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Inverse radiotherapy planning for a concave-convex PTV in cervical and upper mediastinal regions

Inverse Bestrahlungsplanung für ein konvex-konkaves Planungszielvolumen im Halsbereich und im oberen Mediastinum

Simulation of radiotherapy using an alderson-RANDO phantom

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Abstract

Aim

Three-dimensional inverse treatment planning with modulated beams was applied for dosimetric optimization of a lengthy (22 cm) and complex (concave-convex) shaped planning target volume (PTV) in the cervical and upper mediastinal regions.

Material and Method

The planning was done for 9 coplanar beams spaced evenly at 40° intervals. Properties of 15 MV photons from a linear accelerator were simulated. The optimization of the fluence modulation profiles for each beam was based on a definition of the desired/permitted relative dose levels in the PTV and organs at risk, and a definition of the strengths of the constraints to achieve these objectives.

Results

An adequate dose delivery to the PTV and protection of the spinal cord are completely achievable. The dose delivered to the lungs is clinically acceptable with respect to the risk of radiation-induced pneumonitis. For reasons of physics, no further decrease in the radiation burden on the lungs can be attained with X-rays without compromising the PTV coverage. The radiation burden on some critical part of normal tissues was effectively reduced by application of a dummy organ at risk.

Conclusion

The inverse planning is an effective method for conformal radiotherapy of large tumors as well. However, the power of the technique is insufficient when the tolerance dose of the neighbouring normal tissue is too low and its volume effect is high. Although requiring further operator interactions, introduction of dummy organs at risk may be of help in reducing the radiation burden on normal tissues.

Zusammenfassung

Ziel

Die dreidimensionale inverse Bestrahlungsplanung mit einem intensitätsmodulierten Strahl wurde zur Optimierung der Dosisverteilung eingesetzt. Dieses Verfahren wurde bei einem länglichen (22 cm) und komplex (konvex-konkav) geformten Planungszielvolumen (PTV) im Halsbereich und im oberen Mediastinum angewandt.

Material und Methode

Die Bestrahlungsplanung erfolgte für neun koplanare Felder, die gleichmäßig im 40°-Winkelabstand angeordnet waren. Zur Simulation wurden 15-MV-Photonen eines Linearbeschleunigers angenommen. Die Optimierung der Modulationsprofile für jedes Feld erfolgte aufgrund der Vorgabe der gewünschten bzw. erlaubten Dosis im Planungszielvolumen und den Risikoorganen sowie der Definition der Randbedingungen, um diese Ziele zu erreichen.

Ergebnisse

Es ist möglich, eine adäquate Dosis im PTV bei ausreichendem Schutz des Rückenmarks zu applizieren. Die Bestrahlungsdosis im Bereich der Lunge ist klinisch annehmbar unter Berücksichtigung des Risikos einer radiogenen Pneumonitis. Aus physikalischen Gründen kann keine weitere Reduktion der Strahlenbelastung der Lungen erreicht werden, ohne daß Kompromisse hinsichtlich der Erfassung des PTV zu akzeptieren sind. Die Strahlenbelastung einiger empfindlicher Teile des Normalgewebes konnte durch die Anwendung eines “dummy organ at risk” deutlich reduziert werden.

Schlußfolgerung

Die inverse Bestrahlungsplanung ist eine wirkungsvolle Methode zur konformierenden Bestrahlungsplanung auch großvolumiger Tumoren. Jedoch sind die Ergebnisse dieser Technik unbefriedigend, wenn die Toleranzdosis der benachbarten Organe zu niedrig und der Volumeneffekt groß ist. Die Einführung von “dummy organ at risk” könnte hilfreich sein, um die Strahlenbelastung des Normalgewebes zu reduzieren, auch wenn damit die Bestrahlungsplanung aufwendiger wird.

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Authors and Affiliations

  1. Department of Medical Physics, German Cancer Research Center, Heidelberg, Germany

    T. Bortfeld, R. Bendl & W. Schlegel

  2. Department of Radiotherapy, National Institute of Oncology, Ráth György u. 7-9, H-1122, Budapest, Hungary

    O. Ésik & G. Németh

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  1. O. Ésik
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Correspondence to O. Ésik.

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Ésik, O., Bortfeld, T., Bendl, R. et al. Inverse radiotherapy planning for a concave-convex PTV in cervical and upper mediastinal regions. Strahlenther. Onkol. 173, 193–200 (1997). https://doi.org/10.1007/BF03039288

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