Prakash Aryal

Independent dosimetric assessment of the model EP917 episcleral brachytherapy plaque

PURPOSE To investigate the influence of slot design on dose distributions and dose-volume histograms (DVHs) for the model EP917 plaque for episcleral brachytherapy. METHODS Dimensions and orientations of the slots were measured for three model EP917 plaques and compared to data in the Plaque Simulator (PS) treatment planning software (version 5.7.6). These independently determined coordinates were incorporated into the MCNP Monte Carlo simulation environment to obtain dose from the plaques in a water environment and in a clinical environment with ocular structures. A tumor volume was simulated as 5 mm in apical height and 11 mm in basal diameter. Variations in plaque mass density and composition; slot length, width, and depth; seed positioning; and Ag-marker rod positioning were simulated to examine their influence on plaque central axis (CAX) and planar dose distributions, and DVHs. RESULTS Seed shifts in a single slot toward the eye and shifts of the(125)I-coated Ag rod within the capsule had the greatest impact on CAX dose distribution. A shift of 0.0994 mm toward the eye increased dose by 14%, 9%, 4.3%, and 2.7% at 1, 2, 5, and 10 mm, respectively, from the inner sclera. When examining the fully-modeled plaque in the ocular geometry, the largest dose variations were caused by shifting the Ag rods toward the sclera and shifting the seeds away from the globe when the slots were made 0.51 mm deeper, causing +34.3% and -69.4% dose changes to the outer sclera, respectively. At points along the CAX, dose from the full plaque geometry using the measured slot design was 2.4%±1.1% higher than the manufacturer-provided slot design and 2.2%±2.3% higher than the homogeneous calculation of PS treatment planning results. The ratio of D10 values for the measured slot design to the D10 values for the manufacturer-provided slot design was higher by 9%, 10%, and 19% for the tumor, inner sclera, and outer sclera, respectively. In comparison to the measured slot design, a theoretical plaque having narrower and deeper slots delivered 30%, 37%, and 62% lower D10 doses to the tumor, inner sclera, and outer sclera, respectively. CONCLUSIONS While the measured positions of the slots on the model EP917 plaque were in close agreement (<0.7 mm) with the PS values, small differences in the slot shape caused substantial differences in dose distributions and DVH metrics. Increasing slot depth by 0.1 mm decreased outer scleral dose by 20%, yet shifting the Ag rods in the seeds toward the globe by 0.1 mm increased outer scleral dose by 35%. The clinical medical physicist is advised to measure these types of plaques upon acceptance testing before clinical use to inspect slot shape and position for comparison with data used for treatment planning purposes.

Implementation and early clinical results utilizing Cs-131 permanent interstitial implants for gynecologic malignancies.

OBJECTIVE Permanent interstitial brachytherapy is an ideal yet underutilized treatment modality for accessible, small volume gynecological malignancies. We present early clinical results utilizing a new permanent isotope, Cs-131. METHODS A retrospective review was performed evaluating patients treated with Cs-131 permanent interstitial radiation at our institution from July 2011 through June 2013. Doses were most commonly prescribed and calculated to a depth of 5mm using Paterson-Parker planar implant rules for Au-198. This activity was converted to air-kerma strength (U). A conversion factor of 1.1 was applied based on RBE calculations, clinical observation and experience. RESULTS 14 patients were identified among whom 17 Cs-131 implants were performed. Seven patients were implanted as sole therapy, and a median dose of 50 Gy was delivered. Ten implants were performed as boost within a more extensive radiation treatment plan. In these patients, a median implant dose of 27.5 Gy was used and the median total dose delivered in combination was 78.25 Gy. After a median follow up of 12 months, the actuarial local control rate was 84.4%. A very low level of grade 1-3 reactions was observed with no fistula formations or other severe side effects. CONCLUSIONS Permanent interstitial brachytherapy with Cs-131 was well tolerated with favorable early results compared to other series. Cs-131 has multiple favorable properties, including minimal radiation exposure to treating staff, and should be considered as a therapeutic option in appropriately selected patients. A methodology for dose prescription, calculation of radioactivity required and distribution of the isotope is also presented.

Determination of prescription dose for Cs-131 permanent implants using the BED formalism including resensitization correction.

