Zusammenfassung
Klinisches/methodisches Problem
Die Strahlentherapie erfolgt standardmäßig auf Basis von CT-Aufnahmen, welche vor Beginn der Therapie gewonnen werden. Diese dienen auch der Bestimmung von Lage und Form des Zielvolumens. Da der Patient für jede Fraktion neu gelagert werden muss, können Abweichungen der Position des Tumors relativ zum Strahlungsfeld, aber auch interne Bewegungen des Tumors auftreten, welche zu Unsicherheiten führen. Diese Unsicherheiten werden im sog. Planungszielvolumen (PTV) berücksichtigt, indem ein Sicherheitssaum um das klinische Zielvolumen (CTV) im Normalgewebe addiert wird.
Radiologische Standardverfahren
Als Standard wird heute eine CT-basierte Bestrahlungsplanung vor Beginn der Therapie durchgeführt. Die daraus gewonnene Information über Lage und Form des Zielvolumens wird während des oft mehrere Wochen andauernden Bestrahlungszyklus unverändert genutzt.
Methodische Innovationen
Durch wiederholte Bildgebung des Patienten in Behandlungsposition vor jeder Fraktion kann die Lage des Tumors erneut bestimmt und für jede Fraktion korrigiert werden.
Leistungsfähigkeit
Die Reduktion der Lagerungsungenauigkeit kann für eine Reduktion des Sicherheitssaums genutzt werden. Dies führt zu einem reduzierten bestrahlten Normalgewebsvolumen.
Bewertung
Mit der Reduktion des bestrahlten Normalgewebsvolumens wird das Auftreten von Nebenwirkungen gesenkt. Dies bietet wiederum die Möglichkeit, eine erhöhte Tumorkontrolle durch Dosiseskalation zu erreichen.
Empfehlung für die Praxis
Bevor das PTV reduziert wird, muss eine genaue Analyse der Unsicherheiten für das jeweils verwendete bildgebende Verfahren und Bestrahlungstechnik durchgeführt werden.
Abstract
Clinical/methodical issue
As a standard, today’s radiation therapy is based on CT images which are used for therapy planning. These images are obtained once before therapy starts and serve as a basis to obtain the position and shape of the target volume. As the patient has to be positioned anew for each fraction, deviations of the tumor position relative to the radiation field but also internal motion of the tumor may occur. These deviations lead to uncertainties, which are taken into account by adding a safety margin around the clinical target volume (CTV) to create the planning target volume (PTV).
Standard radiological methods
As a standard today, CT-based treatment planning is used, where images are obtained once prior to therapy. The information on tumor position and shape, which is obtained from these images, is used throughout the whole cycle of radiation therapy without any change. This cycle may last several weeks.
Methodical innovations
By repeated imaging of the patient in the treatment position prior to each fraction, the position of the tumor can be assessed and corrected for each fraction.
Performance
A reduction of positioning uncertainty may be used to reduce the safety margin. This leads to a decreased volume of irradiated normal tissue.
Achievements
A reduced volume of irradiated normal tissue leads to reduced side effects and provides the opportunity of increased tumor control by dose escalation.
Practical recommendations
Before the PTV is reduced, a detailed analysis of the uncertainties for the specific imaging method and radiation technique must be performed.
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A. Schwahofer und O. Jäkel geben an, dass kein Interessenkonflikt besteht.
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Schwahofer, A., Jäkel, O. Planungszielvolumen. Radiologe 58, 736–745 (2018). https://doi.org/10.1007/s00117-018-0419-z
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DOI: https://doi.org/10.1007/s00117-018-0419-z