Transitioning from a COMS-based plaque brachytherapy program to using eye physics plaques and plaque simulator treatment planning system: A single institutional experience

The aim of this work is to describe the implementation and commissioning of a plaque brachytherapy program using Eye Physics eye plaques and Plaque Simulator treatment planning system based on the experience of one institution with an established COMS-based plaque program. Although commissioning recommendations are available in official task groups publications such as TG-129 and TG-221, we found that there was a lack of published experiences with the specific details of such a transition and the practical application of the commissioning guidelines. The specific issues addressed in this paper include discussing the lack of FDA approval of the Eye Physics plaques and Plaque Simulator treatment planning system, the commissioning of the plaques and treatment planning system including considerations of the heterogeneity corrected calculations, and the implementation of a second check using an FDA-approved treatment planning system. We have also discussed the use of rental plaques, the analysis of plans using dose histograms, and the development of a quality management program. By sharing our experiences with the commissioning of this program this document will assist other institutions with the same task and act as a supplement to the recommendations in the recently published TG-221.

3D image-guided treatment planning for Ruthenium-106 brachytherapy of choroidal melanomas

BACKGROUND

Current standard treatment procedures for Ruthenium-106 (Ru-106) brachytherapy for choroidal melanomas do not use 3D image-guided treatment planning. We evaluated the potential impact of introducing 3D treatment planning and quantified the theoretical clinical benefits in terms of tumour control probability (TCP) and normal tissue complication probability (NTCP).

MATERIALS AND METHODS

Treatment plans for thirty-two patients were optimized using 3D image-guided treatment planning and compared to the original 2D clinical plans. Optimization of plans was done in an image-based treatment planning system by optimizing the plaque position and treatment time such that the entire tumour received the prescribed dose of 100 Gy. TCP and NTCP for 2D clinical plans and optimized 3D image-guided plans were estimated from published outcome prediction models and compared within patients using Wilcoxon signed-rank test.

RESULTS

The median minimum tumour dose (D_99% ) for 2D clinical plans was 93 Gy (range: 23-158 Gy), corresponding to 5-year TCP of 75% (IQR 61-86%), while median tumour D_99% for optimized 3D image-guided plans was 115 Gy (range 103-141 Gy), corresponding to TCP of 82% (IQR 80-84%). This was a statistically significant increase in estimated TCP (median increase in TCP 8% (IQR: -5-23, p = 0.006). While the dose to normal tissue increased somewhat, there was no significant change in NTCP.

CONCLUSION

3D treatment planning theoretically allows for improved tumour dose delivery for Ru-106 brachytherapy of choroidal melanomas, resulting in a significant increase in expected tumour control compared to traditional approaches using 2D calculations. The deliverability of optimized plans, and potential increased risk of late complications, will have to be confirmed in future clinical studies.

Increasing the efficiency of the retinoblastoma brachytherapy protocol with ultrasonic hyperthermia and gold nanoparticles: a rabbit model

PURPOSE

This study purposed to evaluate the efficacy of brachytherapy with the modality of ultrasonic hyperthermia in the presence of gold nanoparticles (GNPs) on an ocular retinoblastoma tumor in an animal model of the rabbit.

MATERIALS AND METHODS

A retinoblastoma tumor was induced by the injection of the human cell line of Y79 in rabbit eyes (n = 41). After two weeks, tumor size reached a diameter of about 5-7 mm. Seven groups were involved: control, GNPs injection, hyperthermia, hyperthermia with GNPs injection, brachytherapy with I-125, a combination of hyperthermia and brachytherapy, and a combination of brachytherapy, hyperthermia and, GNPs. The tumor area was measured using B-mode ultrasound images on the zero-day and at the end of the third week. The groups were evaluated for a histopathological study of tumor necrosis.

RESULTS

There was a significant difference between the relative area changes of tumor in the combination group with the other study groups (p < .05). The results of histopathologic studies confirmed the necrosis of living retinoblastoma cells.

CONCLUSION

Combination therapy of brachytherapy and hyperthermia with GNPs reduces the relative size of the tumor. This method increases the necrosis percentage of retinoblastoma and significantly reduces the retinoblastoma mass in the rabbit eyes.

