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Multi-dimensional dosimetric verification of stereotactic radiotherapy for uveal melanoma using radiochromic EBT film

2007; Elsevier BV; Volume: 18; Issue: 1 Linguagem: Alemão

10.1016/j.zemedi.2007.08.001

1876-4436

Eva Sturtewagen, Martina Fuß, Leen Paelinck, Carlos De Wagter, Dietmar Georg,

Radiation Therapy and Dosimetry

Since 1997, linac based stereotactic radiotherapy (SRT) of uveal melanoma has been continuously developed at the Department of Radiotherapy, Medical University Vienna. The aim of the present study was (i) to test a new type of radiochromic film (Gafchromic EBT) for dosimetric verification of class solutions for these treatments and (ii) to verify treatment plan acceptance criteria, which are based on gamma values statisitcs. An EPSON Expression 1680 Pro flat bed scanner was utilized for film reading. To establish a calibration curve, films were cut in squares of 2×2 cm2, positioned at 5 cm depth in a solid water phantom and were irradiated with different dose levels (0.5 and 5 Gy) in a 5×5 cm2 field at 6 MV. A previously developed solid phantom (polystyrene) was used with overall dimensions corresponding to an average human head. EBT films were placed at four different depths (10, 20, 25 and 30 mm) and all films were irradiated simultaneously. Four different treatment plans were verified that resemble typical clinical situations. These plans differed in irradiation technique (conformal mMLC or circular arc SRT) and in tumour size (PTV of 1 or 2.5 cm3). In-house developed software was applied to calculate gamma (γ) index values and to perform several statistical operations (e.g. γ-area histograms). At depths of 10 mm γ1% (γ-value where 1% of the points have an equal or higher value in the region of interest) were between 1–3 and maximum γ>1 (% of γ-values >1 in the region of interest) areas were almost 30%. At larger depths, i.e. more close to the isocenter, γ1% was 1 areas were mostly 1 Bereiche lagen bei 30% (der Region of Interest). In größeren Tiefen, d.h. in Isozentrumsnähe, wurde mit γ1% 1<5% meist durchwegs eine bessere Übereinstimmung zischen Messung und Dosisberechnung erzielt. Die mittleren γ-Werte waren meist<0,5. Außer der beeinträchtigten Genauigkeit im Dosisaufbaubereich konnten für die SRT dieselben Akzeptanzkriterien erfolgreich angewendet, die für IMRT Verifikation abgeleitet wurden [Stock et Al, Phys. Med. Biol. 50 (2005) 399-411]. Der Radiochromatische Filmtyp EBT eignet sich in Kombination mit einem Flachbettscanner hervorragend für die mehrdimensionale Dosimetrie zur Qualitätssicherung von Bestrahlungsplänen. Die Zuverlässigkeit machen ihn zu einem geeigneten Ersatz für traditionell verwendete Silberhalogenid-Filme. Die auf die SRT übertragenen und verwendeten Akzeptanzkriterien für Bestrahlungspläne können in zukünftigen Studien mit unterschiedlichem Equipment (z.B. für andere Planungssystem oder Kollimatoren) als Benchmarking-Datensatz verwendet werden.