PURPOSE The current widely used biological equivalent dose (BED) formalism for permanent implants is based on the linear-quadratic model that includes cell repair and repopulation but not resensitization (redistribution and reoxygenation). The authors propose a BED formalism that includes all the four biological effects (4Rs), and the authors propose how it can be used to calculate appropriate prescription doses for permanent implants with Cs-131. METHODS A resensitization correction was added to the BED calculation for permanent implants to account for 4Rs. Using the same BED, the prescription doses with Au-198, I-125, and Pd-103 were converted to the isoeffective Cs-131 prescription doses. The conversion factor F, ratio of the Cs-131 dose to the equivalent dose with the other reference isotope (Fr: with resensitization, Fn: without resensitization), was thus derived and used for actual prescription. Different values of biological parameters such as α, β, and relative biological effectiveness for different types of tumors were used for the calculation. RESULTS Prescription doses with I-125, Pd-103, and Au-198 ranging from 10 to 160 Gy were converted into prescription doses with Cs-131. The difference in dose conversion factors with (Fr) and without (Fn) resensitization was significant but varied with different isotopes and different types of tumors. The conversion factors also varied with different doses. For I-125, the average values of Fr/Fn were 0.51/0.46, for fast growing tumors, and 0.88/0.77 for slow growing tumors. For Pd-103, the average values of Fr/Fn were 1.25/1.15 for fast growing tumors, and 1.28/1.22 for slow growing tumors. For Au-198, the average values of Fr/Fn were 1.08/1.25 for fast growing tumors, and 1.00/1.06 for slow growing tumors. Using the biological parameters for the HeLa/C4-I cells, the averaged value of Fr was 1.07/1.11 (rounded to 1.1), and the averaged value of Fn was 1.75/1.18. Fr of 1.1 has been applied to gynecological cancer implants with expected acute reactions and outcomes as expected based on extensive experience with permanent implants. The calculation also gave the average Cs-131 dose of 126 Gy converted from the I-125 dose of 144 Gy for prostate implants. CONCLUSIONS Inclusion of an allowance for resensitization led to significant dose corrections for Cs-131 permanent implants, and should be applied to prescription dose calculation. The adjustment of the Cs-131 prescription doses with resensitization correction for gynecological permanent implants was consistent with clinical experience and observations. However, the Cs-131 prescription doses converted from other implant doses can be further adjusted based on new experimental results, clinical observations, and clinical outcomes.

SU-E-T-114: Dose Modification for Cs-131 Permanent Implants Using Resensitization-Corrected Normal Tissue BED

PURPOSE To apply resensitization (redistribution and reoxygenation) correction to normal tissue BED calculation and have it verified with clinical outcomes. METHOD AND MATERIALS The BED formalism without resensensitization for permanent implants was BED = D*RE - BF, where D is the prescribed dose, RE = 1 + (β/α)R0/(μr+λ), BF = K*Teff, K = ln(2)/(αTp), and Teff = Taveln(αDTp/T1/2). α and β are LQ parameters, R0 the initial dose rate, μr the repair constant, λ the source decay constant, and Tp the repopulation time. Resensitization can be included in the extended LQ equation (LQR) S = exp[-αD - βG(Tr)D2 + 1/2σ2 G(Ts)D2 + Teff/Tp], where G(Tr) and G(Ts) describe repair and resensitization, and 1/2σ2 represents cell-to-cell diversity. Combining Dales formalism with LQR led to RE = 1+(β/α)R0/(μr+λ)-(1/2σ2 /α)R0/(μs+λ), where μs is the resensitization constant. We used this formula to calculate the BED for normal tissue based on the prescribed dose for Au-198 GYN permanent implants from which we have gained extensive clinical experience. Then, we calculated the dose with Cs-131 which has the equal BED as Au-198. RESULTS The prescribed doses for Au-198 ranged from 10 to 120 Gy. The converted doses for Cs-131 implants ranged from 9 to 161 Gy (without resensitization correction) and 8.9 to 156 Gy (with resensitization correction), which resulted in the average value of dose conversion factor, Fn (no resensitization correction) = 1.14, and 1.10 for Fr (with resensitization correction) which agreed with the results from the calculation for tumor. The doses derived with 1.10 reduced the complications such as brisk moist desquamation in actual clinical cases. CONCLUSION Resensitization correction in BED for normal tissues led to significant reduction in prescription dose and thus in toxicity. The results further show that resensitization correction is needed for permanent implant dose calculation.

SU‐E‐T‐13: Comparison of Dose Rates with and without Gold Backing of USC #9 Radioactive Eye Plaque Using MCNP5

