Equivalent uniform RBE-weighted dose in eye plaque brachytherapy

BACKGROUND

Brachytherapy for ocular melanoma is based on the application of eye plaques with different spatial dose nonuniformity, time-dependent dose rates and relative biological effectiveness (RBE).

PURPOSE

We propose a parameter called the equivalent uniform RBE-weighted dose (EUDRBE) that can be used for quantitative characterization of integrated cell survival in radiotherapy modalities with the variable RBE, dose nonuniformity and dose rate. The EUDRBE is applied to brachytherapy with 125I eye plaques designed by the Collaborative Ocular Melanoma Study (COMS).

METHODS

The EUDRBE is defined as the uniform dose distribution with RBE = 1 that causes equal cell survival for a given nonuniform dose distribution with the variable RBE > 1. The EUDRBE can be used for comparison of cell survival for nonuniform dose distributions with different RBE, because they are compared to the reference dose with RBE = 1. The EUDRBE is applied to brachytherapy with 125I COMS eye plaques that are characterized by a steep dose gradient in tumor base-apex direction, protracted irradiation during time intervals of 3-8 days, and variable dose-rate dependent RBE with a maximum of about 1.4. The simulations are based on dose of 85 Gy prescribed to the farthest intraocular extent of the tumor (tumor apex). To compute the EUDRBE in eye plaque brachytherapy and correct for protracted irradiation, the distributions of physical dose have been converted to non-uniform distributions of biologically effective dose (BED) to include the biological effects of sublethal cellular repair, Our radiobiological analysis considers the combined effects of different time-dependent dose rates, spatial dose non-uniformity, dose fractionation and different RBE and can be used to derive optimized dose regimens brachytherapy.

RESULTS

Our simulations show that the EUDRBE increases with the prescription depths and the maximum increase may achieve 6% for the tumor height of 12 mm. This effect stems from a steep dose gradient within the tumor that increases with the prescription depth. The simulations also show that the EUDRBE increase may achieve 12% with increasing the dose rate when implant duration decreases. The combined effect of dose nonuniformity and dose rate may change the EUDRBE up to 18% for the same dose prescription of 85 Gy to tumor apex. The absolute dose range of 48-61 Gy (RBE) for the EUDRBE computed using 4 or 5 fractions is comparable to the dose prescriptions used in stereotactic body radiation therapy (SBRT) with megavoltage X-rays (RBE = 1) for different cancers. The tumor control probabilities in SBRT and eye plaque brachytherapy are very similar at the level of 80% or higher that support the hypothesis that the selected approximations for the EUDRBE are valid.

CONCLUSIONS

The computed range of the EUDRBE in 125I COMS eye plaque brachytherapy suggests that the selected models and hypotheses are acceptable. The EUDRBE can be useful for analysis of treatment outcomes and comparison of different dose regimens in eye plaque brachytherapy.

Monte Carlo investigation of dose distribution of uniformly and non-uniformly loaded standard and notched eye plaques

To investigate the effect of using non-uniform loading and notched plaques on dose distribution for eye plaques. Using EGSnrc Monte Carlo (MC) simulations, we investigate eye plaque dose distributions in water and in an anatomically representative eye phantom. Simulations were performed in accordance with TG43 formalism and compared against full MC simulations which account for inter-seed and inhomogeneity effects. For standard plaque configurations, uniformly and non-uniformly loaded plaque dose distributions in water showed virtually no difference between each other. For standard plaque, the MC calculated dose distribution in planes parallel to the plaque is narrower than the TG43 calculation due to attenuation at the periphery of the plaque by the modulay. MC calculated the dose behind the plaque is fully attenuated. Similar results were found for the notched plaque, with asymmetric attenuation along the plane of the notch. Cumulative dose volume histograms showed significant reductions in the calculated MC doses for both tumor and eye structures, compared to TG43 calculations. The effect was most pronounced for the notch plaque where the MC dose to the optic nerve was greatly attenuated by the modulay surrounding the optic nerve compared to the TG43. Thus, a reduction of optic nerve D95% from 14 to 0.2 Gy was observed, when comparing the TG43 calculation to the MC result. The tumor D95% reduced from 89.2 to 79.95 Gy for TG43 and MC calculations, respectively. TG43 calculations overestimate the absolute dose and the lateral dose distribution of both standard and notched eye plaques, leading to the dose overestimation for the target and organs at risk. The dose matching along the central axis for the non-uniformly loaded plaques to that of uniformly loaded ones was found to be sufficient for providing comparable coverage and can be clinically used in eye-cancer-busy centers.

Radio-luminescent imaging for rapid, high-resolution eye plaque loading verification

BACKGROUND

Eye plaque brachytherapy is currently an optimal therapy for intraocular cancers. Due to the lack of an effective and practical technique to measure the seed radioactivity distribution, current quality assurance (QA) practice according to the American Association of Physicists in Medicine TG129 only stipulates that the plaque assembly be visually inspected. Consequently, uniform seed activity is routinely adopted to avoid possible loading mistakes of differential seed loading. However, modulated dose delivery, which represents a general trend in radiotherapy to provide more personalized treatment for a given tumor and patient, requires differential activities in the loaded seeds.

