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Design and implementation of a “cheese” phantom-based Tomotherapy TLD dose intercomparison

Design und Anwendung eines „Cheese“-Phantoms basierend auf einem Tomotherapie-TLD-Dosimetrievergleich

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Abstract

Background

The unique beam-delivery technique of Tomotherapy machines (Accuray Inc., Sunnyvale, Calif.) necessitates tailored quality assurance. This requirement also applies to external dose intercomparisons. Therefore, the aim of the 2014 SSRMP (Swiss Society of Radiobiology and Medical Physics) dosimetry intercomparison was to compare two set-ups with different phantoms.

Materials and methods

A small cylindrical Perspex phantom, which is similar to the IROC phantom (Imaging and Radiation Oncology Core, Houston, Tex.), and the “cheese” phantom, which is provided by the Tomotherapy manufacturer to all institutions, were used. The standard calibration plans for the TomoHelical and TomoDirect irradiation techniques were applied. These plans are routinely used for dose output calibration in Tomotherapy institutions. We tested 20 Tomotherapy machines in Germany and Switzerland. The ratio of the measured (Dm) to the calculated (Dc) dose was assessed for both phantoms and irradiation techniques. The Dm/Dc distributions were determined to compare the suitability of the measurement set-ups investigated.

Results

The standard deviations of the TLD-measured (thermoluminescent dosimetry) Dm/Dc ratios for the “cheese” phantom were 1.9 % for the TomoHelical (19 measurements) and 1.2 % (11 measurements) for the TomoDirect irradiation techniques. The corresponding ratios for the Perspex phantom were 2.8 % (18 measurements) and 1.8 % (11 measurements).

Conclusion

Compared with the Perspex phantom-based set-up, the “cheese” phantom-based set-up without individual planning was demonstrated to be more suitable for Tomotherapy dose checks. Future SSRMP dosimetry intercomparisons for Tomotherapy machines will therefore be based on the “cheese” phantom set-up.

Zusammenfassung

Hintergrund

Die einzigartige Bestrahlungstechnik mit Tomotherapie-Bestrahlungsgeräten (Accuray Inc., Sunnyvale, CA, USA) erfordert spezifische Qualitätssicherungsmethoden. Dies trifft auch für externe Dosimetrievergleiche zu. Es war das Ziel des Dosimetrievergleichs 2014 der SGSMP (Schweizerische Gesellschaft für Strahlenbiologie und Medizinische Physik), Testmethoden mit zwei unterschiedlichen Phantomen zu prüfen.

Material und Methoden

Neben einem kleinen, zylindrischen Plexiglasphantom, das vergleichbar jenem des IROC ist (Imaging and Radiation Oncology Core, Houston, USA), wurde das „Cheese“-Phantom verwendet. Letzteres gehört zur Grundausstattung jedes Tomotherapie-Bestrahlungsgeräts. Es wurden dieselben Standardkalibrierungspläne für die TomoHelical- und TomoDirect-Bestrahlungstechniken abgestrahlt, die auch zur Dosiskalibrierung dienen. Insgesamt wurden 20 in Deutschland und in der Schweiz verwendete Geräte geprüft. Das Verhältnis der gemessenen (Dm) zur berechneten Dosis (Dc) wurde für beide Phantome und Bestrahlungstechniken ermittelt. Die Verteilung der Dm/Dc-Werte diente als Vergleichsmaß für die Eignung des Messaufbaus.

Ergebnisse

Die Standardabweichung der mit TLDs (Thermolumineszenzdosimetrie) gemessenen Dm/Dc-Verhältnisse betrug beim „Cheese“-Phantom 1,9 % für die TomoHelical- (19 Messungen) und 1,2 % für die TomoDirect-Bestrahlungstechnik (11 Messungen). Die Werte des Zylinderphantoms lagen bei 2,8 % (18 Messungen) und 1,8 % (11 Messungen).

Schlussfolgerung

Der auf dem „Cheese“-Phantom basierende Messaufbau und die Verwendung von Kalibrierungsplänen weisen Vorteile gegenüber der Zylinderphantom-basierten Methode auf. Für zukünftige SGSMP-Dosimetrievergleiche von Tomotherapie-Bestrahlungsgeräten wird deshalb das „Cheese“-Phantom verwendet.

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Acknowledgments

Dr. C. Pychlau, PTW Freiburg, and his staff are gratefully acknowledged for performing the irradiation of the reference TLDs. We also thank all participants. The Tomotherapy dose intercomparison presented here is part of cooperation between Accuray and the radiotherapy department of the Cantonal Hospital of St. Gallen.

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Authors and Affiliations

  1. Department of Radiation Oncology, Medical Physics Group, Kantonsspital St. Gallen, 9007, St. Gallen, Switzerland

    Hans Schiefer PhD, Konrad Buchauer PhD & Simon Heinze MS

  2. Department of Radiation Oncology, Kantonsspital St. Gallen, 9007, St. Gallen, Switzerland

    Guido Henke &  Ludwig Plasswilm

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  1. Hans Schiefer PhD
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  2. Konrad Buchauer PhD
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  3. Simon Heinze MS
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Correspondence to Hans Schiefer PhD.

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Conflict of interest

H. Schiefer, K. Buchauer, S. Hinze, G. Henke, and L. Plasswilm state that there are no conflicts of interest.

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Schiefer, H., Buchauer, K., Heinze, S. et al. Design and implementation of a “cheese” phantom-based Tomotherapy TLD dose intercomparison. Strahlenther Onkol 191, 855–861 (2015). https://doi.org/10.1007/s00066-015-0850-2

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