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A Novel Ion Chamber Technology for Quality Assurance in External Beam Radiotherapy

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A Novel Ion Chamber Technology for Quality Assurance in External Beam Radiotherapy TECHNISCHE UNIVERSITÄT MÜNCHEN Advanced Technologies in Radiation Therapy A novel ion chamber technology for quality assurance in external beam radiotherapy Michele Togno, M.Sc. Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. Martin Zacharias Prüfer der Dissertation: 1. Univ.-Prof. Dr. Jan J. Wilkens 2. Univ.-Prof. Dr. Franz Pfeiffer Die Dissertation wurde am 15.05.2017 bei der Technischen Universität München eingereicht und durch die Fakultät für Physik am 10.07.2017 angenommen PHYSIK-DEPARTMENT A NOVEL ION CHAMBER TECHNOLOGY FOR QUALITY ASSURANCE IN EXTERNAL BEAM RADIOTHERAPY Dissertation von Michele Togno ̶ April 2017 ̶ TECHNISCHE UNIVERSITÄT MÜNCHEN Abstract Radiation therapy with external beams aims to cure cancer by means of dose deposition in the patient body. The number of cancer patients that could benefit from a radiation treatment is expected to grow significantly in the coming years. A number of different techniques have been recently developed to increase the precision of dose delivery to the target and to minimize the effects of the treatment on healthy organs. Examples of such innovative techniques include stereotactic radiosurgery (SRS) and intensity-modulated proton therapy (IMPT) with scanning pencil beams. As with the accuracy of the treatment, the complexity of the radiation delivery and of the machines themselves is also increased. Thus, high-level quality assurance is required to ensure the consistency of medical prescription to the patient and to improve the dosimetric and geometric accuracy of dose delivery. Moreover, quality assurance procedures play a fundamental role in decreasing the likelihood of accidents and errors, and they are a precondition for intercomparisons of data on a large scale among different radiotherapy centers. A number of different devices have been developed to fulfill the requirements of fast and reliable machine quality assurance and patient quality assurance in external beam radiotherapy. However, the challenges introduced by new treatment practices necessitate the further and continuous development of such devices. The aim of this thesis is to investigate the suitability of an innovative technology to quality assurance applications in complex radiotherapy techniques. The investigated technology is based on ionization chambers, which are arranged in a grid in the final detector design in order to cover a 2D surface. The choice of ionization chambers was driven by their intrinsic properties, such as energy independence, radiation hardness and possibility to measure absorbed dose to water. The main desired characteristics of the technology, besides those typical of standard dosimeters, are high sampling resolution and low sensitivity dependence on dose rate/dose per pulse. These features are essential to properly resolving dose distributions, which are delivered with time-varying parameters, high dose rate/dose per pulse, and steep gradients. The development process is divided into three major phases. In each phase, a detector prototype is built and tested under different types of radiation. The first prototype is tested mainly with continuous gamma radiation in order to optimize performances and design through a step-by-step approach. The second prototype is built to consolidate the achievements made using the first prototype, and it comes with new frontend electronics and a design which allows for independent and reliable tests. The detector is deeply tested with clinical MV X-rays at different beam qualities and dose rates, with clinical proton beams at different energies and currents. It is also benchmarked to state of the art detectors. Moreover, performances of the detector are evaluated in a clinical environment for the sake of machine quality assurance controls (e.g. output factors, beam profiling, depth dose curves…) and pre- treatment patient plan verification. Thanks to the encouraging results of this research phase, a third prototype is designed and the proof of concept started with preliminary validation in clinical proton beams. The investigated technology is proven to have great potential for applications in radiation dosimetry. It is demonstrated to be a valuable option in addressing the need for innovative tools that arises from the increasing complexity and effectiveness of modern radiotherapy techniques. Furthermore, the recommendations of international dosimetric protocols can also be fulfilled. Nevertheless, some improvements and additional developments are required, and these are identified in three main research branches, as described in the conclusion of the