AAPM recommendations on medical physics practices for ocular plaque brachytherapy: Report of task group 221

The American Association of Physicists in Medicine (AAPM) formed Task Group 221 (TG-221) to discuss a generalized commissioning process, quality management considerations, and clinical physics practice standards for ocular plaque brachytherapy. The purpose of this report is also, in part, to aid the clinician to implement recommendations of the AAPM TG-129 report, which placed emphasis on dosimetric considerations for ocular brachytherapy applicators used in the Collaborative Ocular Melanoma Study (COMS). This report is intended to assist medical physicists in establishing a new ocular brachytherapy program and, for existing programs, in reviewing and updating clinical practices. The report scope includes photon- and beta-emitting sources and source:applicator combinations. Dosimetric studies for photon and beta sources are reviewed to summarize the salient issues and provide references for additional study. The components of an ocular plaque brachytherapy quality management program are discussed, including radiation safety considerations, source calibration methodology, applicator commissioning, imaging quality assurance tests for treatment planning, treatment planning strategies, and treatment planning system commissioning. Finally, specific guidelines for commissioning an ocular plaque brachytherapy program, clinical physics practice standards in ocular plaque brachytherapy, and other areas reflecting the need for specialized treatment planning systems, measurement phantoms, and detectors (among other topics) to support the clinical practice of ocular brachytherapy are presented. Expected future advances and developments for ocular brachytherapy are discussed.

A novel quality assurance system for eye plaque brachytherapy

Eye Plaque brachytherapy pre-treatment quality assurance (QA) conducted clinically involves an activity verification of individual seeds via well chamber and does not include a physical measurement of dose-rate of the final assembly. A novel spectroscopic, dose-rate detection system, was evaluated for pre-treatment QA of eye plaque brachytherapy. The system includes a water phantom with sterility management. The system was calibrated using a known-activity I-125 seed, measured at 1 cm in water along the radial axis, compared to TG-43 U1 calculations and verified over a number of distances. A depth dose curve was acquired for a clinical, mixed activity eye plaque and two 'error' plaques. The probe was stepped from a water equivalent source to a detector distance (SDD) of 2.5 to 12 mm along the plaque central axis. The latter measurements aimed to characterise the sensitivity of the system. The calculated and measured single-seed dose-rates agreed to within 0.5 cGy/h from a SDD of 3 mm and above. The clinical plaque showed agreement between measured and treatment planning system (TPS) calculated dose-rates within 2%. Sensitivity testing resulted in a maximum deviation from TPS data of 18%, therefore was able to detect the presence of packing errors. The dose-rate detection system was successfully evaluated for verification of I-125 based eye plaques without compromising sterility, allowing for quick pre-treatment, dose-rate verification of patient-ready plaques. Its agreement with TPS data for the unmodified plaque and its deviation when introducing errors confirms the approach suggested is a viable QA tool.

Treatment planning considerations for 125I eye plaque brachytherapy

Effective cancer brachytherapy requires a treatment plan that delivers high-dose to tumor, while minimizing the dose to critical normal tissues. Therefore, an accurate knowledge of the sources and magnitude of the techniques is essential for producing robust and well optimized-plans.

The purpose of this technical note is to establish general procedures and strategies for optimization and customization of the plaques loaded with radioactive seeds, particularly focusing on the definition of useful tactics to limit high doses to organs at risk and adapt the treatment time to the necessity of institution.

Dosimetric validation of the Theragenics AgX-100® I-125 seed for ROPES eye plaque brachytherapy

With the discontinued distribution of the I-125 Oncura Onco seed (model 6711), the Theragenics AgX100® I-125 seeds were considered as a suitable alternative for eye plaque brachytherapy as their physical properties matched the requirements for use with the ROPES eye plaques. The purpose of this study aims at validating the dosimetry of the AgX-100 loaded ROPES plaques (11 mm diameter, 15 mm diameter with flange, 15 mm diameter with notch, 18 mm diameter) and assess the differences with the discontinued I-125 6711 model. To independently verify the plaque dosimetry, the brachytherapy module of RADCALC® version 6.2.3.6 was commissioned for the new AgX-100 I-125 seed using the published AAPM TG43 data from the literature. Experimental dosimetry verification was performed using EBT3 Gafchromic™ film and TLD-100 micro-cubes in a specially designed Solid Water® phantom. Both RADCALC® and film confirmed the dosimetry calculated by Plaque Simulator (PS) version 6.4.6 The dose calculated by PS agrees with RADCALC® to within 2% for depths of 1-15 mm for the 4 available ROPES plaques. The dosimetric measurements agreed with the calculations of PS for clinically relevant depths (4 mm to 6 mm) within the evaluated uncertainties of 4.7% and 7.2% for EBT3 film and TLDs respectively. The AgX-100 I-125 seed was a suitable replacement for the 6711 I-125 seed. Due to the introduction of the stainless-steel backscatter factor in PS v6.4.6, the department has decided to report both the homogenous dose and heterogeneity corrected dose for each eye plaque patient.

