Multilevel quality assurance aspects for volumetric modulated arc therapy

Die volumetrisch modulierte Rotationstherapie (VMAT) ist eine Erweiterung der intensitätsmodulierten Strahlentherapie (IMRT) die mehrere Vorteile gegenüber der IMRT bietet, wie zum Beispiel geringere Bestrahlungszeiten und reduzierte Monitor Einheiten (MU). Um eine klinisch adäquate Planqualität sowie eine präzise Bestrahlung zu erreichen, sind mehrere Voraussetzungen nötig. Hierzu zählen die Untersuchung der gesamten technischen Therapiekette, sowie die Entwicklung eines geeigneten Qualitätssicherungsprogrammes. Beide Aspekte wurden im Rahmen dieser PhD Arbeit untersucht und mit Fokus auf Strahlentherapie für Brustkrebs weiter entwickelt. Eine offene Frage ist die Anwendbarkeit von VMAT für die Behandlung von Brustkrebs, daher wurden Planungsstudien für linksseitiges Mammakarzinom inklusive Lymphabfluss durchgeführt. Für zehn Patientinnen mit linksseitigem Mammakarzinom inklusive Lymphabfluss wurden Teilrotations-VMAT Pläne sowie 4-Feld IMRT Pläne mit je 6, 10 und 15MV Photonenenergie erstellt. Nachfolgend wurde ein neuer VMAT-Ansatz vorgestellt und mit der obigen Technik verglichen: Bestrahlungspläne bestehend aus Einzelrotationen mit einem großen Winkelsegment (<250) vs. zwei Teilrotationen mit je einem kleinen Winkelsegment (<60) wurden evaluiert. Für die maschinenspezifische Qualitätssicherung wurde ein künstlicher Testplan erzeugt und die Logdateien des Linearbeschleunigers (Linac) ausgewertet. Hierzu wurde ein VMAT Plan, der das volle Potential dynamischer Komponenten abdeckte, entworfen. Weiters wurden die in den Logdateien aufgezeichneten Fehler mit dosimetrischen Ungenauigkeiten korreliert, um die Validität der Linac-internen Logdateien zu demonstrieren. Testläufe und dosimetrische Messungen wurden an vier Instituten an achtzehn Elekta Linacs durchgeführt. Zusätzlich wurden Bestrahlungseffizienz und Robustheit quantifiziert. VMAT ermöglicht, bei gleichbleibender Bestrahlungsplanqualität für die Behandlung von linksseitigem Mammakarzinom inklusive Lymphabfluss die Bestrahlungseffizienz zu erhöhen. Die eingeführte Zweisegment-VMAT Technik erzielte eine bessere Schonung des Normalgewebes, insbesondere in Bezug auf Niedrigdosisbereiche. Die Auswertung der Logdateien erwies sich als geeignete Methode für eine prospektive Qualitätskontrolle des Linacs.Volumetric modulated arc therapy (VMAT) is an extension of intensity modulated radiation therapy (IMRT) that offers various advantages over IMRT such as reduced delivery time and monitor units (MU). Several prerequisites are needed in order to achieve clinically adequate plan quality and precise dose delivery. These include the research on treatment plan quality and dosimetric accuracy of the new method examining the entire technical chain and furthermore the development of a quality assurance program. Within the frame of this PhD thesis, both aspects were evaluated in depth and further developed with a special focus on breast cancer radiotherapy. One open question is whether VMAT is generally applicable for node positive left sided breast cancer, and respective planning studies were performed. For ten patients with left sided node-positive breast cancer single partial arc VMAT plans and four-field IMRT plans were generated using 6, 10 and 15MV photon beam energy. Subsequently, a novel approach was proposed and compared to the previously mentioned technique: VMAT plans consisting of a single partial rotation with a large arc segment (<250) vs. plans with two small tangential arc segments (<60) were evaluated. Machine specific quality assurance was investigated by developing an artificial test plan and tracking log files. More specifically, a benchmark plan was designed to cover the full potential of dynamic parameters, i.e. dose rate, speed of gantry and leaves. Furthermore, log file indicated errors were correlated with dosimetric inaccuracies to demonstrate the validity of the linac internal log files. Tests and dosimetric measurements were performed at four different institutions on eighteen Elekta linacs. Additionally, delivery efficiency and robustness were quantified. VMAT has the potential to retain plan quality for the treatment of node-positive left sided breast cancer while increasing delivery efficiency. The proposed small tangential arc technique improved normal tissue sparing, especially regarding low dose contributions. Log file evaluation proved suitable for a comprehensive tool for prospective machine specific quality assurance. Further research is necessary to compile a proactive quality assurance program that serves as a global baseline for VMAT commissioning.Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersWien, Med. Univ., Diss., 2015OeBB(VLID)171508