present work. Zusammenfassung Perkutane Strahlentherapie dient der Behandlung von Krebs durch Bestrahlung des Patientenkörpers mit hochenergetischer Strahlung, wobei Dosis im Tumor deponiert wird. Die Anzahl der Krebspatienten, welche von dieser Behandlungsmethode profitieren, wird in den nächsten Jahren stark steigen. Verschiedenste Verfahren wurden in letzter Zeit entwickelt, um einerseits die Genauigkeit der Dosisverteilung im Zielvolumen zu verbessern und andererseits die Nebenwirkungen auf gesundes Gewebe und Risikoorgane zu minimieren. Einige Beispiele für diese neuen Behandlungstechniken sind stereotaktische Radiochirurgie (SRS, stereotactic radiosurgery) und Intensitätsmoduliertes Protonenbeam- Scanning (IMPT, intensity modulated proton therapy with scanning pencil beams). Mit der Genauigkeit der Behandlung steigt auch die Komplexität der Bestrahlungstechniken und der Behandlungsgeräte an. Aus diesem Grund benötigt man umfangreiche Qualitätssicherungsmaßnahmen, um Fehlbestrahlungen und Behandlungsunfälle zu vermeiden und die korrekte Dosisapplizierung dosimetrisch und geometrisch für den Patienten sicherzustellen. Außerdem können die QS-Daten zum Vergleich unterschiedlicher Strahlentherapieeinrichtungen verwendet werden. Es wurde bereits eine Reihe von Geräten entwickelt, um den Anforderungen einer schnellen und zuverlässigen Qualitätssicherung in der Strahlentherapie gerecht werden zu können. Die Herausforderungen an die QA durch neue Therapiemethoden erfordern jedoch eine ständige Weiterentwicklung dieser Geräte. Ziel dieser Arbeit ist, die Eignung einer neuen Technologie für die Qualitätssicherung komplexer Strahlentherapie-Techniken zu prüfen. Die untersuchte Technologie basiert auf Ionisatioskammern, welche in einem Raster angeordnet eine zweidimensionale Oberfläche bedecken. Ionisationskammern wurden aufgrund ihrer intrinsischen Eigenschaften wie Energieunabhängigkeit, Beständigkeit gegen Strahlung und der Möglichkeit der Messung von Wasser-Äquivalentdosis gewählt. Die hauptsächlich benötigten Eigenschaften der Technologie, neben denen der typischen Standard-Dosimeter, sind eine hohe zeitliche Auflösung und eine geringe Abhängigkeit der Empfindlichkeit von der Pulsdosis. Diese Eigenschaften sind entscheidend, um Dosisverteilungen, welche mit einer hohen Dynamik, hoher Pulsdosis und steilen Gradienten appliziert wurden, detailgetreu und korrekt darzustellen. Der Charakterisierungsprozess ist in drei Hauptphasen unterteilt. In jeder Phase wird ein Prototyp gebaut und mit verschiedenen Strahlenarten getestet. Der erste Prototyp wird hauptsächlich mit 60Co-Strahlung getestet, um sein Verhalten und Design in einem schrittweisen Prozeß zu optimieren. In den Aufbau des zweiten Prototyps fließen die mit dem ersten Testmuster gewonnenen Erkenntnisse ein und er erhält neue Elektronik sowie ein Design, welches unabhängige und verlässliche Tests ermöglicht. Der Detektor wird intensiv mit klinischen hochenergetischen Photonenstrahlen unterschiedlichster Qualität und mit verschiedenen Dosisleistungen getestet, außerdem mit klinischen Protonenstrahlen, ebenfalls unterschiedlicher Energie und Ströme. Anhand der Ergebnisse lässt sich der Detektor auch mit anderen Detektoren vergleichen. Darüber hinaus wurde die Eignung des Detektors hinsichtlich seines Einsatzes in der Maschinen-Qualitätssicherung (Messung von Output- Faktoren, Strahlprofilen, Tiefendosis-Kurven) und der Verifikation von Bestrahlungsplänen im klinischen Umfeld bewertet. Aufgrund vielversprechender Ergebnisse in dieser Entwicklungsphase wurde ein dritter Prototyp entworfen und gebaut und dieser Detektor an klinischen Bestrahlungsanlagen erprobt. Die mittels ausführlicher Tests untersuchte Technologie erwies sich für ihre Anwendung in der Strahlendosimetrie als überaus geeignet. Die Hauptergebnisse sind: Eine höhere räumliche Auflösung als gängige Detektoren der gleichen Sensortechnologie, und eine geringe Dosispuls-Abhängigkeit. Es konnte gezeigt werden, dass Eignung zur Messung von Dosisverteilungen mit steilen Dosisgradienten sowie hohen Dosisleistungen besteht. Obwohl ausgezeichnete Ergebnisse erzielt wurden, konnten einige Verbesserungsvorschläge erarbeitet werden. Im Ergebnis dieser Arbeit sind weitere Forschungsarbeiten angezeigt. Contents Chapter 1 INTRODUCTION ............................................................................................. 25 1.1 RADIATION THERAPY .................................................................................. 27 1.1.1 Rationale and concept ........................................................................................ 27 1.1.2 Radiotherapy with photons and protons ............................................................ 30 1.2 QUALITY ASSURANCE ................................................................................
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