A Polymer-Gel Eye-Phantom for 3D Fluorescent Imaging of Millimetre Radiation Beams

We have filled a 24 mm diameter glass sphere with a transparent polymer-gel that is radio-fluorogenic, i.e., it becomes (permanently) fluorescent when irradiated, with an intensity proportional to the local dose deposited. The gel consists of >99.9% tertiary-butyl acrylate (TBA), pre-polymerized to ~15% conversion, and ~100 ppm maleimido-pyrene (MPy). Its dimensions and physical properties are close to those of the vitreous body of the human eye. We have irradiated the gel with a 3 mm diameter, 200 kVp X-ray beam with a dose rate of ~1 Gy/min. A three-dimensional (3D) (video) view of the beam within the gel has been constructed from tomographic images obtained by scanning the sample through a thin sheet of UV light. To minimize optical artefacts, the cell was immersed in a square tank containing a refractive-index-matching medium. The 20⁻80% penumbra of the beam was determined to be ~0.4 mm. This research was a preparatory investigation of the possibility of using this method to monitor the millimetre diameter proton pencil beams used in ocular radiotherapy.

Radiochromic film calibration for low-energy seed brachytherapy dose measurement

PURPOSE

Radiochromic film dosimetry is typically performed for high energy photons and moderate doses characterizing external beam radiotherapy (XRT). The purpose of this study was to investigate the accuracy of previously established film calibration procedures used in XRT when applied to low-energy, seed-based brachytherapy at higher doses, and to determine necessary modifications to achieve similar accuracy in absolute dose measurements.

METHODS

Gafchromic EBT3 film was used to measure radiation doses upwards of 35 Gy from 75 kVp, 200 kVp, 6 MV, and (∼28 keV) I-125 photon sources. For the latter irradiations a custom phantom was built to hold a single I-125 seed. Film pieces were scanned with an Epson 10000XL flatbed scanner and the resulting 48-bit RGB TIFF images were analyzed using both FilmQA Pro software andMATLAB. Calibration curves relating dose and optical density via a rational functional form for all three color channels at each irradiation energy were determined with and without the inclusion of uncertainties in the measured optical densities and dose values. The accuracy of calibration curve variations obtained using piecewise fitting, a reduced film measurement area for I-125 irradiation, and a reduced number of dose levels was also investigated. The energy dependence of the film lot used was also analyzed by calculating normalized optical density values.

RESULTS

Slight differences were found in the resulting calibration curves for the various fitting methods used. The accuracy of the calibration curves was found to improve at low doses and worsen at high doses when including uncertainties in optical densities and doses, which may better represent the variability that could be seen in film optical density measurements. When exposing the films to doses > 8 Gy, two-segment piecewise fitting was found to be necessary to achieve similar accuracies in absolute dose measurements as when using smaller dose ranges. When reducing the film measurement area for the I-125 irradiations, the accuracy of the calibration curve was degraded due to the presence of localized film heterogeneities. No degradation in the calibration curves was found when reducing the number of calibration points down to only 4, but with piecewise fitting, 6 calibration points as well as a blank film are required. Variations due to photon energy in film optical density of up to 3% were found above doses of 2 Gy.

CONCLUSIONS

A modified procedure for performing EBT3 film calibration was established for use with low-energy brachytherapy seeds and high dose exposures. The energy dependence between 6 MV and I-125 photons is significant such that film calibrations should be done with an appropriately low-energy source when performing low-energy brachytherapy dose measurements. Two-segment piecewise fitting with the inclusion of errors in measured optical density as well as dose was found to result in the most accurate calibration curves. Above doses of 1 Gy, absolute dose measurements can be made with an accuracy of 1.6% for 6 MV beams and 5.7% for I-125 seed exposures if using the I-125 source for calibration, or 2.3% if using the 75 kVp photon beam for calibration.