PURPOSE To show the effect of gold backing on dose rates for the USC #9 radioactive eye plaque. METHODS An I125 source (IsoAid model IAI-125A) and gold backing was modeled using MCNP5 Monte Carlo code. A single iodine seed was simulated with and without gold backing. Dose rates were calculated in two orthogonal planes. Dose calculation points were structured in two orthogonal planes that bisect the center of the source. A 2×2 cm matrix of spherical points of radius 0.2 mm was created in a water phantom of 10 cm radius. 0.2 billion particle histories were tracked. Dose differences with and without the gold backing were analyzed using Matlab. RESULTS The gold backing produced a 3% increase in the dose rate near the source surface (<1mm) relative to that without the backing. This was presumably caused by fluorescent photons from the gold. At distances between 1 and 2 cm, the gold backing reduced the dose rate by up to 12%, which we attribute to a lack of scatter resulting from the attenuation from the gold. Dose differences were most pronounced in the radial direction near the source center but off axis. The dose decreased by 25%, 65% and 81% at 1, 2, and 3 mm off axis at a distance of 1 mm from the source surface. These effects were less pronounced in the perpendicular dimension near the source tip, where maximum dose decreases of 2% were noted. CONCLUSIONS I 125 sources embedded directly into gold troughs display dose differences of 2 - 90%, relative to doses without the gold backing. This is relevant for certain types of plaques used in treatment of ocular melanoma. Large dose reductions can be observed and may have implications for scleral dose reduction.

SU‐F‐P‐47: Estimation of Skin Dose by Performing the Measurements On Cylindrical Phantom

PURPOSE To evaluate the skin dose by performing the measurements on cylindrical phantom with 6X beam. METHODS A cylindrical phantom was used to best model a patient surface. The source to surface distance (SSD) was 100 cm at phantom surface along central axis (CAX). The EBT2 films were cut into 2×2 cm2 pieces. Each piece of film was placed at CAX on phantom surface for each measurement at 0°, 15°, 30°, 45°, 60°, 75°, and 90° gantry angles for field sizes of 5×5, 10×10, 15×15, and 20×20 cm2 respectively. One hundred monitor units (MU) with 6X beam were delivered for each set up. Similarly, the measurements were repeated using lithium fluoride (LiF) thermoluminescent dosimeter (TLD) chips (1X1X1 mm3 ). Two TLD chips were placed for each gantry angle and field size. The calibration curves were produced for both film and TLD. The computed tomography (CT) was also performed on the same cylindrical phantom and dose was evaluated at the phantom surface using Eclipse treatment planning system (AAA algorithm) for skin dose comparison. RESULTS Data showed small differences at smaller angles among EBT2, TLD and Eclipse treatment planning system. But Eclipse treatment planning system under estimated the skin dose between 20% and 50% at larger gantry angles (between 40° and 80°) at all field sizes before dose differences began to converge. CONCLUSION Given this data, we can conclude that Eclipse treatment planning system under estimated the dose especially between 40 and 80 degrees of obliquity compared to the measurements results. Ideally, this study can be applied largely to head and neck patients where contours differ drastically and where skin dose is paramount.

Erratum: "Independent dosimetric assessment of the model EP917 episcleral brachytherapy plaque" [Med. Phys. 41, 092102 (11pp.) (2014)].

The authors would like to thank Dr. Melvin Astrahan for bringing to our attention that two of the three eye plaque samples reported by Aryal et al.1 that were represented as model EP917 eye plaques (manufactured by Eye Physics, LLC, Los Alamitos, CA) were in fact USC#9 model plaques whose manufacture preceded Eye Physics, LLC, and that the designs of the two plaque models were intended to be different.2 The product manufacturer, Eye Physics, LLC (Los Alamitos, CA), recently produced an extensive online resource that clarifies the differences between these two plaque models. Specifically, the model EP917 plaque has small separators between slots #1 and #2 and between slots #1 and #3, while the model USC#9 has a long, continuous midline channel into which three seeds are placed end-to-end. These dividers are visible in Fig. 1 of Aryal et al. This information may assist users to discern the plaques we investigated, which were not labeled with identifying information such as model or serial numbers. Another aspect in which the designs differ is in the depth of the slots. For the three plaques studied, the mean measured slot (MS) depths were 0.46±0.16 mm (k = 1).1 Separating out by model, the measured slot depth for the model EP917 plaque and the two USC#9 plaques were 0.65± 0.07 mm and 0.36 ±0.08 mm, respectively, as shown in Table I and Fig. 1. The

SU-E-T-15: A Comparison of COMS and EP917 Eye Plaque Applicators Using Different Radionuclides

