, it was required to use a point-source approximationfor 125I seed (models 6711 and 6702 at the time) with the ef-fects of the gold backing and Silastic insert on scatter andattenuation ignored and the plaque was assumed to be waterequivalent.8–10 However, it is now well recognized that the useof line-source approximation and incorporating heterogeneitycorrection to account for the gold plaque and the Silastic in-sert into the dose calculations result in a significant and con-sistent reduction of calculated doses to tumor and structures ofinterest within the eye, as reported in the literature includingthe recently published TG-129 report.6, 7, 11–16 These studieson 125I seed are about models 6711, 2301, 1251L, 3500, andIAI-125, and mostly based on Monte Carlo (MC) simulations.There is no report to our knowledge addressing the hetero-geneity effect specific to model I25.S16 seed, which is alsowidely used in ocular melanoma treatment. As reported byThomson and Rogers, the heterogeneity effect is seed modeldependent. Thus, it is important to quantify such effect for themodel I25.S16 seed.
The dose reduction is attributed to the interaction of thelow energy (about 30 keV) photons with the plaque materi-als and the predominance of the photoelectric effect. For pho-tons of these low energies, photoelectric effect is the dominantradiation interaction process. Because the photoelectric massattenuation coefficient is roughly proportional to the cube ofthe effective atomic number (Zeff) of the absorbing material,inhomogeneities around an 125I seed can significantly affectthe absorbed dose distribution.
Radiochromic film dosimetry has been established as anaccurate measurement method in radiation therapy, includ-ing external beam and brachytherapy.17–26 In particular, Chiu-Tsao et al. demonstrated the viablility of the dosimetry for125I seed of Implant Sciences model 3500 using EBT filmsfor distances down to 1 mm in Solid Water phantom.20
Furstoss et al. reported the EBT film dosimetry for 125I (model6711) seed in liquid water phantom and verified the MC-calculated dose distributions.24 Krintz et al. reported on mea-sured relative dose distributions for COMS eye plaque (14and 20 mm) uniformly loaded with 125I (model 6711) seedsusing GafChromic model MD55-2 films in a Solid Waterphantom.26 They found the measured relative off-axis dosesnear the plaque periphery were lower by an average of 15%compared with the calculated doses using Plaque Simulator(PS) software version 4.27
The relative central axis depth doses and off-axis doseprofiles at depths of 5 and 12 mm using GafChromic EBTfilm dosimetry for COMS eye plaques (14–20 mm) uniformlyloaded with 125I seeds (model I25.S16) were reported byAcar,28 based on the calibration at 6 MV photon beam en-ergy. These relative doses were found to be within 15% fromthe calculated doses using an earlier version of PS (v4). How-ever, absolute doses were not determined due to the lack offilm calibration at low photon energy (∼30 keV) for 125I,and the plaque heterogeneity effect could not be evaluated inthat work. In the current study, expanding from Acar’s work,the absolute doses for 125I seeds (model I25.S16) in COMSplaques were measured using EBT film, based on the cali-bration at 125I energy. This is the first systematic study of
absolute dosimetry using EBT film in an eye phantom forCOMS plaques uniformly loaded with 125I seeds, both alongthe plaque’s central axis and off-axis direction. This paper isthe first report on the evaluation of the plaque material het-erogeneity dosimetric effects specifically for this seed modelbased on EBT film dosimetry.
The PS software is specifically for eye plaque treatmentplanning. This software supports three-dimensional dose cal-culations for a wide range of radioactive nuclides, seed mod-els, and several different designs of plaques (including COMSplaques).27, 29 The algorithm is based on superposition of in-dividual seed dose contributions following the AAPM TG-43U1 formalism.29, 30 There is also a feature for heterogeneitycorrection due to plaque materials. In version 4 and earlier,only a constant dose correction factor of 0.9 was allowed forheterogeneity correction.27 Starting version 5, correction foroblique path length in Silastic and dose correction functionT(r) that varies with the distance from the seed became avail-able for use in heterogeneous dose calculations.29
Monte Carlo codes have been widely used for simulationsof radiation therapy dose calculations in external beam andbrachytherapy.7, 12, 13, 30–33 Specifically, Melhus and Rivard12
used MCNP5 code34 to calculate doses for COMS plaquesloaded with 125I (model 6711), 103Pd (model 200), and 131Cs(CS-1 Rev2). Thomson et al.7, 13 investigated several seedmodels of 125I (6711, 2301, 1251L, 3500, and IAI-125) and103Pd (200, 2335 and IAPd-103) in COMS plaque using MCcalculations with BrachyDose/EGSnrc code.35, 36 They foundthe T(r) values to be seed model dependent for the sameradionuclide.13 The T(r) values for 125I seed (model 6711)and 103Pd seed (model 200) were published in the TG-129report.16 However, the T(r) values for 125I seed model I25.S16have not been reported. In this study, we determined the T(r)values for this seed model for the first time using MCNP5code.34
The EBT film measured absolute doses for uniformlyloaded COMS plaques were compared with those calculatedwith homogeneous water assumption and with the plaque het-erogeneity effect incorporated using PS version 5.3.9 basedon the dose correction function T(r) values specific to modelI25.S16 seed. The PS software version 5.3.9 has been testedagainst MC for 125I seed (model 6711) and 103Pd seed (model200) as reported in a recent paper by Rivard et al.14 The test-ing of heterogeneous dose calculation accuracy of PS v5.3.9against EBT film data performed for the 125I seed (modelI25.S16) is reported in this paper for the first time.