PURPOSE

In this study, a fast and low-cost radio-luminescent imaging and dose calculating system to verify the seed activity distribution for differential loading was developed.

METHODS

A proof-of-concept system consisting of a thin scintillator sheet coupled to a camera/lens system was constructed. A seed-loaded plaque can be placed directly on the scintillator surface with the radioactive seeds facing the scintillator. The camera system collects the radioluminescent signal generated by the scintillator on its opposite side. The predicted dose distribution in the scintillator's sensitive layer was calculated using a Monte Carlo simulation with the planned plaque loading pattern of I-125 seeds. Quantitative comparisons of the distribution of relative measured signal intensity and that of the relative predicted dose in the sensitive layer were performed by gamma analysis, similar to intensity-modulated radiation therapy QA.

RESULTS

Data analyses showed high gamma (3%/0.3 mm, global, 20% threshold) passing rates for correct seed loadings and low passing rates with distinguished high gamma value area for incorrect loadings, indicating that possible errors may be detected. The measurement and analysis only required a few extra minutes, significantly shorter than the time to assay the extra verification seeds the physicist already must perform as recommended by TG129.

CONCLUSIONS

Radio-luminescent QA can be used to facilitate and assure the implementation of intensity-modulated, customized plaque loading.

Eye plaque brachytherapy versus enucleation for ocular melanoma: an analysis from the National Cancer Database

Purpose

There is no current randomized data comparing the efficacy of brachytherapy and enucleation for patients with larger sized tumors. The purpose of the present study was to use a large, contemporary database to determine current practice patterns and compare survival outcomes between different management options for patients with choroidal melanoma of various sizes.

Material and methods

The National Cancer Database was queried (2004-2014) for histologically-confirmed choroidal melanoma for patients treated with brachytherapy versus enucleation. Chi-square test was used to compare categorized demographic and clinical variables in both arms. Kaplan-Meier analysis evaluated overall survival (OS). Cox proportional hazards assessment determined variables associated with OS. Patients were divided into cohorts representing small, medium, and large tumors. Propensity scores matching (PSM) was utilized to compare more similar cohorts.

Results

A total of 7,096 patients met the selection criteria; 5,501 (78%) patients received brachytherapy and 1,595 (22%) patients were treated with enucleation. After PSM, 5-yr OS for small tumors was 87% vs. 64%, for medium tumors was 77% vs. 57%, and for large tumors was 68% vs. 46% for brachytherapy and enucleation, respectively (p < 0.001). Following PSM, multivariate Cox regression found older age (hazard ratio [HR] = 1.76, 95% confidence interval [CI] = 1.51-2.06), more comorbidities (HR = 1.46, 95% CI = 1.25-1.70), extraocular extension (EOE) (HR = 1.25, 95% CI = 1.06-1.48), ciliary body invasion (CBI) (HR = 1.20, 95% CI = 1.02-1.40), and larger size (HR = 1.52, 95% CI = 1.40-1.66) were negative prognosticators of survival. Brachytherapy was a positive prognosticator of survival (HR = 0.45, 95% CI = 0.40-0.51).

Conclusions

Patients selected for brachytherapy had improved survival compared to enucleation in all size cohorts. EOE and CBI are significantly higher in the enucleation cohort and are important negative prognosticators for survival selected against patients having brachytherapy. Brachytherapy is a reasonable treatment option for certain patients with large size tumors.

A comparison of the shielding effectiveness of silicone oil vitreous substitutes when used with Palladium-103 and Iodine-125 eye plaques

PURPOSE

Episcleral eye plaques provide excellent local control of ocular melanoma, but vision sparing remains a significant problem with 30% or more of patients experiencing significant visual acuity degradation. The use of silicone oil shielding with Iodine-125 plaques has previously been reported to improve critical structure sparing. We hypothesized that the use of Palladium-103 would improve the shielding effectiveness of silicone oil due to the strong energy dependence of the photoelectric effect. This Monte Carlo simulation study reports a comparison of the shielding effects of silicone oil when used in conjunction with Pd-103 and with I-125 plaques.

MATERIALS AND METHODS

GEANT4 was used to simulate eye plaque treatments to an eye with either water-equivalent vitreous humor, or silicone oil in place of the vitreous humor. Two solid gold plaques, 15 and 23 mm, were simulated loaded with I-125 and with Pd-103 source seeds. Seed activity was normalized such that 85 Gy was delivered to the tumor apex in the water-equivalent cases. Tumor apex dose, central axis dose, and inner sclera dose reductions with silicone oil were evaluated.