Purpose: To investigate the effect of plaque design and radionuclides on eye plaque dosimetry. Methods: The Monte Carlo N-particle Code version 6 (MCNP6) was used for radiation transport simulations. The 14 mm and 16 mm diameter COMS plaques and the model EP917 plaque were simulated using brachytherapy seeds containing I-125, Pd-103, and Cs-131 radionuclides. The origin was placed at the scleral inner surface. The central axis (CAX) doses of both COMS plaques at −1 mm, 0 mm, 1 mm, 2 mm, 5 mm, 10 mm, 15 mm, 20 mm, and 22.6 mm were compared to the model EP917 plaque. Dose volume histograms (DVHs) were also created for both COMS plaques for the tumor and outer sclera then compared to results for the model EP917 plaque. Results: For all radionuclides, the EP917 plaque delivered higher dose (max 343%) compared to the COMS plaques, except for the 14 mm COMS plaque with Cs-131 at 1 mm and 2 mm depths from outer sclera surface. This could be due to source design. For all radionuclides, the 14 mm COMS plaque delivered higher doses compared to the 16 mm COMS plaque for the depths up to 5 mm. Dose differences were not significant beyond depths of 10 mm due to ocular lateral scatter for the different plaque designs. Tumor DVHs for the 16 mm COMS plaque with Cs-131 provided better dose homogeneity and conformity compared to other COMS plaques with I-125 and Pd-103. Using Pd-103, DVHs for the 16 mm COMS plaque delivered less dose to outer sclera compared to other plaques. Conclusion: This study identified improved tumor homogeneity upon considering radionuclides and plaque designs, and found that scleral dose with the model EP917 plaque was higher than for the 16 mm COMS plaque for all the radionuclides studied.

WE-A-17A-12: The Influence of Eye Plaque Design On Dose Distributions and Dose- Volume Histograms

PURPOSE To investigate the effect of slot design of the model EP917 plaque on dose distributions and dose-volume histograms (DVHs). METHODS The dimensions and orientation of the slots in EP917 plaques were measured. In the MCNP5 radiation simulation geometry, dose distributions on orthogonal planes and DVHs for a tumor and sclera were generated for comparisons. 27 slot designs and 13 plaques were evaluated and compared with the published literature and the Plaque Simulator clinical treatment planning system. RESULTS The dosimetric effect of the gold backing composition and mass density was < 3%. Slot depth, width, and length changed the central axis (CAX) dose distributions by < 1% per 0.1 mm in design variation. Seed shifts in the slot towards the eye and shifts of the 125 I-coated Ag rod within the capsule had the greatest impact on CAX dose distribution, increasing by 14%, 9%, 4%, and 2.5% at 1, 2, 5, and 10 mm, respectively, from the inner sclera. Along the CAX, dose from the full plaque geometry using the measured slot design was 3.4% ± 2.3% higher than the manufacturer-provided geometry. D10 for the simulated tumor, inner sclera, and outer sclera for the measured plaque was also higher, but 9%, 10%, and 20%, respectively. In comparison to the measured plaque design, a theoretical plaque having narrow and deep slots delivered 30%, 37%, and 62% lower D10 doses to the tumor, inner sclera, and outer sclera, respectively. CAX doses at -1, 0, 1, and 2 mm were also lower by a factor of 2.6, 1.4, 1.23, and 1.13, respectively. CONCLUSION The study identified substantial sensitivity of the EP917 plaque dose distributions to slot design. However, it did not identify substantial dosimetric variations based on radionuclide choice (125 I, 103 Pd, or 131 Cs). COMS plaques provided lower scleral doses with similar tumor dose coverage.

SU-E-T-354: Dosimetry Parameters Revisited for the IsoAid Model IAI-125A Brachytherapy Seed

PURPOSE Investigate and analyze TG43 dosimetry parameters for the IsoAid I125 model IAI 125A seedMethods: We revisited dosimetry for the IsoAid advantage I125 model IAI 125A seed to verify the published TG43 parameters. The IsoAid seed was examined based on manufacturer data and detailed information published in literature. The MCNP5 Monte Carlo radiation transport code was used. Sensitivity of these dosimetry parameters was assessed by changing a total of 10 geometric and computational variables, including source coating thickness, capsule design, photon emission spectrum, and cross section library. The calculated dose rate constants and radial dose functions were then compared with published literature values. RESULTS Compared to the reference simulation conditions of a 0.5 um source coating, recent capsule design data, the NNDC photon spectrum, and the ZAID.04p MCNP5 cross section libraries, the highest dose rate constant variation was +3.1% and the lowest was -4.0% compared to our value of 0.922 cGy/h/U. Calculated values by Solberg et al. (2002) and Meigooni et al. (2002) were 0.962 and 0.98 cGy/h/U respectively. There were significant differences (>10%) between the published values for the radial dose function and those calculated herein, becoming increasingly disparate as radial distance increased. These differences could be explained within a few percent by performing simulations with their photon emission spectrum and using older cross section libraries. For the similarly designed model 6711 seed, our dose rate constant results agreed within 0.2% and within 0.5% for gL(0.5<r<10cm) with Kennedy et al. (2010). CONCLUSION A modern investigation of the TG43 dosimetry parameters for the I125 IsoAid seed indicates clinically-significant differences with published results used for the AAPM consensus values. We suggest that the AAPM revisit dosimetry for this and other seeds included in the 2004 and 2007 reports.