II. MATERIALS AND METHODS
II.A. 125I seed and COMS eye plaques
125I seeds model I25.S16 (Bebig GmbH, Berlin, Germany)are available with air-kerma strength up to 32 U each. Theyare identical in design to the model I25.S06,30, 37, 38 but withhigher strength suitable for eye plaque therapy. The inter-nal construction and dimensions of the model I25.S16 125Iseed are described as follows by the vendor. The source cap-sule consists of a 0.05 mm thick titanium tube, density of
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4.54 g/cm3, that is laser welded at both ends. The weldsare spherical with an average thickness ranging from 0.44 to0.48 mm depending on the batch. On average, the weld isslightly thinner on the axis than near the cylindrical wall.The radioactive core consists of a cylindrical ceramic shellwith outer and inner diameters of 0.60 and 0.22 mm, respec-tively, and a length of 3.50 mm. The ceramic has a density of2.88 g/cm3 and consists of alumina (Al2O3) within which125I is uniformly distributed. A gold marker, density of19.32 g/cm3, 0.17 mm diameter, and 3.5 mm long inside theceramic core, permits radiographic localization of the seed.
The individual air-kerma strengths for all the seeds usedin this study were measured using a well-type ionizationchamber (PTW Type 33004 with Unidos Webline electrom-eter) calibrated by an ADCL to check whether the measured-and the manufacturer-stated source strengths were withinthe tolerance limit of AAPM.39 The measured air-kermastrength had a median difference of 2.2% from the manufac-turer’s stated value, with decay correction, and was within theAAPM tolerance range. The air-kerma strength values statedin the Bebig certificate were used.
The COMS plaques from Trachsel Dental Studio(Rochester, MN) are available in seven different sizes from 10to 22 mm in 2 mm increments. In this study, we focus on theplaques with 14, 16, 18, and 20 mm in diameter, loaded with125I (model I25.S16) seeds. The gold-alloy (Modulay) back-ing of the COMS plaque has elemental composition of 77%gold, 14% silver, 8% copper, and 1% palladium by weight.7, 40
The Silastic insert has high percentage of silicon (Si). The el-emental composition of Silastic is 6.3% hydrogen, 24.9% car-bon, 28.9% oxygen, 39.9% silicon, and 0.005% platinum byweight.6
Each calibration film was 2 × 2 cm2 in size. To check theenergy dependence of the film response of this lot of EBTfilm, multiple sets of calibration films were exposed to 6 MVbeam and 125I photon energy with the same set of dose values.There were three unirradiated films serving as the backgroundfilms. Since the film response is nearly energy independentfrom 6 MV photon down to 22 keV,41 the calibration curvefor 6 MV photon served as a check on the 125I calibrationcurve.
For 6 MV calibration, each film was positioned at a depthof 1.5 cm in a Solid Water phantom with 15 cm of surround-ing material to provide sufficient backscatter. Films were ir-radiated (one at a time) in fixed SSD conditions, orientated
normal to the incident beam. EBT film pieces were centeredin a 10 × 10 cm2 field and doses between 0 and 1200 cGyin 11 steps were delivered with a 6 MV photon beam from aSiemens ONCOR linac. The linac output was calibrated fol-lowing IAEA TRS 398 protocol.42
For 125I calibration, a single high-strength seed was used.A groove was machined on a 1 cm polystyrene (RadiationProducts Design, Inc., Albertville, MN) phantom to accom-modate an 125I seed positioned horizontally. A calibrationEBT film was positioned horizontally above the seed with a5-mm polystyrene phantom slab in between. Additional phan-tom slabs (10 cm thick each) were above and below the filmand seed containing slab to provide full scatter condition. Theexposure time for applying a given dose to each film was cal-culated based on the dosimetric parameters for model I25.S06from the TG-43U1 report.30 Multiple calibration films of thesame lot were irradiated in this setup, one at a time, to deliverthe film-center doses to the same linac calibration doses.
