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Surface dose characterisation of the Varian Ir-192 HDR conical surface applicator set with a vertically orientated source

Charakterisierung der Oberflächendosis für den konischen Ir-192-HDR-Oberflächenapplikator vom vertikalen Typ

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Abstract

Background

Conical surface applicators with an Ir-192 high-dose-rate brachytherapy source are a common modality for the treatment of non-melanomatous skin cancer with high tumour control rates. Surface dose characterisation of the Varian Varisource GammaMed+ IX afterloader vertical type surface applicators is performed two dimensionally using high-resolution film dosimetry.

Aim

The focus of this study was to determine if Varian surface applicators with a vertical source suffer from the dose distribution irregularities reported for comparable applicators. Our goal was to evaluate if the irregularities found affected treatment and dose output verification procedures.

Methods

Ionisation chamber-based verification of applicator output was established according to guidelines provided by the manufacturer. For additional measurement of surface dose Gafchromic EBT3 film dosimetry was used. The term “therapeutic dose” was defined as 85 % of the prescribed dose level.

Results

For the 10 different applicator inserts evaluated, cold spots were observed. Mean cold spot size was 2.0 mm × 3.6 mm (± 0.6 mm). The cold spots were dosimetrically well below 85 % of the prescribed dose. The cold spot was situated 2.2 mm (1.4–2.7 mm) unilaterally from the central axis and caused general asymmetry in the dose profiles intersecting the cold spot area. A source tilt of approximately 8° (± 1°) was determined for the source used for irradiation.

Conclusions

A central underdosed area exceeding 15 % of the prescribed dose has not been previously reported. Source tilt was observed and found to affect clinical use and possibly treatment outcome in applicators using a vertically arranged source. Surface applicators with a vertically orientated source were subject to dose irregularities that could impact on chamber-based applicator output verification procedures. We recommend film dosimetry-backed applicator commissioning to avoid systematic errors.

Zusammenfassung

Hintergrund

Konische Oberflächenapplikatoren sind ein etabliertes Mittel um mit Ir-192 HDR-Brachytherapie nichtmelanomatöse Hautkrebserkrankungen mit hohen Tumorkontrollraten zu behandeln. In dieser Arbeit wird eine hochauflösende Analyse der Oberflächendosis eines Varian-Varisource-GammaMed+ IX-Afterloader-Konus-Applikatorsatzes des vertikalen Typs dargestellt.

Ziel der Arbeit

Analyse der Oberflächendosisverteilung des Applikatorsatzes im Hinblick auf Unregelmäßigkeiten, die für ähnliche Applikatoren bereits berichtet wurden. Weiterhin wird untersucht, ob Applikatoren des vertikalen Typs zusätzliche Verfahren als Ergänzung der aktuell vorgeschlagenen Methoden zur Verifikation des Dosisoutputs von konischen Oberflächenapplikatoren benötigen.

Methoden

Die Verifikation der absoluten Dosis wird nach Herstellerangaben durchgeführt. Die Dosisverteilung an der Oberfläche wird zusätzlich mit Gafchromic-EBT3-Filmdosimetrie bestimmt. Der Begriff „therapeutische Dosis“ ist in dieser Arbeit mit 85 % der verschriebenen Dosis definiert.

Ergebnisse

Zehn ausgewertete Applikatoren zeigen Unterdosierungen in der Applikatormitte, mit einer mittlere Größe von 2,0 mm × 3,6 mm (± 0,6 mm), dosimetrisch deutlich unter 85 % der verschriebenen Dosis. Die Unterdosierung liegt 2,2 mm (1,4–2,7 mm) seitlich der Zentralachse und verursacht eine generelle Asymmetrie in der Dosisverteilung. Ein Schiefstand von 8° (± 1°) wurde für die verwendete Quelle bestimmt.

Schlussfolgerung

Eine um mehr als 15 % reduzierte Dosis in der Applikatormitte wurde bei Oberflächenapplikatoren bislang nicht berichtet. Quellenschiefstand wurde als Parameter identifiziert, der die Handhabung dieser Oberflächenapplikatoren und möglicherweise den Therapieerfolg beeinflussen kann. Das beschriebene kleinräumige Dosisartefakt kann systematischen Einfluss auf kammerbasierte Dosisverifikationsverfahren haben. Wir empfehlen daher bei der Applikatorkommissionierung eine zusätzliche Absicherung mit Filmdosimetrie.

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Conflict of interest

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

The accompanying manuscript does not include studies on humans or animals.

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Authors and Affiliations

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

    Konrad Buchauer PhD,  Guido Henke,  Hans Schiefer &  Ludwig Plasswilm

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  1. Konrad Buchauer PhD
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  2. Guido Henke
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  3. Hans Schiefer
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  4. Ludwig Plasswilm
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Correspondence to Konrad Buchauer PhD.

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Buchauer, K., Henke, G., Schiefer, H. et al. Surface dose characterisation of the Varian Ir-192 HDR conical surface applicator set with a vertically orientated source. Strahlenther Onkol 190, 1163–1168 (2014). https://doi.org/10.1007/s00066-014-0713-2

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  • DOI: https://doi.org/10.1007/s00066-014-0713-2

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