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Dosimetric Properties of the Octavius 1000SRS Liquid Filled Ionization Chamber Array

B Poppe

T.S. Stelljes1,2, B. Poppe1,2, H.K. Looe1,2, D. Harder3, K Willborn1, (1) Pius Hospital, Oldenburg, Germany (2) Carl von Ossietzky University, Oldenburg, Germany (3) University of Goettingen, Germany

SU-E-T-62 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The two-dimensional liquid filled ionization chamber array Octavius 1000SRS (PTW-Freiburg, Germany), being the conceptual successor of the LA48 linear array¹, has been introduced recently. Dosimetric properties and the applicability in modern dosimetry were investigated.

Methods: Either a Semiflex 31013 or 31010 ionization chamber (PTW-Freiburg, Germany) was used as a reference dosimeter to check for stability, linearity, saturation effects, output factors and energy dependence. Output factors were additionally measured with a diode (type 60012, PTW-Freiburg, Germany). PDD measurements of the array and a Roos chamber (type 31004, PTW-Freiburg, Germany) were used to determine the effective point of measurement. Dose profiles obtained with the diode and the array were compared and an IMRT verification was performed using the Gamma-Index and visualization of line dose profiles. All measurements were carried out with an Elekta Synergy or Siemens Primus accelerator.

Results: The 1000SRS showed excellent stability, with less than 0.1 % fluctuation, and linearity. At 400 Hz repetition rate the collection efficiency dropped by 3.5 % between 2.15 Gy/min and 11.7 Gy/min. The detector's relative response varied with field size and depth of measurement showing a small energy dependence of the array's response not exceeding a variation of ±2.6 % from the reference conditions (10 x 10 cm² in 10 cm depth). Output factor and dose profile measurements showed the applicability of the array in small field and stereotactic dosimetry. An IMRT field verification yielded excellent results even in regions with steep dose gradients.

Conclusion: This study shows the applicability of the Octavius 1000SRS in modern dosimetry. The high spatial resolution ensures adequate measurements of dose profiles in regular and intensity modulated irradiation fields.

¹C. Martens, C. De Wagter, W. De Neve, "The value of the LA48 linear ion chamber array for characterization of intensity-modulated beams," Phys. Med. Biol. 46 1131-1148 (2001).

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