Comparison of complexity metrics for multi-institutional evaluations of treatment plans in radiotherapy

It is known that intensity-modulated radiotherapy plans that are highly complex might be less accurate in dose calculation and treatment delivery. Multiple complexity metrics have been proposed, but the relationships between them have not been thoroughly investigated. This study investigated these relationships in multi-institutional comparisons of treatment plans, where plans from multiple treatment planning systems (TPSs) are typically evaluated. A program was developed to compute several complexity indices and provide analysis of dynamic plan parameters. This in-house software was used to analyse plans from a recent multi-institutional audit. Additionally, 100 clinical volumetric modulated arc therapy (VMAT) plans from two institutions using different TPSs were analysed. All plans produced satisfactory pre-treatment verification results and, hence, complexity metrics could not be used to predict plans failing QA. Regarding the relationship among complexity indices, some very strong correlations were found (r > 0.9 with p < 0.01). However, some relevant discrepancies between complexity indices were obtained, even with negative correlation coefficients (r ∼ −0.6) which were expected to be positive. These discrepancies could be explained because each complexity index focused on different features of the plan and different TPSs prioritised modulation of different plan parameters. Some complexity indices provided similar information and can be considered equivalent. However, indices that focused on different plan parameters yielded different results and it was unclear which complexity index should be used. Careful consideration should be given to the use of complexity metrics in multi-institutional studies

Novel methodologies for dosimetry audits: Adapting to advanced radiotherapy techniques

With new radiotherapy techniques, treatment delivery is becoming more complex and accordingly, these treatment techniques require dosimetry audits to test advanced aspects of the delivery to ensure best practice and safe patient treatment.This review of novel methodologies for dosimetry audits for advanced radiotherapy techniques includes recent developments and future techniques to be applied in dosimetry audits. Phantom-based methods (i.e. phantom-detector combinations) including independent audit equipment and local measurement equipment as well as phantom-less methods (i.e. portal dosimetry, transmission detectors and log files) are presented and discussed. Methodologies for both conventional linear accelerator (linacs) and new types of delivery units, i.e. Tomotherapy, stereotactic devices and MR-linacs, are reviewed.Novel dosimetry audit techniques such as portal dosimetry or log file evaluation have the potential to allow parallel auditing (i.e. performing an audit at multiple institutions at the same time), automation of data analysis and evaluation of multiple steps of the radiotherapy treatment chain. These methods could also significantly reduce the time needed for audit and increase the information gained. However, to maximise the potential, further development and harmonisation of dosimetry audit techniques are required before these novel methodologies can be applied

Comparison of complexity metrics for multi-institutional evaluations of treatment plans in radiotherapy

Background and purpose: It is known that intensity-modulated radiotherapy plans that are highly complex might be less accurate in dose calculation and treatment delivery. Multiple complexity metrics have been proposed, but the relationships between them have not been thoroughly investigated. This study investigated these relationships in multi-institutional comparisons of treatment plans, where plans from multiple treatment planning systems (TPSs) are typically evaluated. Materials and methods: A program was developed to compute several complexity indices and provide analysis of dynamic plan parameters. This in-house software was used to analyse plans from a recent multi-institutional audit. Additionally, 100 clinical volumetric modulated arc therapy (VMAT) plans from two institutions using different TPSs were analysed. Results: All plans produced satisfactory pre-treatment verification results and, hence, complexity metrics could not be used to predict plans failing QA. Regarding the relationship among complexity indices, some very strong correlations were found (r > 0.9 with p < 0.01). However, some relevant discrepancies between complexity indices were obtained, even with negative correlation coefficients (r ∼ −0.6) which were expected to be positive. These discrepancies could be explained because each complexity index focused on different features of the plan and different TPSs prioritised modulation of different plan parameters. Conclusions: Some complexity indices provided similar information and can be considered equivalent. However, indices that focused on different plan parameters yielded different results and it was unclear which complexity index should be used. Careful consideration should be given to the use of complexity metrics in multi-institutional studies. Keywords: Plan complexity, Complexity metrics, Audits, Clinical trial

Real-time beam monitoring for error detection in IMRT plans and impact on dose-volume histograms

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