Ann A. Lawyer

COMPARISON OF MONTE CARLO CALCULATIONS AROUND A FLETCHER SUIT DELCLOS OVOID WITH RADIOCHROMIC FILM AND NORMOXIC POLYMER GEL DOSIMETRY

The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33μGym2h-1 Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within ±2% or 2 mm for 98% of points compared with RCF measurements and to within ±3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

Development and implementation of a remote audit tool for high dose rate (HDR) Ir-192 brachytherapy using optically stimulated luminescence dosimetry

PURPOSE The aim of this work was to create a mailable phantom with measurement accuracy suitable for Radiological Physics Center (RPC) audits of high dose-rate (HDR) brachytherapy sources at institutions participating in National Cancer Institute-funded cooperative clinical trials. Optically stimulated luminescence dosimeters (OSLDs) were chosen as the dosimeter to be used with the phantom. METHODS The authors designed and built an 8 × 8 × 10 cm(3) prototype phantom that had two slots capable of holding Al2O3:C OSLDs (nanoDots; Landauer, Glenwood, IL) and a single channel capable of accepting all (192)Ir HDR brachytherapy sources in current clinical use in the United States. The authors irradiated the phantom with Nucletron and Varian (192)Ir HDR sources in order to determine correction factors for linearity with dose and the combined effects of irradiation energy and phantom characteristics. The phantom was then sent to eight institutions which volunteered to perform trial remote audits. RESULTS The linearity correction factor was kL = (-9.43 × 10(-5) × dose) + 1.009, where dose is in cGy, which differed from that determined by the RPC for the same batch of dosimeters using (60)Co irradiation. Separate block correction factors were determined for current versions of both Nucletron and Varian (192)Ir HDR sources and these vendor-specific correction factors differed by almost 2.6%. For the Nucletron source, the correction factor was 1.026 [95% confidence interval (CI) = 1.023-1.028], and for the Varian source, it was 1.000 (95% CI = 0.995-1.005). Variations in lateral source positioning up to 0.8 mm and distal∕proximal source positioning up to 10 mm had minimal effect on dose measurement accuracy. The overall dose measurement uncertainty of the system was estimated to be 2.4% and 2.5% for the Nucletron and Varian sources, respectively (95% CI). This uncertainty was sufficient to establish a ± 5% acceptance criterion for source strength audits under a formal RPC audit program. Trial audits of four Nucletron sources and four Varian sources revealed an average RPC-to-institution dose ratio of 1.000 (standard deviation = 0.011). CONCLUSIONS The authors have created an OSLD-based (192)Ir HDR brachytherapy source remote audit tool which offers sufficient dose measurement accuracy to allow the RPC to establish a remote audit program with a ± 5% acceptance criterion. The feasibility of the system has been demonstrated with eight trial audits to date.

Dosimetric evaluation of the Fletcher-Williamson ovoid for pulsed-dose-rate brachytherapy: a Monte Carlo study

We used radiochromic film dosimetry to validate a Monte Carlo (MC) model of a 192Ir pulsed-dose-rate (PDR) source inside a Fletcher-Williamson ovoid. MD-55-2 radiochromic film was placed in a high-impact polystyrene phantom in a plane parallel to and displaced 2.0 cm medially from the long axis of the ovoid. MC N-particle transport code (MCNPX) version 2.4 was used to model the ovoid and the 192Ir source. Energy deposition was calculated using a track-length estimator modified by an energy-dependent heating function, which is a good approximation of the collision kerma. To convert the estimates of the MC dose per simulated particle to clinically relevant absolute dosimetry, additional MC models of an actual and a virtual 192Ir source in dry air were simulated to determine air kerma strength for the penetrating part of the photon spectrum (>11.3 keV). The absolute dose distributions predicted by MCNPX agreed with the film results and were within +/-9.4% (k = 2) and within +/-2% or within a distance to agreement of 2 mm for 94% of the dose grid. Additional MC models characterized the uncertainty resulting from source positioning inside the ovoid. For a worst-case scenario of 1 mm off centre from the nominal source position in the 3 mm diameter ovoid shaft, the average dose deviation over the film plane was +/-5% (1sigma = +/-4%), with maximum deviation near the sharp dose-gradient provided by the shields of -20% to + 26%. A validated MC model is the first requirement to simulate common LDR clinical loadings (5-20 mgRaEq) and, thus, will aid in the transition from the current 137Cs Selectron LDR ICBT to PDR for treatment of gynecologic cancers.

