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Computed Tomography as a Source of Electron Density Information for Radiation Treatment Planning

CT-Systeme als Datenquelle der Elektronendichte in Bestrahlungsplanungssystemen

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Purpose:

To evaluate the performance of computed tomography (CT) systems of various designs as a source of electron density (ρel) data for treatment planning of radiation therapy.

Material and Methods:

Dependence of CT numbers on relative electron density of tissue-equivalent materials (HU-ρel relationship) was measured for several general-purpose CT systems (single-slice, multislice, wide-bore multislice), for radiotherapy simulators with a single-slice CT and kV CBCT (cone-beam CT) options, as well as for linear accelerators with kV and MV CBCT systems. Electron density phantoms of four sizes were used. Measurement data were compared with the standard HU-ρel relationships predefined in two commercial treatment-planning systems (TPS).

Results:

The HU-ρel relationships obtained with all of the general-purpose CT scanners operating at voltages close to 120 kV were very similar to each other and close to those predefined in TPS. Some dependency of HU values on tube voltage was observed for bone- equivalent materials. For a given tube voltage, differences in results obtained for different phantoms were larger than those obtained for different CT scanners. For radiotherapy simulators and for kV CBCT systems, the information on ρel was much less precise because of poor uniformity of images. For MV CBCT, the results were significantly different than for kV systems due to the differing energy spectrum of the beam.

Conclusion:

The HU-ρel relationships predefined in TPS can be used for general-purpose CT systems operating at voltages close to 120 kV. For nontypical imaging systems (e.g., CBCT), the relationship can be significantly different and, therefore, it should always be measured and carefully analyzed before using CT data for treatment planning.

Ziel:

Vergleich verschiedener Computertomographie-(CT-)Systeme zur Bestimmung der Elektronendichte (ρel) für die Bestrahlungsplanung.

Material und Methodik:

Die Relation des CT-Werts zur Elektronendichte wurde an verschiedenen modernen CT-Scannern („single-slice“, „multislice“, „wide-bore multislice“) ermittelt, für die Therapiesimulatoren mit einem „single-slice“-CT und kV-CBCT-(„cone-beam“-CT-)Optionen sowie für Linearbeschleuniger mit kV- und MV-CBCT-Systemen. Vier unterschiedlich große Phantome zweier Hersteller wurden zur Messung der Elektronendichte benutzt. Die Messdaten wurden mit den Standardumrechnungsformeln zweier marktüblicher Therapieplanungssysteme (TPS) verglichen.

Ergebnisse:

Die HU-ρel-Relationen, die in allen modernen CT-Systemen vorhanden sind, waren untereinander sehr ähnlich, ebenso wie zu den vorgegebenen Relationen in den TPS. Einige Abweichungen der HU-Werte in Abhängigkeit von der Röhrenspannung wurden bei knochenäquivalentem Material beobachtet. Bei vorgegebener Röhrenspannung wurden bei den verschiedenen Phantomen größere Differenzen gemessen als in den verschiedenen CT-Scannern. Weniger exakt waren die Informationen über ρel mit den Therapiesimulatoren und KV-CBCT-Systemen aufgrund der mäßigen Uniformität der Bilder. Die Ergebnisse des MV-CBCT unterschieden sich aufgrund des unterschiedlichen Energiespektrums der Röntgenstrahlen signifikant von denen der kV-Systeme.

Schlussfolgerung:

Die im TPS vorgegebene HU-ρel-Relation kann bei modernen CT-Systemen mit einer Röhrenspannung im Bereich von 120 kV genutzt werden. Signifikant unterschiedlich dagegen ist die Relation bei nichttypischen Bildsystemen (z.B. CBCT). Deshalb sollte bei solchen Systemen immer gemessen und sorgfältig analysiert werden, bevor die CT-Daten für die Therapieplanung herangezogen werden.

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Authors and Affiliations

  1. Medical Physics Department, Center of Oncology, Warsaw, Poland

    Witold Skrzyński, Wioletta Ślusarczyk-Kacprzyk & Wojciech Bulski

  2. Medical Physics Department, Holycross Cancer Center, Kielce, Poland

    Sylwia Zielińska-Dąbrowska, Marta Wachowicz & Paweł F. Kukołowicz

  3. Medical Physics Department, Center of Oncology, Roentgena 5, 02-781, Warsaw, Poland

    Witold Skrzyński

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  1. Witold Skrzyński
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  2. Sylwia Zielińska-Dąbrowska
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  3. Marta Wachowicz
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  4. Wioletta Ślusarczyk-Kacprzyk
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  6. Wojciech Bulski
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Correspondence to Witold Skrzyński.

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Skrzyński, W., Zielińska-Dąbrowska, S., Wachowicz, M. et al. Computed Tomography as a Source of Electron Density Information for Radiation Treatment Planning. Strahlenther Onkol 186, 327–333 (2010). https://doi.org/10.1007/s00066-010-2086-5

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  • DOI: https://doi.org/10.1007/s00066-010-2086-5

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