Abstract
Background
Irradiation of intraocular tumors requires dedicated techniques, such as brachytherapy with 106Ru plaques. The currently available treatment planning system relies on the assumption that the eye is a homogeneous water sphere and on simplified radiation transport physics. However, accurate dose distributions and their assessment demand better models for both the eye and the physics.
Methods
The Monte Carlo code PENELOPE, conveniently adapted to simulate the beta decay of 106Ru over 106Rh into 106Pd, was used to simulate radiation transport based on a computerized tomography scan of a patient’s eye. A detailed geometrical description of two plaques (models CCA and CCB) from the manufacturer BEBIG was embedded in the computerized tomography scan.
Results
The simulations were firstly validated by comparison with experimental results in a water phantom. Dose maps were computed for three plaque locations on the eyeball. From these maps, isodose curves and cumulative dose-volume histograms in the eye and for the structures at risk were assessed. For example, it was observed that a 4-mm anterior displacement with respect to a posterior placement of a CCA plaque for treating a posterior tumor would reduce from 40 to 0% the volume of the optic disc receiving more than 80 Gy. Such a small difference in anatomical position leads to a change in the dose that is crucial for side effects, especially with respect to visual acuity. The radiation oncologist has to bring these large changes in absorbed dose in the structures at risk to the attention of the surgeon, especially when the plaque has to be positioned close to relevant tissues.
Conclusion
The detailed geometry of an eye plaque in computerized and segmented tomography of a realistic patient phantom was simulated accurately. Dose-volume histograms for relevant anatomical structures of the eye and the orbit were obtained with unprecedented accuracy. This represents an important step toward an optimized brachytherapy treatment of ocular tumors.
Zusammenfassung
Hintergrund
Die Bestrahlung intraokularer Tumoren erfordert spezielle Techniken wie die Brachytherapie mittels 106Ru-Applikatoren. Alle derzeitigen verfügbaren Näherungen zur Planung der Dosisverteilung beruhen auf der Annahme, das Auge sei eine homogene mit Wasser gefüllte Kugel, und auf simplen Strahlungstransportmodellen. Eine korrekte Berechnung der Dosisverteilung bedarf jedoch weiterentwickelter Modelle sowohl für das Auge als auch für die zugrunde liegenden physikalischen Phänomene.
Methodik
Der Monte-Carlo-Code PENELOPE ist zur Berechnung des Betazerfalls von 106Ru über 106Rh zu 106Pd geeignet und wurde verwendet, um den Strahlungstransport in einem Phantom zu berechnen, das auf der Computertomographie eines Auges basiert. Dabei wurde ein detailliertes geometrisches Modell von 2 Applikatoren für die Brachytherapie am Auge (Modelle CCA und CCB des Herstellers BEBIG) verwendet.
Ergebnisse
Die Computersimulationen wurden zunächst mit Messdaten validiert. Dosisverteilungen wurde für drei unterschiedliche Lokalisationen der verschiedenen Applikatoren berechnet. Isodosen und Dosis-Volumen-Histogramme im Auge und in definierten Risikostrukturen wurden erstellt. So führt zum Beispiel bei der Bestrahlung eines Tumors in der Nähe des Äquators eine geringe anteriore Verlagerung des CCA-Applikators um 4 mm dazu, dass 0% der Papille eine Dosis über 80 Gy erhält, während bei der posterioren Position 40% der Papille mit über 80 Gy belastet wird. Dieser geringe Unterschied in der anatomischen Positionierung des Applikators kann durch den erheblichen Unterschied in der Dosisverteilung einen großen Einfluss auf Nebenwirkungen, insbesondere das Sehvermögen, haben. Es gehört zu den Aufgaben des Strahlentherapeuten, dem Operateur zu verdeutlichen, wie dramatisch die Veränderung der applizierten Dosis an einer Risikostruktur sein kann, wenn Zielvolumen und Risikoorgan sehr nahe beieinander liegen.
Schlussfolgerung
Erstmals wurde es möglich, eine detaillierte Geometrie eines Augenapplikators innerhalb einer computerisierten und segmentierten Tomographie eines realistischen Patientenphantoms präzise zu simulieren. Dosis-Volumen-Histogramme für die relevanten anatomischen Strukturen des Auges und der Orbita wurden mit bisher nicht erreichter Präzision berechnet. Dies stellt einen wichtigen Schritt zur weiteren Optimisierung der Brachytherapie bei Augentumoren dar.
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Acknowledgments
The authors are grateful to Dr. Theodor Kaulich for the experimental data related to the CCB plaque. L.B. acknowledges financial support from the Deutsche Forschungsgemeinschaft project BR 4043/1-1. J.S. also acknowledges partial financial support from the Spanish Ministerio de Economía y Competitividad (Project no. FIS2012-38480), from the Spanish Ministerio de Ciencia e Innovación (project no. FPA2009-14091-C02-01), and from the Spanish Networking Research Center CIBER-BBN.
Conflict of interest
On behalf of all authors, the corresponding author states that there are no conflicts of interest.
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Brualla, L., Sempau, J., Zaragoza, F. et al. Accurate estimation of dose distributions inside an eye irradiated with 106Ru plaques. Strahlenther Onkol 189, 68–73 (2013). https://doi.org/10.1007/s00066-012-0245-6
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DOI: https://doi.org/10.1007/s00066-012-0245-6