Disease control and toxicity outcomes using ruthenium eye plaque brachytherapy in the treatment of uveal melanoma

PURPOSE Ruthenium-106 ((106)Ru) eye plaques have the potential to achieve excellent tumor control with acceptable radiation toxicity. We evaluated our experience in the management of uveal melanoma treated with (106)Ru brachytherapy. METHODS AND MATERIALS The records of 40 patients with uveal melanoma treated with brachytherapy using (106)Ru plaques from 2003 to 2007 at University of Texas MD Anderson Cancer Center were reviewed. Endpoints assessed included tumor control and toxicity. RESULTS Median ophthalmologic follow-up was 67 months. Actuarial 5-year rates of local control (LC), progression-free survival (PFS), and overall survival (OS) were 97%, 94%, and 92%. There were 3 deaths, 2 related to melanoma. Fifteen patients experienced clinically significant visual loss; no patients were diagnosed with neovascular glaucoma, and 1 patient developed a clinically significant radiation-associated cataract. No patient required enucleation. CONCLUSIONS We report the largest published US cohort of patients treated with (106)Ru plaque brachytherapy for uveal melanoma. Tumor control was excellent, and toxicity was acceptably low. These data support the reintroduction of (106)Ru into clinical practice for ocular melanoma.

Monte Carlo model for a prototype CT-compatible, anatomically adaptive, shielded intracavitary brachytherapy applicator for the treatment of cervical cancer

PURPOSE Current, clinically applicable intracavitary brachytherapy applicators that utilize shielded ovoids contain a pair of tungsten-alloy shields which serve to reduce dose delivered to the rectum and bladder during source afterloading. After applicator insertion, these fixed shields are not necessarily positioned to provide optimal shielding of these critical structures due to variations in patient anatomies. The authors present a dosimetric evaluation of a novel prototype intracavitary brachytherapy ovoid [anatomically adaptive applicator (A3)], featuring a single shield whose position can be adjusted with two degrees of freedom: Rotation about and translation along the long axis of the ovoid. METHODS The dosimetry of the device for a HDR 192Ir was characterized using radiochromic film measurements for various shield orientations. A MCNPX Monte Carlo model was developed of the prototype ovoid and integrated with a previously validated model of a v2 mHDR 192Ir source (Nucletron Co.). The model was validated for three distinct shield orientations using film measurements. RESULTS For the most complex case, 91% of the absolute simulated and measured dose points agreed within 2% or 2 mm and 96% agreed within 10% or 2 mm. CONCLUSIONS Validation of the Monte Carlo model facilitates future investigations into any dosimetric advantages the use of the A3 may have over the current state of art with respect to optimization and customization of dose delivery as a function of patient anatomical geometries.

A choice of radionuclide: Comparative outcomes and toxicity of ruthenium-106 and iodine-125 in the definitive treatment of uveal melanoma

PURPOSE Both iodine-125 ((125)I) Collaborative Ocular Melanoma Study and ruthenium-106 ((106)Ru) eye plaques can achieve excellent tumor control in patients diagnosed with uveal melanoma. We analyzed our single institutional experience in the management of ocular melanoma treated with either (125)I or (106)Ru plaque brachytherapy. METHODS AND MATERIALS The records of 107 patients with uveal melanoma treated with either (106)Ru (n = 40) or (125)I (n = 67) plaque brachytherapy between 2000 and 2008 were retrospectively reviewed. Tumor control parameters and toxicity were assessed. RESULTS Actuarial 5-year rates of local control, progression-free survival, and overall survival with (106)Ru were 97%, 94%, and 92%, respectively. For (125)I, these values were 83%, 65%, and 80%. In the subset of patients with tumor apex height ≤5 mm (36 (125)I and 40 (106)Ru), there was no difference in overall survival; however, progression-free survival was significantly improved with (106)Ru (P = .02). Enucleation-free survival was significantly different between the 2 subsets, with no enucleations in the (106)Ru cohort (P = .02). Patients treated with (106)Ru experienced reduced retinopathy (P = .03) and cataracts (P < .01). CONCLUSIONS Both (125)I and (106)Ru eye plaque brachytherapy treatment result in encouraging tumor control for patients with uveal melanoma. We demonstrate that (106)Ru offers these benefits with reduced toxicity in patients treated for uveal melanomas ≤5 mm in apical height.

