An improved treatment planning and quality assurance process for Collaborative Ocular Melanoma Study eye plaque brachytherapy

Novel 3D printed universal conical holder for eye plaque quality assurance

PURPOSE

For the custom-built construction of eye plaques, the iodine (I-125) seeds of different source strengths are recycled in our eye plaque program. To return I-125 seeds to the correct lot, we developed a novel 3D-printed conical plaque QA holder for relative assay for eye plaques.

MATERIALS AND METHODS

A universal 3D-printed conical plaque holder was designed to accommodate six plaque sizes and fit reproducibly in a well-type dose calibrator. A reproducibility test was used to compare the plaque placement consistency in the holder versus without the holder. Plaque assays were performed for assembled plaques both before implant and after explant. The explant reading was compared with the implant reading adjusted for decay, and the relative error was calculated. The plaque response fraction (PRF) is defined as the fraction of well chamber implant reading over the total seed strength for a plaque. The PRF was aggregated for each individual plaque to confirm the seed lot before implant.

RESULTS

The reproducibility test showed the chamber reading's relative standard deviation of 0.40% with the QA holder compared to 0.68% without it. The batch relative assay was performed for 251 plaques. The absolute value of measurement deviation between explant and decay-corrected implant readings is 0.89% ± 0.86% (mean ± standard deviation). The PRFs for individual plaques range from 36.49% to 49.87%, with a maximum standard deviation of 2%.

CONCLUSIONS

This novel 3D-printed QA holder provides reproducible setup for assaying assembled eye plaques in a well chamber. Batch relative assay can validate the seed batch used and plaque integrity during the implant without assaying individual seeds, saving valuable physicist time and radiation exposure from seed handling.

Current and Emerging Radiotherapy Options for Uveal Melanoma

What treatment options are there for patients having uveal melanoma? A randomized, prospective, multi-institutional clinical trial (COMS) showed no difference in survival between brachytherapy and enucleation for medium-sized lesions. With the obvious benefit of retaining the eye, brachytherapy has flourished and many different approaches have been developed such as low-dose-rate sources using alternate low-energy photon-emitting radionuclides, different plaque designs and seed-loading techniques, high-dose-rate brachytherapy sources and applicators, and low- and high-dose-rate beta-emitting sources and applicators. There also have been developments of other radiation modalities like external-beam radiotherapy using linear accelerators with high-energy photons, particle accelerators for protons, and gamma stereotactic radiosurgery. This article examines the dosimetric properties, targeting capabilities, and outcomes of these approaches. The several modalities examined herein have differing attributes and it may be that no single approach would be considered optimal for all patients and all lesion characteristics.

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.

Radiobiological evaluation of organs at risk for electronic high-dose-rate brachytherapy in uveal melanoma: a radiobiological modeling study

Purpose

The objective of this study was to examine feasibility of single- or hypo-fraction of high-dose-rate (HDR) electronic brachytherapy (eBT) in uveal melanoma treatment.

Material and methods

Biologically effective doses (BED) of organs at risk (OARs) were compared to those of iodine-125-based eye plaque low-dose-rate brachytherapy (¹²⁵I LDR-BT) with vitreous replacement (VR). Single- or hypo-fractionated equivalent physical doses (SFEDs or HFEDs) for tumor were calculated from tumor BED of ¹²⁵I LDR-BT using linear-quadratic (LQ) and universal survival curve (USC) models. BED OARs doses to retina opposite the implant, macula, optic disc, and lens were calculated and compared among SFED, HFED, and ¹²⁵I LDR-BT. Electronic BT of 50 kVp was considered assuming dose fall-off as clinically equivalent to ¹²⁵I LDR-BT. All OARs BEDs were analyzed with and without silicone oil VR.

Results

For a single-fraction incorporating VR, the median/interquartile range of LQ (USC)-based BED doses of the retina opposite the implant, macula, optic disc, and lens were 16%/1.2% (33%/4%), 35%/19.5% (64%/17.7%), 37%/19% (75%/17.8%), and 27%/7.9% (68%/23.2%) of those for ¹²⁵I LDR-BT, respectively. SFED tumor values were 29.8/0.2 Gy and 51.7/0.5 Gy when using LQ and USC models, respectively, which could be delivered within 1 hour. SFED can be delivered within 1 hour using a high-dose-rate eBT. Even four-fraction delivery of HFED without VR resulted in higher OARs doses in the macula, optic disc, and lens (135 ~ 159%) than when using ¹²⁵I LDR-BT technique. A maximum p-value of 0.005 was observed for these distributions.

Conclusions

The simulation of single-fraction eBT, including vitreous replacement, resulted in significantly reduced OARs doses (16 ~ 75%) of that achieved with ¹²⁵I LDR-BT.

Local tumor control and treatment related toxicity after plaque brachytherapy for uveal melanoma: A systematic review and a data pooled analysis

Uveal melanoma (UM) represents the most common primary intraocular tumor, and nowadays eye plaque brachytherapy (EPB) is the most frequently used visual acuity preservation treatment option for small to medium sized UMs. The excellent local tumor control (LTC) rate achieved by EPB may be associated with severe complications and adverse events. Several dosimetric and clinical risk factors for the development of EPB-related ocular morbidity can be identified. However, morbidity predictive models specifically developed for EPB are still scarce. PRISMA methodology was used for the present systematic review of articles indexed in PubMed in the last sixteen years on EPB treatment of UM which aims at determining the major factors affecting local tumor control and ocular morbidities. To our knowledge, for the first time in EPB field, local tumor control probability (TCP) and normal tissue complication probability (NTCP) modelling on pooled clinical outcomes were performed. The analyzed literature (103 studies including 21,263 UM patients) pointed out that Ru-106 EPB provided high local control outcomes while minimizing radiation induced complications. The use of treatment planning systems (TPS) was the most influencing factor for EPB outcomes such as metastasis occurrence, enucleation, and disease specific survival, irrespective of radioactive implant type. TCP and NTCP parameters were successfully extracted for 5-year LTC, cataract and optic neuropathy. In future studies, more consistent recordings of ocular morbidities along with accurate estimation of doses through routine use of TPS are needed to expand and improve the robustness of toxicity risk prediction in EPB.

Influence of tumor shape and location in eye plaque brachytherapy dosimetry

PURPOSE

A common treatment planning technique for eye plaque brachytherapy is to model the tumor as an ellipse. For posterior tumors near the optic disc and fovea, this approach may lead to overlap between tumor and the organ at risk (OAR). We hypothesized that a superior plan can be generated by modeling the actual tumor shape.

MATERIALS AND METHODS

Forty eye plaque patients with tumors <1 cm from the optic disc and fovea were selected. Two treatment plans were generated for each patient: an elliptical tumor model plan and a true tumor model plan. Dosimetric data were collected for each plan, and Wilcoxon signed-rank tests were used to asses any statistically significant differences.

RESULTS

Equivalent tumor coverage was confirmed between the elliptical and true tumor plans for all patients. Qualitative analysis showed greater dosimetric differences between plans as the distance from the OARs increased from 0 to 2 mm but the largest differences were observed between 2 and 4 mm. Minimal differences between models were seen beyond 4 mm. Statistically significant dosimetric improvements were found for tumors <4 mm from the fovea and <2 mm from the optic disc.

CONCLUSIONS

Intuitively, accurate modeling of the tumor accounting for irregularities in the shape should result in a more conformal plan and an overall reduction in OAR dose. However, this technique is only beneficial for tumors that are within 4 mm of the fovea or optic disc. An elliptical tumor model allows for an acceptable plan unless the tumor is located posteriorly and has an irregular shape.