II.B.2. Film irradiation in an eye phantom
II.B.2.a. Phantom configuration. A wax head phantomwith overall dimensions corresponding to an average humanhead was made. The wax head phantom was about 7 cm thickfrom anterior to posterior, which is about half the AP sepa-ration of an adult head (Fig. 1). The left-right separation was15 cm. The flat posterior surface of the wax head phantomwas positioned above a wooden table whose thickness was6 cm. This head phantom was machined to accommodate oneplaque (at a time) and an eye phantom in the left eye socketof the wax head phantom to simulate a patient’s head receiv-ing eye plaque treatment (Fig. 1). The eye phantom was asphere of 25 mm diameter made of polystyrene. The spherewas formed by two parts sandwiching a circular EBT film,25 mm in diameter. The EBT film experiments were per-formed with the head phantom in the supine position and theeye phantom above the plaque.
Following the COMS protocol, the origin of the plaque co-ordinate system is taken to be 1 mm away from the concavesurface of the Silastic insert at the center of the plaque co-inciding with the inner sclera. The plaque’s central axis wasalong the z-axis, pointing vertically upward. The depth frominner sclera was z (mm). The horizontal plane perpendicularto z-axis was defined as the x-y plane.
II.B.2.b. Single seed in 20-mm plaque. A single seed wasloaded at the center groove of a 20 mm plaque. EBT film cutto a circular shape with 25 mm diameter was positioned ver-tically in a central transverse plane of the polystyrene eyephantom above the plaque. The film plane (x-z plane) coin-cided with the transverse bisector of the seed and containedthe central axis (z) of the plaque. Three consecutive runs wereperformed with one film at a time using the same seed.
The air-kerma strength of the seed was 4.5 U at the be-ginning and 3.6 U at the end of the series of three runs.The exposure times ranging from 135 to 157 h were calcu-lated based on AAPM TG-43U1 parameters to deliver about400 cGy at 9 mm depth from inner sclera.
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FIG. 1. (a) Schematic diagram for the cephalad-caudad view of the transverse cross section of the phantom, through the center of the eye phantom. The eyeplaque was positioned in the machined cavity and then the eye phantom containing the EBT film was placed above the plaque. The remainder of the cavitywas filled with rice grains. (b) The expanded view of the eye phantom above a COMS plaque of 14 mm diameter. The seeds were indicated by cross-hatchedrectangle or circles. Due to gravity, the seeds were offset from the idealized position by an estimated 0.28 mm radially from the eye phantom center.
II.B.2.c. Uniformly loaded plaques. COMS eye plaques14, 16, 18, and 20 mm in size, uniformly loaded with 125Iseeds were studied. EBT experimental films (25 mm in diam-eter) were positioned in a polystyrene eye phantom above theplaque, in two configurations: (1) vertically in the plaque’scentral transverse plane (x-z plane) for measuring the centralaxis depth doses, (2) horizontally (x-y plane) and perpendicu-lar to the plaque’s central axis at 5 and 12 mm depth (from in-ner sclera) in the eye phantom for measuring the off-axis doseprofiles. For each plaque size, five and three repetitive runswere performed in configurations #1 and #2, respectively, tominimize Type A statistical uncertainties.
The air-kerma strength per seed ranged from 2.4 to 4 U.The exposure times (from 3.5 to 8.7 h) were adjusted to de-liver about 400 cGy at 5 mm depth from inner sclera.
II.B.3. Film scanning and data analysis
All the films (calibration, experimental, and background)were scanned at least 24 h after irradiation using an Epson10000XL scanner with transmission mode, 48 bit color, and75 dpi. The red channel data were analyzed using Mephystomc2 FilmCal software (PTW, Freiburg, Germany). Unirradi-ated films were used to obtained background optical density(OD) and net optical densities (NOD) were obtained by sub-tracting the background value from optical densities. For thetwo sets of calibration films, the NOD values were plottedagainst doses to establish the calibration curves for both 6 MVand 125I photons.
Based on the calibration curve established for 125I energy,the NOD values in the experimental films were converted todoses using the linear interpolation method. The dose valueswere determined at 1 mm intervals along the central axis from
depth z = 0 to 21 mm. Along the off-axis direction, the dosevalues were obtained for the off-axis coordinates x from 0to ±12 mm in 2 mm increments. Only the data points withdose values lower than 1200 cGy (within the calibration doserange) were kept in the subsequent analysis. The dose rate(in cGy/U-h) was determined by dividing the dose value bythe product of initial air-kerma strength and effective expo-sure time (incorporating decay) for each experimental film.For each plaque configuration, the average of the dose ratevalues from all repetitive runs was taken. The dose rates alongthe central axis and the off-axis dose rate profiles at depths of5 and 12 mm were determined and plotted.