RESULTS

Silicone oil resulted in an underdosing to the tumor apex of 6.1% and 7.5% in the 15 mm plaque for I-125 and Pd-103, respectively, and 3.4% and 4.3% in the 23 mm plaque for I-125 and Pd-103, respectively. When renormalized to 85 Gy to the tumor apex in all scenarios, silicone oil reduced the dose to the inner sclera 90° from the plaque by 19-32% for the 15 and 23 mm plaques using I-125, and by 33-65% for the 15 and 23 mm plaques using Pd-103.

CONCLUSIONS

The combination of silicone oil and Pd-103 eye plaques offers the potential for greatly improved sparing to normal structures compared to Pd-103 plaques alone or I-125 plaques with or without silicone oil.

Monte Carlo dosimetry modeling of focused kV x-ray radiotherapy of eye diseases with potential nanoparticle dose enhancement

PURPOSE

Eye plaque brachytherapy is the most common approach for intraocular cancer treatment. It is, however, invasive and subject to large setup uncertainty due to the surgical operation. We propose a novel-focused kV x-ray technique with potential nanoparticle (NP) enhancement and evaluate its application in treating choroidal melanoma and iris melanoma by Monte Carlo (MC) dosimetry modeling.

METHODS

A polycapillary x-ray lens was used to focus 45 kVp x rays to achieve pinpoint accuracy of dose delivery to small tumors near critical structures. In addition to allowing for beam focusing, the use of kV x rays takes advantage of the strong photoelectric absorption of metallic NPs in that energy regime and hence strong radiosensitization. We constructed an MC simulation program that takes into account the x-ray optic modeling and used GEANT4 for dosimetric calculation. Extensive phantom measurements using a prototype-focused x-ray system were carried out. The MC simulation of simple geometry phantom irradiation was first compared to measurements to verify the x-ray optic lens modeling in conjunction with the Geant4 dosimetric calculation. To simulate tumor treatment, a geometric eye model and two tumor models were constructed. Dose distributions of the simulated treatments were then calculated. NP radiosensitization was also simulated for two concentrations of 2 nm gold NP (AuNP) uniformly distributed in the tumor.

RESULTS

The MC-simulated full width at half maximum (FWHM) and central-axis depth dose of the focused kV x-ray beam match those measured on EBT3 films within ~10% around the depth of focus of the beam. Dose distributions of the simulated ocular tumor treatments show that focused x-ray beams can concentrate the high-dose region in or close to the tumor plus margin. For the simulated posterior choroidal tumor treatment, with sufficient tumor coverage, the doses to the optic disc and fovea are substantially reduced with focused x-ray therapy compared to eye plaque treatment (3.8 vs 39.8 Gy and 11.1 vs 53.8 Gy, respectively). The eye plaque treatment was calculated using an Eye Physics plaque with I-125 seeds under TG43 assumption. For the energy spectrum used in this study, the average simulated dose enhancement ratios (DERs) are roughly 2.1 and 1.1 for 1.0% and 0.1% AuNP mass concentration in the tumor, respectively.

CONCLUSION

Compared to eye plaque brachytherapy, the proposed focused kV x-ray technique is noninvasive and shows great advantage in sparing healthy critical organs without sacrificing the tumor control. The NP radiation dose enhancement is considerable at our proposed kV range even with a low NP concentration in the tumor, providing better critical structure protection and more flexibility for treatment planning.

Initial evaluation of Advanced Collapsed cone Engine dose calculations in water medium for I-125 seeds and COMS eye plaques

PURPOSE

To investigate the dose calculation accuracy in water medium of the Advanced Collapsed cone Engine (ACE) for three sizes of COMS eye plaques loaded with low-energy I-125 seeds.

METHODS

A model of the Oncura 6711 I-125 seed was created for use with ACE in Oncentra^® Brachy (OcB) using primary-scatter separated (PSS) point dose kernel and Task Group (TG) 43 datasets. COMS eye plaque models of diameters 12, 16, and 20 mm were introduced into the OcB applicator library based on 3D CAD drawings of the plaques and Silastic inserts. To perform TG-186 level 1 commissioning, treatment plans were created in OcB for a single source in water and for each COMS plaque in water for two scenarios: with only one centrally loaded seed, or with all seed positions loaded. ACE dose calculations were performed in high accuracy mode with a 0.5 × 0.5 × 0.5 mm³ calculation grid. The resulting dose data were evaluated against Monte Carlo (MC) calculated doses obtained with MCNP6, using both local and global percent differences.

RESULTS

ACE doses around the source for the single seed in water agreed with MC doses on average within < 5% inside a 6 × 6 × 6 cm³ region, and within < 1.5% inside a 2 × 2 × 2 cm³ region. The PSS data were generated at a higher resolution within 2 cm from the source, resulting in this improved agreement closer to the source due to fewer approximations in the ACE dose calculation. Average differences in both investigated plaque loading patterns in front of the plaques and on the plaque central axes were ≤ 2.5%, though larger differences (up to 12%) were found near the plaque lip.

CONCLUSIONS

Overall, good agreement was found between ACE and MC dose calculations for a single I-125 seed and in front of the COMS plaques in water. More complex scenarios need to be investigated to determine how well ACE handles heterogeneous patient materials.