Loading technique comparison in permanent 125I prostate implants

Treatment of prostate cancer utilizing iodine 125 (125I) interstitial seed implants has become an accepted and widely used modality. Numerous variations in 125I seed implant loading distribution techniques have developed as a result of the preferences of individual institutions implementing the modality. No particular universal standard is currently used for 1251 seed implants. A major concern with 125I seed implants is coverage of the prostate with the desired dose and the minimization of dose to the urethra. A variation of seed distribution per individual anatomy is desirable. Historically, brachytherapy relied on dosimetry systems, such as the Paris, Quimby, and Manchester systems to achieve the desired dose distribution. Use of various peripherally loaded 125I seed implant distributions to accommodate anatomic variations within the same institution prompted the interest of how the results compare to the Manchester system.

SU-FF-T-09: Verification of Dose Point Kernels for Ir-192 Brachytherapy

Purpose: A simple method for verifying the dose point kernel of Ir-192 source used in HDR brachytherapy using Kodak EDR2 film in a spiral phantom. Method and Materials: A spiral solid water phantom available from Gammex International for IMRT QA was modified for this study. A CT scan image of the phantom was acquired with a 10 × 12 inch film loaded in the spiral groove. A 3.5 mm long 8.5 Ci pellet of Ir-192 in a Nucletron HDR system was driven to a snugly fitting catheter located at the center of the phantom. The film loaded in the groove was exposed for a dwell time calculated to deliver doses within the linear range of the film. A calibration film was processed with each film exposed in the groove. The film was scanned with a Vidar 16-bit scanner and analyzed using the RIT film analysis software. The isodose map and dose profile along the length of the spiral were calculated to a number of selected distances from the center of the source. The conversion of film optical density to dose was procured by measuring the dose at selected points in solid water sheets with a 0.6 cc Farmer ion chamber. The nC recorded at selected distances from the source was converted to cGy using the TG-21 formalism. Results: The measured dose point kernel measured with the film was compared with published data. Corrections for obliquity and sensitivity of the chamber with energy were ignored since these are less than 2 %. The dose profile for Ir-192 was found to be in close agreement with published data. Conclusion: A simple film exposed in a spiral phantom can be employed to verify the dose point kernels required for Ir-192 and other radionuclides being considered for HDR and LDR brachytherapy.

TU‐D‐T‐617‐01: Comparison of LDR to PDR Dose Distributions: A Monte Carlo Study

Purpose: 1) To validate a Monte Carlo (MC) model of the Ir‐192 pulsed‐dose rate (PDR) source inside the Fletcher‐Williamson (FW) ovoid using radiochromic film measurements. 2) To compare the FW dose distributions in water to those of Cs‐137 low‐dose rate (LDR) pellet inside the Selectron Fletcher‐Suit‐Delclos (FSD) ovoid. Method and Materials: Detailed mechanical drawings of the FW colpostat were obtained from the vendor. MCNPX 2.5e MC code was used to model the small right FW ovoid and an Ir‐192 source (mHDR v2) centered within the ovoid. MC models of actual and virtual Ir‐192 source in air were also run to derive the conversion from contained to apparent activity. MD‐55 radiochromic film was placed in a polystyrene phantom at a plane parallel to and displaced medially 2.0 cm from the long axis of the colpostat. To compare with the FSD ovoid, MC runs in a 30 cm water sphere were run for common ovoid loadings of 5, 10, 15, and 20 mgRaeq. Results:MCNPX calculated dose relative to film measurements is within ±2% or 2mm distance‐to‐agreement for 92% of the dose grid. For the FW simulations in water, the overall shapes were found to be similar in regions away from the shields to those obtained for the FSD ovoid; however, large but localized deviations were found in high dose‐gradient regions close to the rectal and bladder shields, mainly due to differences in source and shields geometry. Conclusion: The dose distributions predicted by MCNPX are in good agreement with the film results. Dose‐rate atlases calculated in clinically‐relevant 2D planes around the FW and FSD ovoids provide information to aid in the transfer of current LDR intracavitary brachytherapy practice to that using PDR for treatment of gynecological cancers.Conflict of Interest: Partial support by Nucletron Corporation.