II.C. Monte Carlo calculation for a single 125I(model I25.S16) seed
Dose distributions for a single 125I seed (model I25.S16)at the center groove of 20 mm COMS plaque on an eye weresimulated using the MCNP5 MC code. The geometry and theelemental compositions of the seed and plaque componentswere set up using the visual editor.34 The eye center was at thecenter of a large spherical phantom, 30 cm in diameter. Thephantom material was either liquid water or polystyrene. Thecomposition (C8H8)n, and density 1.05 g cm−3 were used forpolystyrene in the MC simulation.43 Seed was taken as a vol-umetric source.31 Simulations were performed in both hetero-geneous (Hetero) and homogeneous (Homo) environments,where the latter were in homogeneous water (or polystyrene)medium and the former included the Silastic seed carrier in-sert and gold-alloy backing. The absolute (reference) doserate values per unit air-kerma strength were calculated. ModePE and *F8 energy deposition tally44, 45 were used with theAAPM TG-43U1 125I photon source spectrum.30 Dose was
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scored in (0.5 mm)3 voxels with 1 mm spacing betweenneighboring voxel centers. The voxel centers were alongthe plaque’s central axis (coinciding with the seed’s trans-verse axis) starting from the inner sclera point (2.4 mmfrom the seed center) to the depth (from inner sclera) of22 mm. The central axis dose rates up to 22 mm depth werecalculated. In the homogeneous water (polystyrene) environ-ment, the dose was scored in the same set of voxels alongthe seed’s transverse axis. Totally 2 × 109 histories were sim-ulated for each phantom configuration. The statistical uncer-tainties at the depths of 0, 5, 10, and 22 mm were, respectively,0.17%, 0.52%, 0.98%, and 1.86% (0.18%, 0.53%, 1.01%, and1.96%) in homogeneous water (polystyrene) medium, and0.18%, 0.53%, 1.01%, and 1.94% (0.20%, 0.56%, 1.03%,and 1.98%) with the presence of the plaque material hetero-geneities in water (polystyrene) medium.
The calculated dose rate values along the seed’s trans-verse axis in the homogeneous water medium were com-pared with those reported MC results in the literature for125I (model I25.S06) seed,30, 31, 37 which is identical in de-sign to model I25.S16 seed. The calculated dose rate val-ues along the plaque’s central axis for single seed in 20 mmplaque were compared with those measured using EBT film(Sec. II.B.2.b).
II.D. Heterogeneity correction function for single seed
The MC calculated dose rate values with and without theplaque were compared and the ratios of these values at indi-vidual depths were obtained for T(r) derivation in both wa-ter and polystyrene phantom.29 This ratio was plotted as afunction of depth z along the central axis, and demonstratedthe dose reduction effect due to the plaque heterogeneity formodel I25.S16 seed. These ratio values were also comparedwith those for model 6711 seed reported by Thomson et al.7
and by Melhus and Rivard.12
II.E. Dose calculations using Plaque Simulator
PS software for dose calculations is based on superpositionof individual seed dose contributions following the AAPMTG-43U1 formalism,30 with options to correct for plaque het-erogeneity effect.27, 29 In this study, dose calculations wereperformed using PS version 5.3.9 using line-source approx-imation with homogeneous water assumption and with theplaque heterogeneity correction. The dosimetric parametersfor model I25.S06 (identical to model I25.S16) from theTG43U1 report were used in the PS calculations. The het-erogeneity correction was achieved by enabling the option ofcarrier factor t(r, d, μ) and the feature of dose collimation bythe gold-alloy backing.29 The carrier factor was the productof T(r) specific to model I25.S16 seed and a factor that cor-rects for the additional attenuation due to oblique path lengthin Silastic beyond 1 mm. The values entered into PS for T(r)were based on those obtained from the MCNP5 calculationfor single seed (model I25.S16) in 20 mm plaque (Sec. II.D).The seed coordinates were based on the offset positions in theplaques as in Fig. 1(b).
The PS dose calculations were performed for all four uni-formly loaded plaques, 14, 16, 18, and 20 mm in diameter.For comparison with the EBT film data, initial dose rates (incGy/U-h) were calculated along the central axis and in off-axis directions at depths of 5 and 12 mm. The dose rate valuesobtained with the homogeneous water assumption and withthe plaque heterogeneity incorporated were referred to as “PSHomo” and “PS Hetero” in this report. The PS Homo valueswere the basis of comparison with the EBT film data in orderto demonstrate the dose reduction by the gold-alloy backingand Silastic insert. The air-interface option in the dose calcu-lation was disabled.
II.F. Uncertainty analysis
II.F.1. EBT film measurement data
The evaluation of the uncertainty in dose conversion fromoptical density in the calibration procedure was shown inTable I. All values henceforth are with a k = 1 statisticalcoverage factor. The conversion uncertainty was estimatedas 5.7%. In Table II, the uncertainty analysis for dose rateper unit air-kerma strength was tabulated. The uncertaintyin central axis dose rate value (cGy/U-h) for single seed in20 mm plaque was about 6.2%. For the uniformly loadedplaques, the uncertainty in dose rate values (cGy/U-h) wasestimated as 6.1% and 6.2% for central axis and off-axis,respectively.
