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A Method for Improved Verification of Entire IMRT Plans by Film Dosimetry

Eine Methode zur verbesserten Dosisverifikation von Gesamt-IMRT-Plänen mittels Filmdosimetrie

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Background and Purpose:

In one entire intensity-modulated radiation therapy (IMRT) plan, beams coming from different directions have different penetration depths in the phantom. Therefore, verifying an entire IMRT plan with a calibration curve for a single depth is error-prone. The aim of this study is to improve the quality of the dose verification of entire IMRT plans with film dosimetry.

Material and Methods:

The dose response of the Kodak EDR2 film to 6-MV photon beams was investigated in a solid-water phantom for different field sizes and depths. A method is proposed for evaluating measurements of the optical density with a calibration curve that takes the response at different depths into account. The described method was tested for three entire IMRT plans. For this purpose, calculated and measured dose distributions were compared, where the measured dose distribution was evaluated using a calibration curve for one depth (5 cm) and a calibration curve derived according to the proposed method.

Results:

All measurements suggest that the dose response significantly depends on the depth in the phantom, while dependencies on field size and off-axes distance are smaller. For tested plans, gamma index < 1 (using 3 mm distance and 3% dose as constrains) was reached for 81% and 91% of the points for one plan (prostate), 81% and 86% for the second plan (prostate), and 74% and 85% for the third plan (liver) when using two calibration curves, respectively.

Conclusion:

The method described here corrects for the change of the film response due to the variation of the scattered radiation with the penetration depth. This is achieved by a modified calibration curve, which can enhance the accuracy of the verification of entire IMRT plans.

Hintergrund und Ziel:

In einem Gesamt-IMRT-Plan (intensitätsmodulierte Radiotherapie) kommen Felder aus verschiedenen Winkeln und haben verschiedene Eindringtiefen im Phantom. Deshalb ist die Verifikation eines Gesamt-IMRT-Plans mit einer Kalibrierungskurve für eine Eindringtiefe nicht fehlerfrei. Ziel dieser Arbeit ist die Verbesserung der Dosisverifikation von Gesamt-IMRT-Plänen mittels Filmdosimetrie.

Material und Methodik:

Das Ansprechvermögen des EDR2-Films (Kodak) auf 6-MV-Photonen wurde für verschiedene Feldgrößen und Tiefen in einem Festwasserphantom untersucht. Es wird eine Methode zur Verifikation der Messungen unter Verwendung einer Kalibrierungskurve, die verschiedene Tiefen berücksichtigt, vorgeschlagen. Die beschriebene Methode wurde an drei Gesamt-IMRT-Plänen getestet. Dazu wurden gemessene und gerechnete Dosisverteilungen verglichen, wobei die gemessene Dosisverteilung mit einer Kalibrierungskurve für eine Eindringtiefe (5 cm) und mit einer auf der vorgeschlagenen Methode beruhenden Kalibrierungskurve ausgewertet wurde.

Ergebnisse:

Alle Messungen weisen darauf hin, dass das Ansprechvermögen des EDR2-Films signifikant von der Tiefe im Phantom abhängt, während die Abhängigkeiten von der Feldgröße kleiner sind. Bei den getesteten Plänen wurde jeweils ein Gamma-Index < 1 (mit 3 mm Abstand und 3% Dosis) für 81% bzw. 91% der Punkte in erstem Plan (Prostata), 81% bzw. 86% in zweitem Plan (Prostata) und 74% bzw. 85% in drittem Plan (Leber) erreicht.

Schlussfolgerung:

Die hier beschriebene Methode kompensiert die Änderung des Ansprechvermögens radiographischer Filme aufgrund der Variation der Streustrahlung mit der Eindringtiefe. Dies wird mit einer modifizierten Kalibrierungskurve erreicht, die die Genauigkeit der Verifikation von Gesamt-IMRT-Plänen erhöhen kann.

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Authors and Affiliations

  1. Department of Radiotherapy and Radiooncology, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany

    Peter Kneschaurek

  2. Klinik für Radiotherapie und Radiologische Onkologie, Klinikum rechts der Isar, Technische Universität München, Ismaninger Straße 22, 81675, München, Germany

    Vesna Jacob

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Correspondence to Vesna Jacob.

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Jacob, V., Kneschaurek, P. A Method for Improved Verification of Entire IMRT Plans by Film Dosimetry. Strahlenther Onkol 185, 34–40 (2009). https://doi.org/10.1007/s00066-009-1879-x

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  • DOI: https://doi.org/10.1007/s00066-009-1879-x

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