The uncertainty in the depth determination along the cen-tral axis was estimated as 0.6 mm, which is sum of the fol-lowing uncertainties: (1) 0.2 mm film cutting uncertainty,(2) 0.2 mm uncertainty in film positioning in the eye phan-tom, and (3) 0.2 mm uncertainty in identifying the film edgefrom the scanned film image. The uncertainty in the off-axiscoordinates was estimated as 0.3 mm (one pixel). The toler-ances of the Silastic insert and gold backing dimensions wereestimated as 0.2 mm.
The dosimetric effect of the positional uncertainty in thedepth determination for the single seed film data were evalu-ated following the recommendation of TG-138.46 They variedwith the depth along the central axis from 16.2% at z = 5 mmdown to 4.9% at z = 22 mm, based on the inverse square law.Combining with the dose rate uncertainty of 6.2% (Table II),the total uncertainty in central axis dose rate values for singleseed ranged from 17.4% at z = 5 mm to 7.9% at z = 22 mm.Taking into account the uncertainties of the MC results, thecombined uncertainties of the values of ratio (EBT/MC) werebetween 17.4% at z = 5 mm and 8.3% at z = 22 mm.
II.F.2. PS dose calculation
Based on the recommendation from TG-138 report,46 theuncertainty in homogeneous dose calculation using PS (PSHomo) would be about 3.8% (Type B). Based on the resultsfrom Sec. II.C, the uncertainty in T(r) was estimated as max-imum 3.0%. When T(r) for model I25.S16 seed was enabledin the dose calculation accounting for the plaque heterogene-ity, the uncertainty in heterogeneous dose calculation using
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TABLE I. Uncertainty in dose conversion from optical density in the calibration procedure.
Type A Type B
Film noise contribution from all sourcesto single measurement
1.9% (Ref. 20)
Scanner consistency 1.0% (Ref. 20)Film to film reproducibility 0.75% (Ref. 20)Uncertainty in OD from scanning a singlecalibration film
2.3% (Quadrature sum of 1.9%,1%, and 0.75%)
Uncertainty in OD from scanning threebackground films
1.6%
Uncertainty in NOD of a calibration film 2.8% (Quadrature sum of 2.3%and 1.6%)
Uncertainty in film/seed positioning during calibrationfilm irradiationPositioning uncertainty, ud = 0.015 cmDistance between film and seed center, d = 0.55 cmUncertainty in the calibration film dose (dose changebetween d and d ± ud)
5.0% (Ref. 20)
Total conversion uncertainty 5.7% (Quadrature sum of 2.8%and 5%)
PS (PS Hetero) would be 4.8%, which is quadrature sum of3.0% and 3.8%.
II.F.3. Ratio (film/PS) for uniformly loaded plaques
The uncertainties in the film/PS Homo and film/PS Heteroratios were about 7.2% (quadrature sum of 3.8% and 6.1%)and 7.8% (quadrature sum of 4.8% and 6.1%), respectively,for central-axis points, and 7.3% (quadrature sum of 3.8% and6.2%) and 7.8% (quadrature sum of 4.8% and 6.2%), respec-tively, for off-axis points.
III. RESULTS
III.A. Calibration curve
The calibration curves are shown in Fig. 2. The two cal-ibration curves for 6 MV photon beam and 125I energy arevery close (within 2%) to each other, indicating weak energydependence of the film response for this film lot.
III.B. EBT film data and MC simulation results forsingle seed
The MCNP5 results for single seed dose rates along theseed’s transverse axis in homogeneous water medium were
TABLE II. Uncertainty analysis for the experimental films in eye phantom.
Type A Type B
Uncertainty in OD from scanning a singleexperimental film
2.3% (From Table I)
Uncertainty in OD from scanning threebackground films
1.6%
Uncertainty in NOD for a single data point fromexperimental film scanning
2.8% (Quadrature sum of 2.3%and 1.6%)
Uncertainty in NOD for five data points (fiverepetitive runs) from experimental film scanning
1.4% Central axis doses,(2.8%) / sqrt(5-1)
Uncertainty in NOD for three data points (threerepetitive runs) from experimental film scanning
2.0% Off-axis doses, (2.8%)/sqrt(3-1)
Uncertainty in dose conversion from NOD 5.7% (From Table I)
Uncertainty in seed’s air-kerma strength, Sk,(incorporating uncertainty in half-life)
1.6% (Refs. 20 and 46)
Uncertainty in central axis dose rate values per unitair-kerma strength for single seed in 20 mm plaque
6.2% 3 repetitive runs (quadrature sumof 2.0%, 1.6%, and 5.7%)
Uncertainty in central axis dose rate values per unitair-kerma strength for uniformly loaded plaques
6.1% 5 repetitive runs (quadrature sumof 1.4%, 1.6%, and 5.7%)
Uncertainty in off-axis dose rate values per unitair-kerma strength for uniformly loaded plaques
6.2% 3 repetitive runs (quadrature sumof 2.0%, 1.6%, and 5.7%)
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FIG. 2. Calibration curves for 6 MV photon beam (open square symbols)and 125I energy (closed diamond symbols) for EBT film (lot #36076-003AL).The ratios of the NOD values of 125I to 6 MV are also plotted and shown byopen triangles and scaled by the right vertical axis. The lines between thesymbols are for guiding the eye.
compared to published MC results.30, 31, 37 Agreement waswithin 2%. The single-seed MC uncertainties are presentedin Table III.
The standard deviation of the single seed EBT film dataamong the three runs was within 11%. The dose rate valuesas a function of depth along the plaque’s central axis for a sin-gle seed in 20 mm plaque measured using EBT film and cal-culated with MCNP5 code in polystyrene and water phantomwere plotted in Fig. 3. The MC results of absolute dose ratesin polystyrene phantom were about 4%–6% higher than thosein liquid water phantom, varying with the depths. Agreementwithin 9% was found between EBT film data and MC resultsin polystyrene phantom. The ratio values of the EBT filmdata to the MC results (both in polystyrene phantom) werebetween 0.93 and 1.09, and were plotted and scaled by theright axis in Fig. 3. The error bars for the ratio values werealso shown, demonstrating the agreement between EBT filmdata and MC results (both in polystyrene phantom) within theuncertainty of measurement and calculation.
The ratios of MCNP5 calculated dose rate values for singleseed in 20 mm plaque (Hetero) and in homogeneous medium(Homo) were determined as a function of depth from innersclera in both water and polystyrene phantoms. These ratio
TABLE III. Single-seed MC dosimetric uncertainties for dose rate at a givenposition (such as r0 = 1 cm and θ0 = 90◦).
d (1 cm, θ0)
Component Type A Type B
Source: capsule geometry 0.17%Dynamic internal components 0.022%Source radiation spectrum 0.1%Phantom composition 0.012%Physics of MC code 0.3%Cross section in phantom 0.2%μen/ρ for dose calculation 1.2%Tally volume averaging 0.006%Tally statistics 0.82%
Quadrature sum 0.82% 1.4%Total (k = 1) uncertainty 1.6%
FIG. 3. Plot of dose rate (cGy/U-h) vs depth from inner sclera for a sin-gle seed in 20 mm plaque, obtained from EBT film dosimetry (open circlesymbols), MCNP5 calculation in polystyrene medium (open square sym-bols), and in water medium (plus symbols). The values of ratios of EBTfilm/MCNP5 (both in polystyrene phantom) are also plotted (with error bars)and shown by open triangles and scaled by the right vertical axis. The linesbetween the triangle symbols are for guiding the eye.
values were the same in both phantom media. These T(r) ra-tios were plotted in Fig. 4. The dose reduction along the cen-tral axis, relative to the homogeneous case, was about 8%,10%, 12%, and 17% at 0, 5, 10, and 22 mm depths, respec-tively. The corresponding results reported by Thomson et al.7
and by Melhus and Rivard12 for model 6711 seed were alsoplotted for comparison. The differences between the ratios formodel I25.S16 and 6711 seeds were within 2%.
The depth z (mm) from inner sclera is related to the dis-tance from the seed center by a shift of 2.4 mm, i.e., r (mm)= z (mm) + 2.4. A table of T(r) values were extracted fromthe above plotted ratio (Hetero/Homo) values for single seedto be used in PS v5.3.9 calculations as listed in Table IV.
III.C. EBT film data and PS calculated doses foruniformly loaded plaques
III.C.1. Central axis depth dose distribution
The repeatability of film data for uniformly loaded plaquesat all central axis depths and off-axis coordinates was within2%. The dose rate values obtained using EBT film for the
FIG. 4. Plot of ratio (Hetero/Homo) values for single seed in 20 mm plaquevs depth from inner sclera.
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TABLE IV. The values of ratio (Hetero/Homo), T(r), values for single seedin 20 mm plaque obtained from MCNP5 calculation on the central axis. Thenumbers in square brackets were extrapolated values for use in PS software.
Distance r (cm) Depth z (cm) Ratiofrom seed center from inner sclera (Hetero/Homo)
uniformly loaded plaques along the central axis are shown inFig. 5(a), compared with those PS generated values with ho-mogeneous water assumption (PS Homo) and heterogeneitycorrection (PS Hetero). The dose ratio (film/PS Homo) and(film/PS Hetero) values along the central axis were shown inFig. 5(b).
The dose ratio (film/PS Homo) values were substantiallylower than unity (mostly between 0.8 and 0.9) for all plaquesizes studied, indicating dose reduction by COMS plaquecompared with homogeneous assumption. The ratio (film/PSHomo) varies with depth and the plaque size. The Silastic andModulay combination results in dose decrease relative to ho-mogeneous water of ∼10% and 20% at z = 10 and 20 mm,respectively, along the plaque’s central axis.
The dose ratio (film/PS Hetero) values were within 5% ofunity for most depths, indicating the PS Hetero calculationsagree with those from the film study. The exception was ob-served at shallower depths within 5 mm for 16 and 20 mmplaque sizes. [Fig. 5(b)] The authors cannot provide explana-tion for such variation with plaque sizes. However, the differ-ence between the film data and PS Hetero calculated valuesappears to be within the uncertainty of both the measurementdata and calculated values.
The central axis data presented here were for the pointsat more than 3 mm from the eye-air interface. No definitiveresults for the air interface effect can be demonstrated fromthis study.
III.C.2. Off-axis dose distribution
The off-axis dose rate values (in cGy/U h) at depths of 5and 12 mm were averaged over the two symmetric sides andplotted against the off-axis coordinates in Figs. 6(a) and 7(a),respectively, for the four plaque sizes, showing general agree-ment with the Hetero values from PS v5.3.9, and substantiallylower than those with homogeneous water assumption.
In off-axis directions, the dose rate (from film data) pre-sented a sharp drop in the penumbra region confirming colli-mation of radiation by the plaque’s lip. The collimating effectwas more pronounced at 5 mm depth. Compared with the doserates calculated with PS Homo, the dose reduction due to theplaque was found to depend on both the depth, z, and the off-axis coordinate, x. For a given depth, as x increases from zero(i.e., moving away from central axis), the dose ratio (film/PSHomo) decreases. [See Figs. 6(b) and 7(b).] Near the plaqueperiphery, these dose ratio values made a sharp drop in thepenumbra zone.
Almost all the ratio (film/PS Hetero) values were within7% from unity for all plaque sizes, indicating the verifica-tion of the PS Hetero values by the film data. Deviations fromunity were observed at the peripheral point with the off-axiscoordinate of 12 mm, namely, 17% higher in the penumbrazone at 5 mm depth for 18 mm plaque [see Fig. 6(b) lowerleft graph], and 9% lower at 12 mm depth for 20 mm plaque.[See Fig. 7(b) lower right graph.] These differences near theplaque periphery may be partially due to (1) the uncertaintyin the film data in the penumbra zone, and (2) the algorithm inPS v5.3.9 for correcting the collimating effect by the plaquelip.
IV. DISCUSSION
The film raw data were obtained at about 0.3 mm pixel sizewith 75 dpi. Using the PTW mc2 software, the dose data weresampled and determined at discrete intervals of 2 mm in theoff-axis direction. Since the PS software allowed tabulationof off-axis dose (or dose rate) values in 2 mm intervals, theauthors decided to plot the film data with PS calculated valuesin 2 mm intervals in Figs. 6 and 7 for direct comparison.
The difference between EBT film data and PS Hetero re-sults can be attributable to possible difference of the TG-43parameters of seed models I25.S06 and I25.S16. Since thereare no data in the literature showing the TG-43 parametersof I25.S16 model seed, data for the I25.S06 model seed wereused in the PS calculation.
The 125I seeds in the eye plaque and the detector (EBTfilm) in the polystyrene eye phantom were at distances lessthan 5 cm from the boundary of the head phantom, which sim-ulated the patient receiving eye plaque treatment. Hence, thefull scatter condition was not met in such configuration. How-ever, the MCNP5 and PS dose calculations performed in this
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FIG. 5. (a) Central axis dose rate values plotted against depth from inner sclera in logarithmic scale obtained from EBT film measurement, compared with thosecalculated using Plaque Simulator v5.3.9 with homogeneous water assumption (PS Homo) and with heterogeneity correction (PS Hetero). (b) The values ofdose ratio (film/PS Homo) in closed circle symbols and (film/PS Hetero) in open square symbols vs depth along the central axis. The error bars are also shown.
study were based on the assumption of liquid water mediumin the full scatter condition. Therefore, some difference be-tween the EBT film measurement and PS calculation may bepartially due to the difference in the phantom materials andradiation scatter environment.
The heterogeneity effects of COMS plaque componentson 125I dose distribution have been reported in the lit-erature, mostly based on the MC calculation.6, 7, 12, 13, 33
There were several publications on the measurement dataof relative or absolute (reference) dose distributions for
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FIG. 6. (a) Off-axis dose rates at depth of 5 mm obtained from EBT film measurement, compared with those calculated using PS v5.3.9 with homogeneouswater assumption (PS Homo) and with heterogeneity correction (PS Hetero). (b) The values of dose ratio (film/PS Homo) in closed circle symbols and (film/PSHetero) in open square symbols vs off-axis coordinates at depth of 5 mm. The lines between the scatter symbols are for guiding the eye.
COMS plaques, either with a single 125I seed or uniformlyloaded.6, 11, 15, 26, 28, 40, 47 Wu and Krasin reported relative dosedistributions for 12 and 16 mm COMS plaques uniformlyloaded with model 6711 125I seeds using radiographic XVfilm study, and also demonstrated the collimating effect dueto the plaque lip.40 Knutsen et al. reported diode measure-ment data of relative dose distributions in liquid water phan-tom for COMS plaques (12 and 20 mm) loaded with model6711 or 6702 125I seeds.47 Krintz et al. presented GafChromicMD55-2 film measurement data of relative dose distributionsin Solid Water phantom for COMS plaques (14 and 20 mm)
uniformly loaded with model 6711 seeds.26 Acar28 reportedrelative dose distributions for COMS plaques (14–20 mm)uniformly loaded with model I25.S16 125I seeds using EBTfilm dosimetry. Chiu-Tsao et al.6 and de la Zerda et al.11 re-ported on the heterogeneity effect for COMS plaques (12 and20 mm) loaded with single model 6711 125I seed in Solid Wa-ter phantom using TLD dosimetry. Trichter et al.15 presentedabsolute (reference) dose measurement for 20 mm COMSplaque uniformly loaded with model 6711 125I seeds in SolidWater phantom using prototype EBT-1 film with single emul-sion layer, and demonstrated the plaque heterogeneity effect
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FIG. 7. (a) Off-axis dose rates at depth of 12 mm obtained from EBT film measurement, compared with those calculated using PS v5.3.9 with homogeneouswater assumption (PS Homo) and with heterogeneity correction (PS Hetero). (b) The values of dose ratio (film/PS Homo) in closed circle symbols and (film/PSHetero) in open square symbols vs off-axis coordinates at depth of 12 mm. The lines between the scatter symbols are for guiding the eye.
on dose reduction. The AAPM Task Group 129 recently pub-lished a report addressing the dose evaluation for COMS eyeplaques loaded with 125I (model 6711) or 103Pd (model 200)seeds.16
However, none of the above mentioned reports addressedabsolute dosimetry for model I25.S16 125I seed in eye plaque.Since the 125I seed dose distributions are dependent on theseed model, this work evaluated the plaque heterogeneity ef-fects specific to model I25.S16 seeds in COMS plaques. Thesingle seed MC result was verified by the EBT film data. TheMC generated dose correction function for single seed of thismodel was entered into PS v5.3.9. The EBT film measured
doses were lower by about 10%–20% compared with the cal-culated homogeneous doses (PS Homo), demonstrating theplaque heterogeneity effect on dose reduction. With properheterogeneity correction feature enabled in PS v5.3.9 calcula-tions, the film-measured doses agree with the calculated het-erogeneous dose values (PS Hetero) within the uncertainty ofmeasurement data. Thus, the heterogeneous dose calculationof PS v5.3.9 was tested and found to be accurate for this seedmodel.
The methodology used in this study should be applicableto the eye plaques (not limited to COMS plaque) containing106Ru, 103Pd, 131Cs or other models of 125I seeds for ocular
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melanoma treatment. New models EBT2 and EBT3 films48, 49
introduced recently should also be feasible for such mea-surements. TLD dosimetry and MC calculation for COMSplaques uniformly loaded with model I25.S16 125I seeds arealso useful. This topic is currently being investigated.
V. CONCLUSION
Evaluation of material heterogeneity dosimetric effects forCOMS eye plaques loaded with 125I seeds (model I25.S16)was performed. Dose distributions in eye phantom for sin-gle seed and realistic multiple-seed arrangements loaded inCOMS eye plaques were measured using radiochromic EBTfilm. Significant heterogeneity effect on dose reduction due tothe plaque components was observed, along the central axisand off-axis directions (including the penumbra region). Sin-gle seed dose correction as a function of distance from theseed specific to model I25.S16 125I seed was obtained fromMC calculation and entered into PS software. The calculateddoses for uniformly loaded plaques using PS v5.3.9 with het-erogeneity correction option enabled were corroborated bythe EBT film measurement data. This lends confidence to theuse of PS software (v5.3.9) for estimating the doses to tumoras well as neighboring critical structures, such as macula andoptic nerve, as long as the proper heterogeneity correction op-tions are enabled.
The authors would like to thank Dr. Melvin Astrahan for atrial license of Plaque Simulator software version 5.3.9 usedin this research study. The helpful comments from Dr. MarkRivard were critical in improving this paper. The authorswould also like to thank Dr. Samil Gürdal for his valuablecontributions in estimating the uncertainties in MCNP5 cal-culations.
a)Electronic mail: [email protected])Author to whom correspondence should be addressed. Electronic mail:
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